ASTRONOMICAL INSTRUMENTS.

There are two principal objects to be accomplished by astronomical instruments; the one is the extension and improvement of the science; and the other its application to geography, navigation, and the ordinary wants of society. Here we give the name astronomical instruments to such as in their application are directed to the heavenly bodies, as a telescope is directed to a star, and the axis of a sun-dial to the pole; but we do not consider as astronomical instruments orreries, and machines composed of wheels and pinions, such as exhibit imperfectly representations of the celestial motions. Globes are indeed appropriate furniture in an observatory, because they truly exhibit the relative positions of the stars and the different countries; and they serve to resolve approximately the different problems of the sphere. But complex orreries and planetary clocks are mere playthings. They excite admiration by the ingenuity displayed in their construction, but they are of no practical use. Showmen pretend to teach astronomy by their assistance; but their inutility in giving just notions of the dimensions and magnitudes of the bodies which form the solar system may be easily conceived by reflecting, that if in an orrery the earth be represented by a sphere one inch in diameter, the representation of the sun should be nine feet; also, that if the representation of Mercury describe a circle of about four inches radius, then the orbit of Uranus should have its radius sixteen feet.

Judicious teachers of astronomy may, however, employ with advantage simple contrivances to facilitate the acquiring of correct notions of the celestial motions; but they will direct the attention to a single object at a time, and not attempt to exhibit all the phenomena of the heavens at once. Wooden wheels and catgut bands are just as useful for the purpose in question as metallic wheels and pinions. A globe moved by the hand round a candle will serve to show the changes of the seasons; and in like manner the phases of the moon, her nodes and their motion, and the nature of eclipses, may be all explained by simple and easy contrivances.

1. Astronomical Telescope.

In a subsequent part of this work the theory of the telescope, and the various kinds of telescopes, will be fully explained. As, however, the telescope forms an essential part of almost all complex astronomical instruments, it will be proper to explain here in a general way its principles and use.

The astronomical telescope is composed of two principal parts, the object-glass, and the eye-glass or eye-piece. These are in opposite ends of a tube; and in its application the former is next the object, and the latter next the eye. In telescopes of the best construction the objectglass is composed of two and sometimes of three pieces. (See Achromatic Glasses.) We shall here, however, suppose it of the simplest form; that is, a very thin double convex glass, the opposite sides being portions of spherical surfaces. -

Let ABDE (Plate XCIII. fig. 131) be a lens of this form, C being the centre of a sphere, of which ABE is a portion of the surface. Let L be any point in an object to be viewed with the telescope, and let a ray of light proceeding from L fall perpendicularly on the convex surface in B: it will pass through the glass without being turned out of its direction, and will proceed straight forward on the prolongation of the line LBD.

Let another ray LI fall obliquely on the convex surface at I: by the principles of optics this will be refracted, that is, turned out of the direction LI, and take a new direction H'P. Draw CI to the centre; the new direction will be such that the sine of the angle of incidence MII. or CII. will be to the sine of the refracted angle CIP or CIP in the constant ratio of a given number η to 1.

We have therefore sin. CII. sin. CII. sin. ICP 'CL IP

γt ““ - - — - ∙ —— - ∙ ,. sin. CIP sin. ICI. sin. CIP “IL CP’

Let us suppose the point I to be near B; then IP=BP nearly, and IL = BL nearly, and we have

CL ∙ BP 1 z , x n ~ BL ~∙~CP , near y (ŋ

When the ray arrives at the concave surface ADE, it suffers a second refraction in passing into the air, and changes its direction from PP into a new direction I'L'. Let C' be the centre of a sphere, of which ADE is a part of the surface: draw the radius C'I', and we shall have sin. C'I'Ľ: sin. C'I'P:: n : I; and therefore sin. C'I'L' sin. C'I'Ľ sin. PC'P C'Ľ I'P γt -. — — sin. C' I' P sin. I'C'L'" sin. C'I'P " I'Ľ’ C'P’

We suppose the arcs IB, I'D, to be small; therefore I'P = PD nearly, and I' Ľ = DĽ nearly; hence

C'L'∙PD 1 η = DĽ ∙ CT’ nearly ∙

Since the thickness of the lens is supposed to be inconsiderable, we may assume that PD = PB nearly, and that DĽ = BL nearly, and then we have

_ C’Ľ-PB n ~ BĽ ∙ C'P

Let r = CB, √ = C'B, ∆ - BL, ∆' = BĽ, v = PB, and equations (1) and (2) become v(r+∆) r(r , + ∆')

n — ∖ ; n — ~~Γ7 —;—λ:

Δ (v — r) ∆! (v + r ’ ) from these we obtain η n — 1 1 1 n 1 n — 1

- — , and - = — —; v r Δ. v ∆! τ and hence

1’ , 1 _ , n ∫ι , iļ

When a ray of light passes out of air into glass, the sine of the angle of incidence is to the sine of the angle of refraction as 3 to 2 nearly, or as 11 to 1. In this case n = IL, and

— + — = ⅜ f- + -Y

Δ Δ' 2 ∖r ι j )

This expression will be the very same if we put r instead of r ’ , and r ’ instead of r; also, if we put Δ for Δ', and Δ' for ∆. Hence we may infer, that whichever of the two convex sides of the object-glass be turned towards the object L, the value of Δ' will be the same for a given value of Δ; also, that if L' be the focus to which rays issuing from L converge after refraction, then L will be the focus to which rays issuing from L' would converge after passing through the object-glass.

If Δ, the distance of L from the object-glass, be very great, then — will be very small in respect of —; and in the case of the heavenly bodies ∙^∙ vani∙ lies, and we have simply l = √l + ±γ

Δ' 2 ∖r ι j J

If we suppose both sides of the object-glass to be alike convex, that is, r — r , , then ∆' = r ; hence we learn, Ľí, that all rays which come from any point whatever of a [7:4:102]very remote object,' and which traverse a double convex •glass of equal curvature on both sides, are united by refraction about its centre of sphericity, which is called its principal focus ; 2d , that rays which proceed from a point at the centre of sphericity do, after refraction, proceed ,in lines which may be considered as parallel. All these conclusions are only true approximately. They suppose the convex arcs ABE ADE small, and the thickness of the - glass very little; nevertheless they differ but little from truth: the focus L is not indeed a mathematical point, but has a certain magnitude, which varies with the distance of the object and the breadth of the lens.

If we now suppose that two lenses, BD, bd (fig. 132), are adapted to the extremity of a tube, so that their centres of sphericity coincide at the same point F, and that this point and the centres of the lenses are in the same 'straight line, then, from what has been explained, it follows that rays coming from a distant object L, after passing through the lens BD or object-glass, will be collected at F (which is therefore called the focus of parallel rays), and will there form an image of the object L: also, that all the rays, after crossing in the focus F, will proceed forward; but in passing through the second lens or eye-glass bd, they will be again refracted, and emerge on the other side in parallel lines; and if they enter an eye now situated at 0, these parallel rays will produce distinct vision.

The eye does not see directly the remote object AC, but only its image formed at the focus F, and this in respect to the object is inverted; for the rays proceeding from A and falling on the object-glass at B, are by refraction turned into the direction BF, and meet the glass at δ. In like manner, the rays which are emitted from C, and pass through the object-glass at D, meet the eye-glass on the opposite side at ß: thus the object AC and its image formed at F have opposite positions.

