The daily morning journal and courier., 1900-04-05

stttoM Glass--Thc Worlds Greatest
 TeIcscope-Astronom Predictions
 for April, 1900.
 There are many interesting practical things about an astronomical observa- tory with which the public seldom has an opportunity to become acquainted
 Among these perhaps the details con- nected with setting up great telescope take first rank The writer happened to be present at the Cape of Good Hope observatory when the photographic equatorial telescope was being mounted and the operation of putting it in po- sition may be taken as typical of simi- lar processes elsewhere.
 In the first place, it is necessary to explain what s meant by an equator ial' telescope One of the chief dit1l- culties in making ordinary observations arises from the rising and setting ot the stars. They are all apparently moving across the face of the sky, usually climbing up from the eastern horizon, only to go down again and set in the west. It therefore we wish to scruti- nize any given object for a considerable time we must move the telescope con- tinu0usly so as to keep pace with the motion of the heavens. For this pur- pose the tube must be attached to aX- les, so that it can be turned easily in any direction. The equatorial mount ing is a device that permits the tele- scope to be thus aimed at any part Of the sky, and at the same time facili- tates greatly the operation of keeping it pointed correctly after star has once been brought into the field of view.
 To understand the equatorial mount- ing 1t is necessary to remember that the rising and setting motions of the heavenly bodies are apparent ones only and due in reality to the turning of the earth on it's own axis. As the earth goes around, it carries observer, telescope and observatory past the stars fixed up- Ion the distant sky. Consequently, to keep a telescope pointed continuously at a given star, it Is merely necessary to rotate it steadily backward upon suit able axis just fast enough to neutralize exactly the turning of our earth.
 By suitable axis of this purpose we mean one so mounted as to be exactly parallel to the earths own axis of rota- ton. A little reflection shows how simply such an arrangemeN will work. All the heavenly bodies may be regard- ed for practical purposes, as excess ively remote in comparison with the di- mensions of our earth. The entire planet shrinks into absolute insignifi- canoe when compared with the dis tances of the nearest objects brought under observation by astronomers. It follows that if we have our telescope attached to such a rotation-axis as we have described, it will be just the same for purposes of ubservation as though the telescopes axis were not only paral- Tel to the earths axis, but actually CO., incident with it. The two axes may be separated by a distance equal to that between the earth's surface and its cen ter; but as we have said, this distance is insignificant so far as our present ob- ject is concerned.
 There is another way to arrive at the same result. We know that the stars in rising and setting all seem to revolve about the polar star, which itself seems to remain immovable. Consequently, if we mount our telescope so that it can turn about an axis pointing at the pole, we shall be able to neutralize the rota- tion of the stars by simply turning the telescope about the axis at the proper speed and in the right direction As tronomical considerations teach us that an axis thus pointing at the pole will be HAL ,,,tI I,te t~ we arrive at the same fundamental principle for mounting an astronomical telescope, from whichever point of view we consider the subject.
 Every large telescope is provided with such an axis ot rotation; and for the reason stated it is called the polar axis The telescope itself is then called an "equatorial." The advantage of this method of mounting is very evident. Since we can follow the stars motion by turning the telescope about one axis only, it becomes very simple matter to accomplish this turning auto matically by means of clockwork.
 The Following" of star being thus provided for by the device of polar axis, it is of course, also necessary to supply some other motion so as to en- able us to aim the tube at any point in the heavens. Tor it is obvious that if it was rigidly attached to the polar axis we could indeed follow any star that happened to be in the field of view, but We could not change this field of view at will so as to observe other stars or planets. 10 accomplish this the tele- | scope is attached to the polar axis by means of pivot. By turning the tee- 1 scope around It's polar axis, end also on | this pivot, we can find any object in the heavens; and once found. we can leave 10 the polar axis and it's automatic | clockwork the task of keeping thal ob- feet before the observers eye
 In setting up the Cape of Good Hope the astronomers were | obliged to do a large part of the work | of adjustment personally. Far away 1 from European instrument makers, the | parts of t1e mounting and telescope had to be assembled or put together, by the astronomers of the Cape observa- tory. A heavy pier of brick and ma sorry had been prepared in advance. Upon this was placed massive iron case, intended to support the super structure of polar axis and telescope This base rested on three points, one of