Morning journal and courier., 1880-02-25

| ng the transit over one line, exactly when the
 star crosses his approximate meridian. say approximate meridian, because the middle ! line only marks the meridian within certain limits which are set by the principle which underlies all practical astronomy, that after human art is exhausted in rendering an in strument as perfect as possible, there are still
 measurable deviations from perfection itself. | In the case of our transit we are unable to
 have 1t perfect in 8 single point; we cannot | put the middle line of this group exactly in | the center of the true telescopio field of view. | We cannot make this axis about which the | telescope turns perfectly horizontal, we can | not bring the telescope so that it shall move exactly in the meridian; and even If we could
 accomplish either of these three things, we
 should hardly have time to announce it before | that unseen motion which is constantly going
 on among the molecules of every body in the
 visible universe, would have disturbed it to
 an extent apprsciable by the application Of | some refined method of investigation.
 In order to measure one day, therefore, we | must assure ourselves of the amount of change our instrument has undergone iu hatinterval. | This is accomplished by the observation of a | number of stars distributed over the northern
 and southern heavens. We can compute for | the discrepancies among such a series Of stars | the correction we should apply to each one of | them, to get the same result we should have | obtained in our measure of the days length, If we had used perfect instrument moving pre- cisely in the meridian. Let us briefly con- | slider the principles we should adopt to free | our observation of star of the errors spring | ing from the three sources I have mentioned. First we can determine how much that middle | line is away from the true center of the teles- cope, by observing some pretty slow moving | star near the pole as it crosses the first two | wires of the group, we will say. Now we know the distance which these two wires are II from the middle one, and we can therefore determine the time at which the star would | ! 11Ek -.-kor n1Io l ly moving, and after it has passed the second wire; now suppose that we skilfully and rap Idly lift our instrument out of its bearings and replace it with it's horizontal axis end for end, and point the telescope upon the same star which has yet to cross these two wires. Now if you reflect moment, you will see that these are the same two wires across which we first observed the stars transit, only they are now on the other side of the field of view, and if we compute at what time the star crossed the middle wire from
 these last two transits we shall find that it differs from the time derived from the first two transits by twice the amount the middle line is from the true center. Secondly, we can determine how nearly level this horizontal
 axis is by the means of this very delicate spirit level, which you see rests only on the axis it self And thirdly, we can determine how much
 our instrument still deviates from the meridian, by combining star transits taken near the pole and the equator. It is the habit of astronomers to consider series of stars observed in this manner constituting one
 time determination. And if our clock shows by it's face that the sidereal day just measured
 has ended ten minutes too soon, then we say that our clock is ten minutes slow.
 think now you will understand me when I say that the first objection the astronomer
 has to observing the sun for time, is that it is
 difficult to get enough stars in the day time to determine the position of the instrument, and another objection is found in the greater un- certainty attending the transit of the suns limbs, which I think we can see on the screen. We have here beautiful photograph taken from the sun directly, and for which we are indebted to the skill of Lewis M. Rutherford, Esq You will notice that the rounded limb of the sun cannot be SO nicely bisected as can the image of this star which follows after wards.
 Let us now examine the method of noting
 the transit of star across wire. If I take this chronometer or that clock, I can count the beats as I sit with my eye to the tele- scope ]; and as the star crosses each wire, I can note the second and the fraction of ond; and . skillful observer will only on rare occasions estimate this fraction fifth of @ second in error. It is better, however, to les | sen the errors which depend upon the pc rson- | ality of the observer, such as his observing ! too fast or too slow, and to economize the time of writing down the observations, to re- | cord them automatically, by means of the chronograph, an instrument first used in this connection by an American astronomer. We | have beautiful one before us, and you see it consists of 8 metallic cylinder around which | sheet of paper is coiled which is revolved uni formly by clock work. A fountain pen rests | upon the surface of the paper, and as the
 cylinder revolves the pen draws line upon it. Now, if you conceive that this cylinder be slowly moved along at the same time it re- l wolves, you will understand that the pen never marks over the same part of the paper. Suppose that the cylinder rotates just once in sixty seconds, and suppose that I cause this clock, by means of an electric circuit to slight | ly move the pen at the beginning of each
 ond; this will cause slight notch in the line, which registers upon the paper the beginning
 of each second, and if we omit the slight notch which would be made by the. fifty ninth
 second we can thus register the beginning of
 each minute. This telegraph key which I hold in my hand is in the same electric circuit with the clock and chronograph, and as this star is passing over the screen I can register it's transit upon the chronograph by simply causing the pen to make notch in the line by breaking the electric circuit. We
 have put telegraphic sounder in the same circuit, so that think you will be able to hear the beats of the clock quite to the other
 end of the hall. There comes our star, and as
 it crosses each wire you will hear the familiar telegraph tick which tells us that we have made the slight notch on the chronograph which records the stars transit. Now, SUD- pose that this slight notch we afterward find occurs six tenths of the way between the
 thirtieth and thirty first second. Then we
 know that the stars transit occurred at thirty one seconds and six-tenths of a second of a
 particular minute. After an evenings ob- servation, the sheet is removed from the cy- finder, labelled, and filed away with the erecords of the Observatory.
 Having obtained the error of our time-piece to within a twentieth of single second the nua6tIOn ia " hQ irna piece so that it will have the same error to- morrow night that it has tonight; or failing in this, how shall we preserve the same rela- tion between the errors on consecutive nights? In other words, how shall we know that the time-piece gains or loses regularly? This leads us to speak of the way clocks and watches keep the time from day to day. You all know that the test of the performance of any time- piece is found in noting the regularity with
 which it gains or loses. Thus clock which
 gained ten seconds day might be a very
 much better one than another which gained and lost alternately ten seconds day. Though
 at the.end of the week the better clock would
 be a minute more in error than the second
 one. The point I wish to illustrate is that
 with an accurate hme-piece we can always
 predict what its error will be, for some days
 in advance, while with poor time piece we
 can form no idea, from the determination of
 it's error on two nights, what it will be on B
 third one. Now it is to the elimination of
 the sources or error Tn CLOCKS ana WaLcneS that
 the attention of the artisan is directed; and
 the practical form which successive improve
 ments take is in more perfectly protecting our
 time pieces from the effects of temperature
 changes, and from those resulting from varia-
 tions of friction in the movement. In the
 clock we endeavor to guard against the effects
 of temperature on the pendulum by uniting
 two metals in such way that one expands up-
 wards while the other expands downwards,
 and they are so adjusted that the center of the
 pendulum stays very nearly in the same posi-
 ton. Now although the clock is the most
 perfect time-piece we have, yet it is still lia-
 ble to the theoretical objection that It's pen
 dulum swings in a circular instead of Q cycloi-
 dal arc. You will be interested to know that
 the finest clocks for astronomers uses are so
 sensitive to external influences that if the ba-
 rometer were to change an inch in height, it
 would cause variation in the clocks rate of
 about 8 quarter of a second per day, and I
 might mention that in some large observatories
 the standard clock is kept in cellar vault to
 avoid changes of temperature and in her
 medically sealed glass case, from which the
 air has been partially exhausted. At the
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