The astronomical telescope, then, differs from the common telescope for viewing objects at a distance, inrevers-mg the position of objects seen through it, also the direction of their motions, the upper limb of the sun or moon appearing the lower, and all the heavenly bodies appearing to move from west to east. This, which would be an inconvenience with terrestrial objects, is of no consequence in viewing the stars.

The surface of the object-glass being always much greater than that of the eye-glass, which has a shorter focus, and all the rays which fall on the surface BD of the former being collected on the surface bd of the latter, they are there condensed, and the illumination is increased in the inverse proportion of the areas of the glasses; so that if the intensity of the light which falls on the object-glass be represented by 1, that on the eye-glass will , ∕BD∖ 2 1 1 . 1 , v .

be ĩ hence telescopes m general render objects more luminous and more easily distinguished. They also magnify objects; for let A be the centre and B the border of an object (fig. 133). The point A is visible to the eye 0, by the ray ADćzEO which traverses the objectglass D and eye-glass E, but suffers no refraction. (We here do not consider the oblique rays, which, proceeding from A, are collected at the principal focus.) The border B is visible by the ray BD6 at the focus b of the objectglass. This ray meets the eye-glass at d, and is there turned by refraction into the direction de ; and, in emerging from the glass at e, is again refracted to 0, its focus, so that OE is parallel to Eδ. The image is seen under the angle eOE = 6E«: but ab = Dα tan. D, therefore tan. E = γy-tan. D = — tan. D, or E = — D;

Lα r ’ r

R being the radius of sphericity of the object-glass, and τ that of the eye-glass. The angle under which the object is seen is therefore increased in the proportion of the two radii, and the magnifying power is the greater, as the ra-diùsof the eye-glass is less than that of the object-glass.

Common astronomical telescopes generallymagnify from 70 to 100 times; some even magnify 300 times. This, however, must not be understood in a rigorous sense; if, for example, we expect to see the moon 100 times larger with a telescope which is said to magnify 100 times, we may be disappointed. To produce this effect, the telescope ought to magnify more: it only represents the moon under an angle 100 times greater; but it is not by the visual angle alone that we judge of magnitude; our opinion is greatly influenced by the distance at which we suppose the object. When we see an object under the angle AKB (fig. 134), nothing determines whether this object is truly AB, or CD, or EFF; and according as we judge it to be in the first, or second, or third of these positions, or in one more remote, we assign to it magnitudes always increasing although the angle is still the same. But this judgment being uncertain, and such as cannot be subjected to calculation, the magnifying power of a telescope is in practice estimated by the angle of vision, which can always be exactly determined.

Let F be the principal focus of the object-glass C (fig. 135). If the angle HCG is such, that HG (=2 HF)=2 CF tan. HCF is equal to the diameter of the interior tube of the telescope, the angle HCG is called the field of view : every object whose focal image is greater than HG cannot be seen entirely in the telescope. This happens in the case of the sun and moon when viewed with telescopes of about 8⅛ feet, such as are used in considerable observatories. In these, the sun’s image will be about 9∣ inches. This exceeds the diameter of the tube. But the opening is yet more contracted by a perforated diaphragm, which, besides other purposes, serves to cut off the rays irregularly reflected from the inside of the tube, also those which produce colour in the image. To determine the field in view as limited by the diaphragm, then, we have , . . o ττ r , r HG 2 HF „

this equation; 2 tan. HCF = 77w = . If we put r

. Cr Cr for HF, the radius of the diaphragm, and R for CF, the radius of sphericity of the object-glass, the field of view . ; . 2 r m seconds is 1 ς—:——.

R sin. 1"

2. Dorpat Telescope.

The late Joseph Fraunhofer of Munich, a most skilful artist and experimenter in optics (whose demise in 1826, in the prime of life, was a great loss to science), constructed a magnificent refracting telescope for the observatory of the Imperial University at Dorpat. It was received by Professor Struve in the year 1825, and has since been found to fulfil most satisfactorily his expectation and the intentions of the maker. As this is one of the most magnificent instruments of the kind that has hitherto been constructed, and described by a figure, we have given an engraving of it, copied from the Memoirs of the Astronomical Society. (See Plate XCV.)

The object-glass of this telescope is about 9L inches in diameter, and its focal length about 14 English feet. The main tube is 13*8 feet; and, in addition, there is the small tube which holds the eye-pieces. Of these there are four; the least magnifying power is 175, and the greatest 700. After the telescope was received at Dorpat, a perfect micrometrical apparatus was ordered to be made for it. This was to consist of four annular micrometers, of which two were to be double; a lamp circular micrometer, with four [7:4:103]eye-pieces; a refracting lamp net micrometer, with position circles, and four eye-pieces.

The frame-work of the stand is made of oak, and the tube of deal, veneered with mahogany. The whole weight of the telescope and its counterpoises is supported at one point, namely, at the common centre of gravity of all the ponderous parts. These weigh 3000 Russian pounds, of which the frame-work contains 1000; the remaining 2000 are so balanced in every position, that the telescope may be turned, with ease and certainty, in every direction towards the heavens.

The basis of the frame is formed of two cross beams, each nine feet seven inches long. The ends of these are seen in the figure at A, B, C, D. They are braced by four smaller bars forming a square, one of which is seen at E. This braced cross is fastened to the floor by eight screws, six of which are seen in the figure. A perpendicular post, about six feet high and seven inches square, is fixed over the centre of the cross, and is propped at the north, east, and west sides by three curved stays, denoted by G, G', G", which are fixed at their lower ends to the beams of the cross, and at the upper to the vertical post. An inclined beam H of the same thickness rests on the southern end of the meridian beam of the cross, and is attached to the vertical beam in a position parallel to the polar axis. This axis, shown in the figure at I, is a cylinder of steel 39 inches long, and proportionally thick. It turns in two collars, and its lower end, which is rounded and polished, rests on a steel plate attached to the bearing piece K, which is secured to the inclined beam H, and has therefore very little friction, the weight being supported by friction rollers near the common centre of gravity; and a counterpoise L is applied to support the axis in any position. There is a circle 13 inches in diameter, graduated to minutes of time, fixed to the lower end of the axis, and furnished with verniers. The axis of vertical motion of the telescope, which has nearly the dimensions of the polar axis, passes through a brass tube at right angles to the latter; the tube, which is seen at Μ, forms a part of the frame, and is fastened to the upper end of the polar axis by twelve screws. This axis carries the circle of declination, which is 19 inches in diameter, and is divided to every 10', with a vernier reading 10" or 5" by estimation. The tube of the telescope is fixed to the frame-work nearer to the eye end than the middle, and has two counterpoises attached to levers, which balance the two ends, and prevent the natural tendency of the longer end to bend. The brass frame holding the two axes appears on the figure clamped to the tube by two strong rings, one at each end of the centre of motion. A bent lever, carrying the weight O, embraces by a double ring the near end of the axis of the declination circle. The axis itself carries another weight; and by this and the weight O it is counterpoised. The slow motion in altitude is given to the telescope by a Hooke’s joint applied to the screw of the clamp, which has a spring urging it against a strong iron bar P, attached to the end of the cylinder Μ, that forms a stop to the circle; and a slow equatorial motion is given by a second Hooke’s joint taking hold of an endless screw, acting with the racked edge of the hour circle, while a spring presses it into action uniformly, and a lever is employed to raise it out of the rack when necessary. The handles taking hold of these screws extend to the reach of the observer, who can thus point his telescope in right ascension and declination with the same certainty as the best meridian instrument.

