Heights of the principal Hills of Wensleydalc, Yorkshire. 353 



ciently visible on entering the field of view of the telescope, 

 became gradually more indistinct as it approached the wire, 

 and wholly faded out of view at a distance from it of one or 

 two minutes*. 



Another annoying impediment to accurate bisection, the 

 consequence of some defect in the tangent screw, or clamp, of 

 the horizontal circles, requires to be noticed. Occasionally, on 

 turning the screw in the proper direction to rectify an ap- 

 proximate bisection, the vertical wire of the telescope, after 

 refusing for some time to obey its action, would be displaced 

 in azimuth, not gradually, but at once, the quantity due to the 

 degree of revolution of the screw. 



In the registers of the measurement of the horizontal angles, 

 there is given for every signal observed the mean of the various 

 corresponding readings, reduced, when requisite, to the centre 

 of the signal. At several of the stations some few of the more 

 distant signals were never sufficiently clear for bisection; but 

 the consequent blanks in the registers, marked M, or C, have 

 been supplied, the former from the accurate data of Colonel 

 Mudge, and the latter, by the 

 following method, from the re- 

 gisters of the other stations. At 

 A the signals C and D were ob- 

 served, but B could not be seen. 

 From the registers of C and B 

 the angles CBA and BCA are 

 extracted, and the angle CAB p 

 obtained by subtracting their 

 sum from 180°. Adding this 



angle to the reading for the signal C, as given in the register 

 of the observations made at A, (the graduations of the theo- 

 dolite being numbered from left to right,) their sum is sub- 



* I have lately had a telescope fitted up with two horizontal thick wires, 

 to each of which is attached a delicate filament, both 

 placed, as exhibited in the figure, in the same vertical 

 line. In makinj; an observation the telescope is 

 moved until the signal appears to be equidistant from 

 the horizontal wires, and as exactly bisected by an 

 imaginary line connecting the two vertical filaments 

 as the eye can estimate. In measuring vertical aniilcs 

 the telescope is to be half inverted within its Ys, and 

 the filaments reiulered truly horizontal. 



Captain Kater and his associates experienced in hazy we:Uher the same 

 difficulty, or rather irnijossibility, of l)isecting distant signals by the inter- 

 section of the (three) wires of their poweH'ul telescope, but succeeded on 

 making use of a minute particle of dust, fixed by Mr. Gardner, on the hori- 

 zontal wire. (Phil. Trans. JH2H, p. 1!)4). This method, it is to be doubted, 

 would scarcely answer if applied to an ordinary telescope. 



N.S. VoL.G. No. 29. May 182y. 'J Z slitutcd 



