OBSERVATORY CLOCKS.] 



ASTRONOMY. 



1001 



adjustments, the line of collimation describes the meri- 

 dian. (See, also, the folio plate\ 



The other instrument, equally important in connec- 

 tion with a transit instrument, is a sidereal clock, beat- 

 in',' seconds, and so regulated that the hour-hand will 

 describe a complete revolution, or 86,400 seconds, from 

 the time of a star's transit over the meridian fibre, to its 

 return to the same point. The difference of the clock's 

 gain or loss from this number is termed its rate : the 

 excellency of a clock, therefore, depends on the steadi- 

 ness of its rate, and every means are taken by mechani- 

 cal refinements to preserve its uniformity. Astronomical 

 clocks are generally furnished with mercurial pendulum 

 for the purpose of compensating for extremes of tempo 

 ratnre. 



The transit clock requires to be mounted on a pier o 

 solid masonry for the purpose of insuring steadiness. 



The method of observing transits is as follows : The 

 observer, being comfortably seated (the telescope having 

 been previously directed to the collimating star), note 

 carefully the minute, second, and fractional part of a 

 second at which a star transits each wire. The hour i 

 afterwards noted, and the counting of the clock verifiet 

 when the transit is completed. This is done for severa 

 objects, the mean of the wires is taken, and thus, th 

 differences of their right ascensions, subject to the rate 

 of the clock, can be found. The fractional part is esti 

 mated by carefully noting the position of an objecl 

 before its passage over the wire, and after its passage 

 over the same wire. By a little practice, the instan 

 when it was exactly behind the wire can be very accu- 

 rately proportioned. In good observations the mean ol 

 wires is seldom liable to an error greater than one-tenth 

 part of a second. 



Recently, a new method of recording transit observa- 

 tions, by the agency of galvanism, has been brought into 

 use at the Royal Observatory, Greenwich, which pro- 

 mises to supersede the old systems of observing. Its 

 first introduction appears to have been in the coast sur- 

 vey of the United States ; and in the bands of Mr. 

 Walker, Dr. Locke, and Professor Mitchell, it has been 

 considerably improved. The form of the invention, as 

 described by the Astronomer Royal, is as follows : 

 From the principal instruments of the observatory, gal- 

 vanic wires are connected with a recording surface placed 

 in another part of the building. The transit clock, at 

 each beat, completes a galvanic circuit, which is regis- 

 tered by proper apparatus on the recording surface, con- 

 ng of a cylindrical barrel, covered with prepared 

 paper. Above the barrel (which is driven by clock- 

 work regulated by a conical pendulum), a system of 

 prickers is placed, carried by a travelling frame, moving 

 slowly in the direction of the barrel axis. Thus, the 

 pricker, which is connected with the transit clock, will 

 register seconds on this barrel, and will form a series of 

 spiral lines as it revolves. The method of arrangement 

 is as follows : A wheel of 60 teeth is fixed on the escape- 

 wheel axis, and the teeth of this wheel, in succession, 

 make momentary contacts of the galvanic springs. The 

 position of the springs is so adjusted, that the effect of 

 the wheel-tooth upon them occurs only when an escape- 

 tooth has passed the sloping surface of the pallet, and 

 the other escape-tooth is dropping upon its bearing ; and 

 thus the resistance of the springs does in no way affect 

 the legitimate action of the train upon the pendulum. 

 . Another pricker, carried by the same travelling frame, 

 is connected, by arbitrary touch, with an index at the 

 eye end of the transit circle. At the instant of a star's 

 passage behind the wire, the observer touches this index, 

 which will, therefore, register on the barrel a series of 

 punctures equivalent to a transit observation. An ad- 

 vantage is gained by this method, insomuch that the 

 equatorial intervals of the wires may be reduced to three 

 < of time the duty of the observer merely con- 

 sisting in writing down, in addition to the preceding 

 signals, the name of the object observed. For the pur- 

 pose of registering on the barrel the beginnings of some 

 -, and hours and minutes, from which the rest 

 may be inferred, a provision is made by certain pre- 



VOL. I. 



arranged signals at known instants of the transit clock. 

 In this manner the paper, when taken off the barrel, 

 may be easily translated. This system has so far suc- 

 ceeded admirably at Greenwich, requiring only the use 

 of the nerves of sight and sensations of touch, of which 

 there is a more intimate connection than those of the 

 eye and the ear. It will, therefore, most probablv 

 reduce the amount of "personal equation." 



It frequently happens that two observers will not give 

 the same time of the occurrence of the same phenome- 

 non ; as, for instance, the passage of a star over the 

 wires of a transit instrument. The amount of this 

 difference, which is termed "personal equation," varies 

 from two-tenths to half a second, though instances have 

 been known of this quantity exceeding one second of 

 time. Dr. Maskelyne, and Kinnebrook his assistant, 

 differed in this manner seven-tenths of a second. In 

 modern times, Bessel and Argelander have differed 

 upwards of a second. The method of determining it is 

 to compare the "clock error" of one observer with that 

 of another independently obtained (at a short interval of 

 time). Or it can be deduced by taking transits of the 

 same star over some wires of a transit instrument, by 

 one observer, compared with the transits over other 

 wires by another observer. Reducing each separately to 

 the middle wire, the difference of their methods of 

 observing may be at once deduced. We refer to the 

 annual volumes of the Greenwich observations for ex- 

 amples of the first of these methods. 



It can also be obtained by a binocular eye-piece, an 

 instrument which permits two observers to see the star 

 in its passage over the wires at the same time, and thus 

 to observe the whole transit. This instrument is the 

 invention of the late Mr. J. Jones. 



In order to determine the relative accuracy of transit 

 observations at the separate wires, the following method, 

 first introduced by Bessel, and recently used by Struve 

 and Dr. Oudemans, may be applied. Let x be the 

 mean error of one observed wire, arising from a defect 

 of hearing, y the mean error produced by an imperfec- 

 tion in the sight, which may be caused by a wrong esti- 

 mation of the time at which the star passes behind the 

 wire (varying with the thickness of the wires) ; then the 

 mean error of an observed transit at one wire will be 



The error of hearing, or x, will arise from the estima- 

 tion of the precise instant of the origin of second, and 

 will, therefore, be the same for all stars. The other will 

 vary as the secant of the declination. 



Then, if the transits of several stars are reduced to the 

 middle wire by the known intervals of the wires, and 

 the difference of each result from the mean is taken, the 

 squares of these differences will be = (n m) M 2 , where 



= the number of wires, m = the number of observed 

 culminations. The probable error of M, determined by 



'4769 

 this equation, will be M , the probable error of 



the value M 2 will be M 2 = 



Forming for each culmination S (t 2 ), and their sums 

 :or all the culminations of the same star, the equation 

 will become 



(n m) x 2 + (n m) j/ 2 sec. 2 S = 52 (t 2 ). 



Proceeding in this manner, Dr. Oudemans has found, 

 'rom 228 culminations, observed from October, 1847, IQ 

 October, 1848, the mean error of a single wire = 



B. s. 



or M = V (0-1242 2 + 0-0003 2 sec. 2 ). 



But the probable error 



W = V (0-838 2 + 0-0407 2 sec. 8 ). 

 A similar investigation from October, 1848, to October, 



6n 



849, gave a mean error, or, 