A regular sidereal motion is communicated to the instrument by clock-work, which keeps a star apparently at rest in the centre of the field of view; and there .is a contrivance by which the sidereal can be changed into a solar, also to a lunar angular motion.

This almost invaluable instrument cost 10,500 florins (about 950 pounds sterling). The price, although it may appear considerable, yet barely covered the expense of the workmanship of its construction. This relinquishment of the profit of trade does great credit to the ingenious and liberal-minded artists, Fraunhofer, and Utzchneider, the chief of the optical establishment at Munich.

3. Sir James South’s Telescope.

Since the fabrication of the Dorpat telescope, an English astronomer, Sir James South, has been so fortunate as to procure, in France, an object-glass of even larger dimensions than that of the Dorpat instrument. The diameter is, we believe, about 12 inches, and the focal length 20 feet. The liberal proprietor has been for a considerable time past engaged in fitting it up, with the able assistance of the very ingenious artists Trough ton and Simms, and has just finished an observatory for its reception. It will of course be immediately applied to the exploration of the heavens; and considering what Sir James South has done with instruments of much inferior power, we look forward with confident expectation to the extension of our knowledge of some of those sidereal systems which have so much engaged the attention of astronomers.

4. Professor Barlow s Telescope.

The Dorpat achromatic telescope, and the still larger and probably more powerful telescope which Sir James South is so happy as to possess, are of the ordinary construction. The great difficulty was to obtain pieces of flint glass of such a size as to form the object-glass. Notwithstanding the belief (we fear delusive) that this difficulty was overcome by the ingenuity of artists in Switzerland, and the hope entertained from the partial success of experiments in this country, the difficulty of obtaining plates of glass of a proper size does still, and may perhaps always exist. Taking this view, Professor Barlow of the Royal Military Academy at Woolwich turned his attention to a mode of construction, in which the use of flint glass, the great desideratum, might be dispensed with; and from what he has already done, we have no doubt whatever of his ultimate success. Upwards of forty years ago, Dr Blair attempted to improve the common construction of the achromatic telescope, by the introduction of a fluid into the combination of glasses which formed the object-glass; but Mr Barlow has proceeded in a different way: he placed an object-glass of plate glass (which without much difficulty may be obtained good of considerable magnitude) in the farther end of the telescope; and in order to correct the colour, he placed, about midway between the object-glass and its focus, a fluid lens, through which the rays pass, and are refracted to a focus, so as to produce a correct and colourless image.

A telescope thus constructed has two advantages. The fluid lens, which is the most difficult part of the construction, is reduced to one half, or less than one half, of the plate lens at the end of the tube; and, what may be considered as of still more importance, a telescope of this kind, of 10 or 12 feet in length, will be equivalent in its focal power to one of 16 or 20 feet of the ordinary construction. (See Mr Barlow’s papers in the London Phil. Trans, for 1828-29-31.)

The fluid which has been selected is sulphuret of carbon. This has a refractive index about equal to that of the best flint glass, with a dispersive power more than double; it is perfectly colourless, beautifully transparent, and, although very expansible, possessing the same, or very nearly the same, optical properties, when hermetically sealed, [7:4:104]under all temperatures to which it is likely to be exposed for astronomical purposes.

Mr Barlow has actually constructed, and mounted in an observatory, a telescope with a clear aperture of 7∙8 inches, which, before the introduction of Sir James South’s telescope, exceeded by about an inch the largest in this country. Its tube is 11 feet, which, together with the eyepiece, makes the whole length 12 feet; but its effective focus is 18 Feet, and it carries a power of 700 on the closest double stars in South’s and Herschel’s catalogue. The ingenious inventor of this valuable instrument read a paper to the Royal Society in December 1830, On the performance of fluid refracting telescopes, and on the applicability of this principle of construction to very large instruments. From what he stated in this paper, it appears, that although his telescope does not equal in power either Sir James South’s 20-feet telescope, or the fine new reflecting telescope with a 20-inch speculum, constructed by Mr Herschel, yet it makes a nearer approach to them, we believe, than any other achromatic telescope now in Britain.

Finally, Mr Barlow declares that he is willing, with proper aid, to undertake the construction of a telescope of much greater dimensions: he suggests an aperture of 2 feet, and a length of 24 feet. We believe this is not one of the innumerable projects which commonly end in disappointment; and we hope that William IV., our present gracious king, to whom Mr Barlow’s powers are well known, may give him such countenance and support as his father George III. afforded to the late Sir William Herchel.

5. Mr Rogers s Telescope.

The difficulty of procuring disks of flint glass of the requisite magnitude, for the object-glass of achromatic telescopes of the common construction, and their great expense when obtained, has induced an ingenious mathematician, Mr Alexander Rogers of Leith, to attempt a construction in which the flint glass part of the telescope is not required to be so large as that composed of the more easily procured material, plate glass. He proposes to interpose between a single object-glass, formed of plate glass, and its focus, a solid compound lens divested of refraction by the opposing powers of a convex of plate and a concave of flint glass, but possessing a dispersion equal to the difference of the dispersions of its component lenses. This construction appears also to possess the great advantage, that the perfection of the telescope does not require any very accurate knowledge of the refraction and dispersion of the kinds of glass employed, or any extreme coincidence of the foci and curvatures of the lenses with those proportions which theory requires, as the correction of the optical aberrations is completed by certain adjustments of the positions of the lenses. This construction is somewhat like Mr Barlow’s; in both, the correction of colour is produced, not at the object-glass, but at a considerable distance from it, in a position where the cross section of the rays, by their convergence, has been diminished, and therefore where the interposed correcting lens admits of a like contraction in size, in comparison with the objectglass.

Mr Rogers has actually procured a telescope to be constructed on his principles: we have seen it compared with a common achromatic telescope of like dimensions, in looking at terrestrial objects, and it seemed to perform well. I he proper test, however, of a telescope, is a double, triple, or quadruple star. In a telescope of moderate length the comes of the pole-star answers very well. Mr Barlow, with a telescope of his construction, having an aperture of three inches, could see this small star with a magnifying power of 46. We know that a most competent judge entertains a favourable opinion of Mr Rogers’s construction; but unless a man has leisure, and can perform the most important manipulations himself, or else has ample means of paying skilful workmen, the invention or improvment of instruments is any thing but a profitable speculation to him. (For an account of Mr Rogers’s telescope, see Mem. Astr. Soc. Lond. vol. iii. part 2.)

The account here given of Messrs Barlow and Rogers’s telescopes is to be considered merely as a notice : we shall have occasion again to recur to them. We might have adverted to two powerful telescopes, one a 20-feet reflector, with an aperture of 18 inches, constructed by J. F. W. Herschel, Esq., and the other, also a reflector, with an equal aperture, but 25 feet in length, constructed by Mr John Ramage of Aberdeen; but after what has been said, it seems sufficient to refer for these to the Mem. Astr. Soc. Lond. vol. ii. part 2, where they are fully described.

