1000 



A.8TEOKOMT. 



[THE TRANSIT INSTRUMENT. 



to tlio horizon ; and by sliding tin- other parts of the roof, 

 the whole of which is placed on friction rollers, ami f ur- 



Ftg. 108. 



nishcd with a crank, K, for the pxirpose of moving it, 

 every portion of the lieavoiis can be swept with the 

 instrument. The equatorial, consisting, as we have 

 ecn, of one circle, parallel to thu plane of the equator, 

 and of another circle which follows the axis of the earth, 

 is arranged RO as to coincide with any circle of declina- 

 tion, and is well adapted for measuring the differences of 

 rijht ascension and declinations of two neighbouring 

 stars. 



TUB TRANSIT INSTRI'MENT. The place of an object in 

 the heavens is mostly defined by two elements ; 

 viz., by its right ascension and north polar 

 distance. In the first approaches to accurate 

 results of observation, these elements were 

 g-nenilly determined by a mural arch or quad- 

 r.mt, firmly fixed in the direction of the meri- 

 dian, the position of which was frequently 

 checked by equal altitudes of stars. It was by 

 means nearly similar, that Flamsteed observed 

 his catalogue of three thousand stars, wliich 

 has been so ably reduced and edited by the late 

 Mr. Baily, and is generally known as the Uritish 

 (,./.,'.,,. i . . 



La Caille, at the Cape of Good Hope, ob- 

 served bis zones in like manner, using certain 

 well-determined stars as "zero points," whose 

 positions hail been obtained by independent 

 methods. The first astronomer was content 

 with noting his observations to seconds of time 

 an accuracy, which, from the imperfection 

 <>f his instruments, ho considered to be quite 

 Kiitlicient. The invention of the. transit instru- 

 ment by Roemcr, in the year 1000, enabled as- 

 t P .in .mem tii make great advances in the accurate 

 determinations of right ascensions. This ele- 

 , the most delicate and important, essen- 

 tially depends upon time, ami is, therefore, 

 more difficult of direct estimation than tlio 

 other, which consists of angular measurement, 

 nod off on graduated instruments, which 

 modern artist* have been enabled to bring to 

 an a-itoniOiii: , degree of perfection. The tran- 

 sit instrument has been successively improved 

 the fint idea of Roemer. Dr. H alley, 

 when Astronomer Royal, introduced its use at 

 Greenwich, the construction being similar to 

 that t'iven I . vr in his Praxi* Atl 



In thu arrangement the telescope was placed twenty - 

 ii inches nearer one Mid of the axis, instead of being 



central a construction which evidently rend -red the de- 

 termination of error of coll; 



The form of the transit instrun 

 Bradley (the next Astronomer I: >\.,!i (Hirers 

 little from that adopted in modern times viz., 

 a telescope fixed at right angles to a cross axis, 

 and capable of taking all positions in the meri- 

 dian of the place. This axis terminates in two 

 cylindrical pivot*, resting in Y*a so that they 

 (inly touch in two points for the purpose of 

 diminishing the friction. One of those Y's is 

 ' in such a manner as to correct either 

 an error of horizontally of the axis, or an error 

 of deviation from the meridian. The, instru- 

 ment is furnished with a graduated netting 

 circle, sometimes placed at one extremity of 

 the axis concentric to this axis (as was the case 

 formerly with Troughton's Greenwich transit 

 instrument), or two small divided circles placed 

 near the eye end of the telescope. In the com- 

 mon focus of the object-glass and eye-glass, a 

 system of wires is placed for the purpose of de- 

 fining the position of an object as it posses 

 through the telescope. These wires are of ex- 

 treme tenuity, and generally consist of li 

 seven vertical wires and two horizontal (Fig. 

 204), the central vertical wire 

 closely approaching to the 

 meridian. The object of the 

 lateral wires is to diminish 

 error of estimation of a star's 

 transit, by taking the mean of all, 

 since the time of transit over a single 

 wire would be liable to some uncer- 

 tainty. To prevent any error arising 

 from parallax in the observation of the wires, theeye- 

 ]iioco is movable by a screw, so that the transit over 

 e.ieh wire maybe observed in the centre of the field. 

 The adjustments of a transit instrument are, first, that 

 its optical axis, or the lino joining this centre of the 



Fig 203. 





the 



object-glass and the middle wire, be. perpendicular to the 

 axis of rotation ; secondly, that the axis 1>e ln.i i 

 and, lastly, that when corrected for the two \ '. 