6. Transit Instrument, Meridian Circle, and Astronomical Clock.

The primary problem in geography is to determine the exact position of any proposed point on the earth’s surface in respect of the equator and some assumed meridian, as that of Greenwich or Paris; that is, to find its latitude and longitude. The corresponding problem in astronomy is to determine the position of every fixed star, and in general of any celestial phenomenon, in respect to the equinoctial circle in the heavens (or else the pole) and a circle passing through the pole and the intersection of the equinoctial and ecliptic; that is, to find its declination or polar distance, and its right ascension (chap. i. sect. i.). The former of these is found in great observatories by the mural quadrant or mural circle, and the latter by the transit instrument or meridian circle, and the clock.

The first transit instrument of which we have any account was that of Roemer, which he described in 1700. (Miscel. Ber. tom. iii.) Dr Halley placed a transit instrument in the Greenwich observatory in 1721. The axis was iron, and the telescope about five feet in length. This has been long laid aside, but we believe it has been preserved as a relic. Transit instruments of the present day are of two forms; one, the most common, is adapted to the determination of the right ascension only, the other to the determination of both right ascension and declination, either at once or by separate observations.

We have selected one of the most modern of the first kind for description. It is now in Sir James South’s observatory at Kensington, and was constructed for him in 1820, by his friend Trough ton, with all the care he could bestow on it; and, as far as the just proportions of its parts are concerned, he regarded it as his happiest production. The instrument, with its various parts, is represented in Plate XCVI. Figures 1 and 3 represent two views of it. EO is an achromatic telescope, of which E is the end next the eye; AA ʹ is its axis of motion, with which the tube is closely united at their junction, so as to form but one body. The extremities of the axis rest in notches formed on two cheeks of metal at A A', which are firmly attached to the inner faces of two stone pillars PP'; and B, B', B' ʹ, B", are four braces connecting the tube and axis. These are the parts of the instrument which, on inspection, immediately meet the eye. When adjusted, the axis of motion of the instrument is truly horizontal and perpendicular to the plane of the meridian, and the optical axis of the telescope is in the plane of the meridian; the object of the whole construction is to keep it precisely in that plane, whatever position be given to the telescope by turning on its axis.

The object-glass of the telescope is four inches in clear, and its focal length seven feet two inches. The body of the telescope and the axis are formed of conical [7:4:105]tubes firmly united in a spherical centre-piece, on which • their wider ends rest, and cover two thirds of its surface: thus the tube of the telescope is formed of two pieces, which taper towards its extremities, where their diameters are the same as that of the object-glass. The axis in like manner tapers equally towards its extremities.

The centre-piece is perforated in the direction of the telescope, and also in that of the axis; the width of the first opening being a little more than the radius of the object-glass, and that of the second just enough to allow the light of a lamp placed near the end of the axis to pass uninterruptedly to the centre illuminator. The ends of all the four cones, where they join the sphere, are strengthened by circular pieces of cast brass, which extend full four inches into the cones, and are fixed by solder and pins. They are turned concave in front, so as to fit the surface of the sphere into which they are rabbeted, and serve to keep the opposite branches of the axis and telescope straight and perpendicular to one another; and to these pieces are attached rings for the reception of the screws which bind the whole together. The four branches of the axis and telescope are solely united by tension bars. These pass through the sphere, six in the direction of the axis and four in that of the telescope. They are arranged at equal distances between corresponding parts, but so as neither to obstruct the rays of the object-glass nor the light of the lamp that falls on the illuminator. They screw into the rings of the brass pieces which enter the cones. The tension bars serve a most important purpose in giving stiffness and permanence of form to the instrument; and he that would imitate it, would do well to study Sir James South’s description in the Phil. Trans. R. S. 1826. Fig. 3 is a section through the axis, and exhibits the six bars which bind together the cones of the axis, and also the places of the four which are perpendicular to them, and which connect the tubes of the telescope. Fig. 4 is a section through the telescope; the bars of the telescope are shown lengthwise, whilst those of the axis are perpendicular. In both figures the illuminator within the telescope is shown, in one the polished surface, and in the other the back of the plate. The illuminator crosses the tube of the telescope at an angle of 45°. This position requires that the opening in it, through which the light coming through the object-glass passes, should be an ellipse. The braces B, B', B", B'", extending from the cones of the axis to those of the telescope, are attached to the former about two inches from the pivots, and to the latter about ten inches from the centre-piece. They exert but a very slight pressure, and might have been omitted in this instrument. They were added in imitation of the Greenwich transit, to which they are essentially necessary.

The apparatus for giving the telescope any required altitude is shown at the eye end in figures 1 and 2, but on a larger scale in fig. 5. It consists of two complete circles, six inches in diameter, firmly attached to the eye end of the telescope: each is provided with two opposite verniers, subdividing its divisions into minutes of a degree. The indices have clamps and slow moving screws, and microscopes are attached to the verniers; a spirit level is also attached to the index of each circle. The apparatus is adjusted by setting the index to the place of the star, and then, the telescope being moved round till the bubble of the level stands in the middle of its range, the star will traverse the field between the two horizontal wires. If two stars differ but little in right ascension, as Capella ∙and Rigel, so as not to allow time for changing the index which was set to the altitude of that which came first, then the index of the other circle may be set to the altitude of the following star, and both observed. When the same object is to be observed by direct vision, and also by reflection, then one of the indices may be set to point the telescope to the direct place of the star, and the other to its reflected image.

Figures 7 and 8 exhibit the plates or side-pieces, and Ys in which the pivots of the axis rest. The plates, which are semicircular, are imbedded in the stone piers, and are firmly screwed to them. Figure 7 represents the eastern plate, in which the contrivance for placing the axis truly level is contained. This adjustment is made by a piece of which the upper end is formed into a Y, and which may be moved vertically, but not laterally. To raise or depress it gradually, there is a piece having a short cylindrical part in the middle; also a fine screw at its upper end, which works in the movable piece, and a coarse screw at its lower, which works in the fixed plates. The cylinder has holes by which it can be turned round by a capstan pin. By the ingenious contrivance of the two screws, the sliding piece is moved vertically, but slowly; for the space gone through is only the difference of the spaces through which it would have been pushed or drawn by each screw acting by itself. Fig. 8 shows the western plate. In this the Y piece admits only of a horizontal motion for the purpose of placing the instrument in the meridian. The adjustment is effected by two screws, which work in the opposite sides of the piece, and whose heads abut against the fixed plate. To produce motion, one of them must be screwed and the other unscrewed by equal quantities while the observer’s eye is at the telescope; and to effect this the screws are connected by pinion work put in motion by a handle hanging down close to the inside of the western pier. (See fig. 1.)

Fig. 9 is a bird’s-eye view of the head of one of the piers. This is meant to show the apparatus for relieving the pivots of the axis and the Ys from a great part of the weight which would otherwise bear on them. Immediately behind the adjustable Y r piece, but rather broader, is a plain piece of brass having a Y cut in its upper end: a lever also is seen, one extremity of which passes into a hole made in this Y piece, while the other end carries a weight. The bar of the lever is expanded into a circle whose centre is about one third of the lever’s length distant from the pivot of the axis. The circle admits the illuminating lantern. Two steel screws, with blunted, hard, and polished points, are inserted in the diameter at right angles to the direction of the lever: these rest on hardened and polished planes, which are let into the stone pier, and together form the fulcrum, in the manner of a balance. The weight is a short cylinder hooked on the end of the lever: it is hollow to receive small shot, introduced, as a counterpoise, to relieve, more or less at pleasure, the instrumental portion of the pivot, also the instrument Y piece, of weight.

Fig. 6 is a perspective view of the eye end of the telescope. In it a micrometer is shown, which moves a plate contiguous to that in which the five transit wires are inserted: one wire is contained in the movable plate, and is intended to facilitate the observation of the pole-star and others near it.

In fig. 1, on the eastern side of the telescope, a projecting finger-screw is seen. This gives motion to an apparatus within the tube of the telescope for regulating the quantity of light projected by the illuminator on the transit wires.

. The Greenwich transit instrument, also the workmanship of Troughton, and one of his much-admired productions, was placed in that observatory in 1816. In its construction it is nearly the same as Sir James,South’s, but of different dimensions. The object-glass of the telescope is 5 inches in clear aperture, and the focal length 10 feet: its horizontal axis, including the pivots, is 3 feet 10 inches. The semicircles at the eye end of the telescope being insufficient to enable the observer to direct the instrument [7:4:106]to the reflected image of a star, a divided circle, two feet in diameter, is attached to one end of the axis. The pivots, originally of hard bell-metal, having suffered an alteration of figure from constant use, were replaced in 1825 by others of hardened steel. There is no apparatus whereby an observer can give a small azimuthal motion to the instrument with his eye at the end of the telescope. The piers are the same which supported the transit which preceded this. They are two feet square, and were formerly 6 feet 2 inches high; but to adapt them to the present instrument their height was augmented one foot, by placing on their top a semicylindrical piece, which projects 3 inches over the old piers, and inwards, because the axis of the present instrument is six inches shorter than was that of the former.

There are seven fixed vertical wires in the focus of the telescope, and two horizontal ones. Each of the latter is placed about a minute and a half from the centre of the field, and between them a star passes during an observation: an interval of 18 ’ 3 seconds of time elapses while a star in the equator passes between each adjoining two vertical wires. Besides these, there is attached to the eye-piece a fine micrometer, which carries a single vertical wire through a large range. This serves various practical purposes, particularly in observing the pole-star, or a star very near it.

The Greenwich instrument has no plumb-line; the axis is adjusted by a fine ground spirit-level, which, when in use, stands above the axis, upon the pivots.

All transit instruments have a meridian marh, that is, a mark on some remote object, by which it may be ascertained at any time whether the instrument be truly in the meridian.

Transit or meridian circles are in their nature quite analogous to the instruments we have described; with, however, the important addition of a graduated circle of considerable dimensions as a principal part of their construction. The meridian circle which Troughton constructed for Mr Groombidge, but which afterwards passed into the hands of Sir James South, is a fine example of this kind of instrument. A figure and description of it may be seen in Dr Pearson’s valuable work on Practical Astronomy. The observatories of Königsberg and Göttingen possess very fine meridian circles, constructed by the late Reichenbach of Munich.

A clock of the very best construction is an indispensable companion of the transit instrument. This may be regulated so as to show either mean solar or sidereal time, according to the principal objects in view, the one being always easily convertible into the other. The attention of ingenious men has been long directed to the construction of the astronomical clock; but on this subject we shall have occasion to treat fully elsewhere in our work.

The transit instruments here described are of the most expensive kind, and adapted to the higher efforts in the cultivation of astronomy; but there are also portable transits which travellers may use, and which may serve to determine true time. In all applications of the transits, the axis must be placed truly horizontal by means of the level; the line of collimation, that is, a line between the centre of the object-glass and the centre of the cross wires, must move in a great circle, or, which is the same thing, it must be perpendicular to the axis; and the vertical circle which it describes must be the meridian: these are the three principal adjustments. The Nautical Almanac gives the exact time when the sun or certain considerable stars pass the meridian. The observer gives the telescope such a position that the star must appear in the field of view in its passage. When it is seen, he notes the exact time, by the beat of the clock, when it crosses each wire. As there are always pairs of wires equally distant from the middle wire, the instant of passing the middle wire and the middle instant between the times of passing any pair· will be the same. The exact time of the passage is thus found, not only by the middle wire, but also by each pair equally distant from it.

As an example of an observation, the transit of the star α Aquilae was observed to pass the wires of a small instrument at the following times, 12th September 1827.

H. μ. s.

1 wire 8 20 21

2 8 20 41∙5

3 8 21 1

4 8 21 21∙5

5 ....8 21 41

5) 105 6∙0

21 1∙2

The middle interval between the first and fifth wire is 8 h ∙ 20 m ∙ 1 3 ∙, and between the second and fourth 8 h ∙ 20 m ∙ 1 s ∙5; one agreeing exactly, and the other differing half a second from the observed passage over the middle wire. The mean of the whole is 8 h ∙ 21 m ∙ l s ∙2, and is obtained by taking one fifth of the sum of all the observed times. It was found from the right ascension of the sun and star given in the Almanac, that the star passed the meridian that day at 8 11 ∙ 18 m ∙ 7 s ∙9, hence it appeared that the clock was 2 m ∙ 53 s ∙3 fast.

In observing the sun’s transit, the time of the passage of the preceding and following limbs must be observed; and the middle instant is that of the passage of the sun’s centre, or apparent noon. To this, the equation of time being applied, the result will be the time before, or after mean noon. As an example, the sun’s transit was observed, July 25, 1827.

O 1 Limb. Θ 2 Limb.

II. M. S. H. M. S.

1 wire 0 6 0∙0 0 8 14∙0

2 0 6 22∙0 0 8 35∙5

3 0 6 43∙0 0 8 56∙5

4 0 7 4∙5 0 9 17∙5

5 0 7 25∙5 0 9 39∙Q

Mean.. . 0 6 43∙0 0 8 56∙5

Θ centre passed 7 49∙7

Eq. time 6 7∙6

1 42∙1

Hence it appears that the clock must have been 1 m - 42 s ∙ fast.

7. Mural Quadrants and Circle.

The Royal Observatory at Greenwich has two mural quadrants, each about eight feet radius; these are fixed on a massive structure of hewn-stone, in the form of a parallclopiped, one on each side, and hence their name. That whose telescope is directed towards the north is chiefly of iron, and was erected by Graham in 1725, for the lunar observations of Halley: it was, however, redivided by Bird in 1753. The other, for the southern part of the meridian, is of brass, and was constructed by Bird, and placed in its position in 1750: with this Bradley and Maskelyne successively made their observations for forty-six years.

Experience has now shown that entire circles have a great advantage over quadrants; accordingly, the use of the Greenwich quadrants has been discontinued, and in their stead a mural circle six feet in diameter, constructed by Troughton, was placed in the observatory on 12th June 1812. A second mural circle, nearly a copy of the other, constructed by Thomas Jones (another eminent artist), was, in 1825, placed in the observatory, fronting the former, and at a distance of seven feet; and with these [7:4:107]two instruments simultaneous observations are daily made on the polar distances of the heavenly bodies. The stars are observed both directly, and by reflection from a surface of quicksilver; sometimes directly or by reflection with both instruments, and sometimes directly with the one and by reflection with the other.

Figs. 1 and 2 of Plate XCVII. give two views of Troughton’s mural circle. In fig. 1 the circle is seen obliquely on the front or eastern face of the wall, with the greater part of its apparatus. The breadth of the wall from north to south is seven feet, its thickness from east to west four feet, and its height ten feet. It is formed of four stones laid one on another. The third stone has in its under side a semicircular groove, cut in its middle from west to east, six inches radius. The upper side of the second stone being worked level, forms the diameter of this semicircular arch-hole, and supports the axis work of the instrument at about five feet above the floor. The real centre of the instrument is about five inches higher. The nucleus of the Greenwich circle is an octagon of eight inches diameter at the corners; its depth is three inches, and a circular perforation of six inches and a half is made through its whole depth. The outward faces of the octagon, which are each three inches square, support eight of the circle’s conical radii, to which they are screwed and steady-pinned. The other eight radii are fitted in closely, each one between two of the former, so that their lower ends come down on the corners of the octagon. The limb of the circle consists of two rings, the interior one having its plane parallel, and the exterior perpendicular, to the plane of the circle, so that, when united, their section will be represented by the letter T. The interior or flat ring has in the engraving the appearance of passing through clefts in the middle of the outer ends of the sixteen radii, which are there solid. The perpendicular ring is fitted close on the exterior edge of the other, to which it is screwed, and also to the ends of the conical radii. The cones are bound together at half the distance from the double ring to the centre, by a circle of interposed bracing bars. The circular aperture of the octagon is shut up by plates before and behind, which are fastened to the octagon by strong steel screws. The posterior plate has a large circular hole, and the anterior a smaller one, both truly wrought. Into these the axis of motion is fitted, and united to the octagon and circle by means of screws. The axis is a cone of brass nearly seven inches in diameter in front, but behind only half as much, and nearly four feet long; this works in a socket, which at each end receives it, and in which it fits with the greatest possible exactness. The two parts which fit the axis are soldered into a strong brass tube, larger than the tube of the axis, but nearly of the same shape. On the tube of the sockets in front is soldered a strong perforated plate or upright bearing piece, at right angles to the axis, which nearly fills the semicylindrical aperture in the wall; and at the remote end is soldered a short cylinder, the use of which will be explained. It is there that the adjustments for placing the circle in the meridian, and for levelling the axis, are performed. Two strong horizontal plates are fastened on the lower surface (which is flat) of the perforation through the wall, one before and the other behind. The bearing piece of the socket in front only rests upon the plate, but behind, the bearing cock and plate are screwed together. In front, the plate and bearing piece are connected by a conical piece of hardened steel, which is fixed under the middle or this piece, and fits nicely into a hole in the plate, but so as to revolve. At this end of the axis these parts do not come quite in contact; for there are fixed under the bearing piece, at each extremity, about ten inches apart, two short props, like buttons of hardened steel, the spherical surfaces of which rest upon planes of hardened steel fixed in the plate. The central conical piece prevents the circle· from sliding sideways when angular motion is given round this conical piece to bring the instrument into the plane of the meridian. It has been stated that a short cylinder was soldered on the remote end of the cone of the sockets. This passes into a perforation in the cock behind, which perforation is greater than that of the cylinder. Two fine threaded screws at right angles to each other work in the cock, one vertically for levelling, and the other horizontally for meridian adjustment. The two screws only press with their points against the sides of the short cylinder; but opposite to them are the ends of two small cylinders standing in the same line, which are urged forward with spiral springs, and thus force the short cylinders into contact with the screws. The telescope is seen on the face of the instrument; its focal length is six feet two inches, which is the exact outer diameter of the circle; the aperture is four inches, and its common magnifying power about 150. The telescope is attached to the circle at the centre by a steel axis, which passes through the proper axis of motion from end to end, and was indeed the arbor on which the axis was turned. The weight of the telescope is supported on its own axis, and it may be fixed to the circle in any position, by means of two clamps which keep hold of the border of the circle. The graduation of the instrument is on the convex cylindrical surface of the exterior ring, therefore the reading microscopes have their direction parallel to the plane of the instrument. The divisions on Troughton’s circle arc made on a narrow ring of white metal, composed of four parts gold to one of palladium; and the figures which count the degrees are engraved on a like ring of platina. In Jones’s circle the divisions are on gold. None of these metals tarnishes in the least degree. The divisions are by lines, and suited to wires which cross in an acute angle in the reading microscopes. The degrees are cut into 5' spaces, and are numbered from the pole southward to the same pole again, viz. from 0° to 360°. The 5' spaces are subdivided by the microscopes to single seconds; and a division representing this quantity on the micrometer head may be easily estimated to the tenth of a second. There are six reading microscopes; but in general the two horizontal ones only are used.

In order that the circle may move easily round on its axis, there is an apparatus for counterpoising it, or for lifting the whole weight, without which the load would press altogether on the lower side of the front socket. This is effected by means of two large rollers, shown below the axis in fig. 1. The rollers, set in a double frame, act on the edge of the centre flanch, nearly in contact with the radial cones. Two perpendicular bars of steel, at about the height of the centre, are connected with the frame of the rollers by hook and eye; and these bars are in a similar manner suspended by two beams, each resembling a common balance, at the top of the wall. The part which appears in front is shown in fig. 1, and one of the beams, its fulcrum and counterpoising weight, near the top of the wall, in fig. 2. This apparatus produces a simple lift, without any tendency to affect the due motion of the circle’s axis.

There is another flat, circular ring, somewhat larger than the graduated one, fastened at several places to the wall, and nearly touching it. On this ring the clamp and screw for slow motion slide, and may be clamped to it at any part of the ring.

The plumb-line of the mural circle, seen in fig. 2, is for placing the axis truly horizontal. The apparatus by which the plummet is suspended applies by dovetail fittings occasionally to the wall near its top, see fig. L The apparatus itself is shown in fig. 2. In fig. 1 fixed micro [7:4:108]scopes are seen on the telescope, near its end for viewing •the plumb-line.

The wires of the telescope are illuminated by a diagonal reflecting plate in the middle of the tube, which receives the light by a circular aperture, seen in fig. 1, in a line with the centre of the circle. A lantern, at four or five feet distance, placed in the line of the axis, throws light on the field of view.

The instrument seen in a vertical position on the back of the wall is a zenith micrometer. It was erected to discover, if possible, the parallax of a star that passes very near the zenith; but it has never been used.

The mural circle has as an accompaniment a clock; and since all observations made with it require to be corrected for refraction, which depends on the state of the atmosphere (see table in page 100), every recorded observation must have annexed to it the height of the barometer and thermometer.

We have now described the instruments which constitute the principal furniture of an observatory, and which are all that the present state of the science absolutely requires. We proceed next to describe some others which are extremely convenient, and articles of luxury in the science.

8. Equatorial Instruments.

An equatorial instrument is of great value to a practical astronomer, for by means of it he can direct his telescope at once to any phenomenon, however minute, whose right ascension and declination are known; and reversely, he can determine the right ascension and declination of any phenomenon to which it is directed, although out of the meridian. There is a fine instrument of this kind in the Greenwich observatory. It was made by Ramsden, for Sir George Shuckburgh.

Plates XCVIII. and XCIX. give a representation of Sir James South’s five-feet equatorial instrument. The greater part of this instrument is composed of tinned iron plate; and its characteristics are lightness, steadiness, promptness in answering to its adjustments, and capability of retaining them. Fig. 1 of Plate XCVIII. represents the instrument as viewed at right angles to the declination circle. The polar axis is about 101 feet long; the lower end is a pivot attached to a cone, which, reckoning upwards, is about a fourth of the whole length. The higher side of the cone is cut in a sloping direction, as seen in the figure, for the purpose of more conveniently observing the vicinity of the pole. From the upper end of the cone the polar axis branches into two parts, between which is room for the declination circle and the head of the observer. These two branches are again united at the top by an open frame of bell metal, represented in fig. 2, to which the upper pivot is attached. This frame, as well as the iron-work which composes it, is so contrived as to present the least possible surface to obstruct the telescope; for the same reason the pivot at the top of the telescope is made as small as possible, whilst that at the lower end is considerably larger. Both ends of the axis are supported on stones, the northern end rising within about four inches of the level of the declination circle, the rest of the support being of wrought iron. At the southern end the stone rises very little above the floor, but a cast iron frame supports the pivot at the height of about two feet. The Y, or angle which supports the lower pivot, is placed upon the frame, and provided with two Screw adjustments, one for giving the axis its due elevation, and the other for bringing the instrument to the meridian. The form of the iron-work will be understood by consulting the different figures. The two branches of the polar axis on their upper sides are formed of broad planes, making one continued plane. On these the axis and reading microscopes of the declination circle are fixed. 1 The instrument is self-balanced by the position and figure of its parts, and the addition of a weight fixed to the conical part of the polar axis. The diameter of the declination circle is four feet, the length of the telescope five feet, and of the axis about thirty-two inches. In Plate XCVIΠ. fig. 1, the declination circle appears quite plain, like the head of a drum, with the telescope directed towards the equator. In Plate XCIX. fig. 1, the polar axis is considerably fore-shortened, from the position of the draughtsman in making the drawing of the instrument. In this figure the edge of the declination circle is shown as a short cylinder, with the telescope protruding beyond it. In this figure, also, the shape of the declination axis, and the two principal microscopes for reading the declination, arc shown. There is a third microscope, which indicates zenith distances. This is seen in Plate XCVHI.' fig. 1, between the eye end of the telescope and the instrument’s elevated pole. In the same figure is shown a narrow brass ring, whereon the graduation is made.

The hour-circle, two feet in diameter, is fastened to the lower end of the polar axis; its edge is seen in Plate XCVIII. fig. 1, and its under side in Plate XCIX. fig. 1 and fig. 3. One of the reading microscopes is well seen in fig. 1 of Plate XCVΠI., and both of them less perfectly in the other two figures. The circle is of brass, and the divisions (fine lines) are on an inlaid ring of platina, corresponding to twenty seconds each; these are subdivided by the microscopes to tenths of seconds. The declination circle is divided to five minutes, which are subdivided by the micrometer screw of the microscopes to single seconds. The instrument is furnished with two ground levels. The divided side of the declination circle is quite flat; but the opposite face is articulated, showing how the parts are united. It is on this side that the levels would be seen; one of them is parallel to the telescope, and the other to the declination axis.

The clamps and screws for slow motion are unusual, but remarkably good. Instead of the common mode of clamping the circle, in this instrument the clamp is made to grasp the axis. There is soldered on each axis a ring of brass, the outer edge of which is broad and cylindrical. On this fixed ring a movable one is fitted, and afterwards cut into three equal parts: these are again united in two of the three sections by joints, like those which bind the different parts of a watch chain together. At the third juncture the clamping takes place, a projecting part of the ring having been there cut through, leaving one half on each side of the section. Here the ring gapes, but a screw passes through the projecting pieces or ears on each side of the disjunction, and, in bringing them together, grasps the axis with a firm embrace. To the middle of the tripartite rings are attached long arms of tinned iron plate, at the extremities of which the slow moving screws have their places. The fixed stud is in the lower screw, planted in the iron support; that of the upper one is in the polar axis. The long screw for slow motion in right ascension is acted on by a contrate wheel and a pinion at right angles to the plane of the circle, as shown in fig. 1, Plate XCVIII.; a long handle is attached to it, and shown leaning against the northern pier. A similar screw for declination, but without the contrate part, is seen in Plate XCIX. fig. 1.

The illumination of the wires of the telescope is made by a small lantern placed at one end of the declination axis; and there is a contrivance between the nozle of the lantern and the end of the axis, by which the light is adapted to different observations.

The eye-piece of the telescope is represented in Plate XCIX. fig. 2, in which there is seen the edge of a gra [7:4:109]duated circle, the front of a quadrant, and two small spirit ’ levels. There is likewise shown, but partially, a double parallel line micrometer, which also measures angles of position.

This instrument, when first constructed, was designed to be placed where a meridian mark could not be obtained. A mark, however, could be placed and seen to the westward; and to take advantage of this, the axis of the declination circle was converted into a telescope with two object-glasses of equal focus, two sets of cross wires, and an eye-glass that might be placed in either end; a mark was then built up to the level of the axis, and in a line at right angles with the meridian, and this formed a substitute for a meridian mark. The instrument bears no maker’s name, but the scheme of its fabric was devised by the late Captain Huddart, F. R. S. The brass-work, &c. was made by J. and E. Troughton, under his direction; and the object-glass for the telescope of 3^ inches aperture by P. and J. Dollond. To preserve the tinned work from oxidation, it is well covered with white paint, and varnished; thus it has not only a neat appearance, but can be easily cleaned at any time.

With this instrument, and another equatorial of sevenfect focal length, the object-glass being made by the late Tully, Sir James South and Mr J. F. W. Herschel made observations on the apparent distances and positions of 380 double stars; a labour which they have since greatly extended, to the no small advancement of this part of astronomy.

9. Azimuth, and Altitude Circle.

An azimuth and altitude circle, called also an astronomical circle, is in itself a complete astronomical apparatus, when combined with a good clock or chronometer; for by means of it the latitude of the observatory may be found with great accuracy, and afterwards the declination or polar distance of any celestial phenomenon. It may be used also as a transit, and applied to the determination of right ascensions; and, when portable, it serves as a surveying instrument. The transit instrument and mural circle can only be applied to meridian observations; but the azimuth and altitude circle, like the equatorial instrument, may serve to determine the position of a star in any quarter of the heavens. To a traveller who wishes to avail himself of all opportunities of improving geography and astronomy, this instrument is of the greatest value; and it gives the cultivator of astronomy who happens to possess it the means of gratifying his taste in making all kinds of observations. It was with an instrument of this kind, constructed by Troughton, that the present astronomer royal, Mr Pond, made his valuable catalogue of the polar distances of 44 principal stars, deduced from 1452 observations, which detected the defects of the Greenwich mural quadrants, and established his fame as an astronomer. The observations were made at Westbury, and the instrument has hence acquired the name of the Westbury circle. Its fame recommended it to the Glasgow Astronomical Institution, who purchased it and placed it in an observatory; but this scientific project failed, and we regret to say that its funds were reduced to such a state as to induce the institution to accept of fifty pounds for the Westbury circle. It passed into the hands of an eminent English astronomer who knew its value, and who put it into complete repair, and reckoned it a great acquisition. He, however, was rich in other instruments, and was mduced to part with it for (as we have heard) four or five hundred pounds.

Azimuth and altitude circles are of various sizes. Troughton has constructed many portable ones, with the vertical circle eighteen, and the horizontal fifteen, inches in diameter. Sir Thomas Brisbane has a fine instrument of this kind (by Troughton) in his observatory at Brisbane Castle; but it is not portable, the axis being fixed in a stone pier. Its vertical circle is two feet in diameter.

The Astronomical Institution of Edinburgh last year (1830) placed an azimuth and altitude circle in the observatory on the Calton Hill. It is the workmanship of Troughton, and another most ingenious artist, Mr Simms, now associated with the veteran astronomical engineer. The pier on which it stands is a frustum of a cone, having for its base a square prism six feet ten inches in the side, and rising nine inches above the floor of the observatory. The part of the pier above the base is composed of eight stones, each of which is a frustum of a cone, and the whole height of the pier from the floor is nineteen and a half feet. The diameter of the circular section of the pier at the base is equal to the length of the side of the square on which it stands, and at the top it is about two feet. The instrument is represented as it stands on its pier under the dome of the observatory, and is composed of two principal parts, the azimuthal or horizontal circle, and the vertical circle with its telescope. By this last the altitude of any celestial object is taken, and by the former azimuthal angles are measured. The axis, which is not seen in the figure, is firmly attached by screws to the cross horizontal bar, or thick plate, which bears the upright pillars of brass that support the vertical circle; and it is also made fast to the centre of the azimuthal or horizontal circle. The stone pier is hollow to a certain extent downwards, and receives a brass conical socket, which passes also through the hexagonal stone on the top. The socket is suspended in the cavity, without touching it, by a projecting flanch at its open end, which is the base of the cone; and this is firmly screwed down to a flat brass ring fixed in the upper surface of the hexagonal stone, which is in fact a part of the instrument. The axis of the horizontal circle goes into the socket without touching its sides, and it rests with its lower end, a blunt steel point, in a corresponding steel cavity at the bottom; thus the socket bears the whole weight of the instrument.

The upper end of the axis is kept in its place by a right-angled hole, having two springs opposite the points of contact, which press it against its bearings, while it turns in contact with only four points, with a steady and easy motion. The bar in which the vertical axis is thus centred is acted on by two adjusting screws that are independent of each other, and stand at right angles the one to the other, by means of which the axis is adjusted to its true vertical direction, while the blunt point continues in its subjacent cup at the bottom of the socket. The frame to which this apparatus is attached is composed of an hexagonal central piece, from which six strong conical tubes of brass proceed, and these are screwed to as many bearing pieces or cocks, standing on and made fast to the hexagonal stone, as shown in the engraving.

The azimuthal circle is composed of ten smaller conical radial tubes, and a circular limb of two feet diameter, divided into spaces of 5' all round, and, being firmly attached to the vertical axis, turns round between the radiated frame and the upper face of the stone pedestal. Its divisions arc read off to seconds by two microscopes, one of which is seen in the figure, the other is hid by the instrument. The circle has a slow motion, regulated by a screw with two milled heads, which may also be turned by a handle with a Hooke’s joint, which is seen near one of the angles of the hexagonal stone. The screw acting in the outward end is a compound bar, that terminates in a ring on the axis of the circle. The ring is cut into three portions, which are again united by joints at two of the sections, leaving the third open and gaping. There are two ears on the ring, one at each side of this opening, through which a strong screw passes. By turning the screw the opening in the ring closes, and the three parts of which it is composed, by moving on the joints, embrace the axis firmly and clamp it; the axis may be released by the reversed [7:4:110]motion of the screw. There is another compound frame, one half of which is seen to the right of the former. This carries the microscopes at its opposite ends. It surrounds the central part of the main frame of radial cones that carries the adjusting screws, and is fixed over the opposite zeros of the horizontal circle when the instrument is placed in the meridian.

The vertical circle is 3 feet in diameter, and is divided into spaces of 5'. Its horizontal axis is supported by the two strong vertical pillars that turn with the concealed vertical axis, and is composed of three strong tubes; the middle one being cylindrical, and the two end ones conical, admitting of the transmission of light like the axis of a transit instrument. The axis is two feet long; its pivots are of bell-metal, and rest in adjustable Ys similar to those of a transit instrument. The circle is composed of two limbs, connected with its axis by conical radii, each limb having its own, twelve in number. The limbs are united by bars crossing obliquely, like net-work, from the one to the other, as shown in the figure. The divided face is read by a pair of opposite microscopes, which are supported in a horizontal position by a bar turned up at the ends, and fixed to one of the upright pillars; and a revolving level, seventeen inches long, hangs constantly on this piece, and is seen in the figure. The telescope has an aperture of 3∣ inches, and a focal length of 51 inches. The clamp of the vertical circle is seen on the inside of one of the upright pillars: and this, by turning the milled head of a screw, closes on the opposite flat sides of a portion of the limb that is not divided, and holds it fast, but subject to the slow motion which is produced by a double-headed miłled screw, or by a handle with a Hooke’s joint, which fits upon the axis of the screw. The upright pillars, which are hollow, contain within them strong spiral springs; these, by pressing upwards, support a large portion of the weight of the circle, and thus relieve the Ys, which are supported by gibbet-pieces attached to the pillars, as shown in the engraving.

The axis of the vertical circle is placed exactly horizontal by means of a spirit level, which when used passes between the cones of tĥe circle, and rests with its reversed Y feet on the pivots. But this level is not shown. A truly vertical position is given to the axis of the instrument by means of a plumb-line suspended from an apparatus made-fast to the upper end of the long tube, seen to the left of the farther upright pillar, to which it is made fast at two places. The tube is 4 feet 6 inches long, and this is about the length of the silver wire by which the plummet is suspended; the plummet is a perforated cylindrical vessel containing lead shot, and is suspended in water contained in a cylinder, which fixes on the bottom of the tube, and may be taken off. The weight of the plummet is just as much as the wire will bear out of the water without breaking. The upper end of the wire is fixed in the angular point of an adjustable bearing piece of metal, that is moved by screws acting at right angles to each other. A microscope enters horizontally the side of the long tube, and with this the plumb-line is viewed, and at the same time the image of a luminous disk, formed by a perforation through a minute circle of metal, placed in a circular surface of mother of pearl. This image will be exactly bisected by the plumb-line in every position, if the axis of the horizontal circle be truly vertical; otherwise the adjusting screws must be applied to the axis until ∙the image remains bisected, while the instrument is turned completely round. There are seven fixed vertical and three horizontal wires in the focus of the telescope; and the eye-piece is made to slide to the right and left, so that the eye may view directly the image of a star all the way in its passage across the field of view.

In selecting instruments for description in the general article Astronomy, we have chosen the principal, such as are used at this time for the extension of the science. There are others of secondary importance. These will be explained in their alphabetical place. See Circle, Collimator, Dynameter, Level, Micrometer, Observatory, Sector, Telescope, Vernier, &c.

Writers professedly on Practical Astronomy are, Ludlam, Astronomical Observations, &c; Vince, A Treatise on Practical Astronomy ; Pearson, Introduction to Practical Astronomy ; Francoeur, Astronomic Pratique. See also Account of a Trigonometrical Survey of Britain ; Ease du Systeme Métrique, par Delambre; Recueil d ’ Observations Géodesiques, &c. par MM. Biot et Arago; and in general all Collections of Astronomical Observations.
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