TRANSIT INSTRUMENT. ^91 



Method of adjusting a Transit Instrumeut in a Plane of the Me' 

 vidian. Bj/ Sir H. C. Englefield, Bart. M.P. F.R.S, 



To Mr. NICHOLSON. 

 SIR, 



BOOKING over some papers the other day, I found the iptrodnctioii. 

 inclosed, which I had drawn up some years since. I do not 

 know that the method of placing a transit instrument, therein 

 described, has been made public. If it has not, I think its ease 

 and accuracy renders it not unworthy of publication. Should 

 you be of a different opinion, you will be so good as to return 

 it to me. 



I am. Sir, 



Your obedient Servant, 



H. C. EJS[QLEFIELD. 

 Tilney-Street, March 11, 1807. 



Let Z (PI. 8) be the zenith ; P, the pole ; HO, the horizon ; Adjustment of 

 ^PI, the meridian circle; ZK, a circle of altitude distant from ^^^ ^■"^"^^'^ i*^- 

 the meridian by a small quantity IK (suppose a degree) ; 12 3 4, levSlfns' its 

 the diurnal circle of the pole star, whose radius is P 45' »^is? and ob, 

 nearly; and let the altitude of the pole^Be M030'. Then, gliefoAime 

 when the pole-star is on the northern meridian, its altitudes I, between the 



will be 490 45', and its zenith distance Z 3, 40» 15' ; and let ^^^"^^'^ "^ *^^ 

 Ar>rvu ri-j- \ ■ ^ r pole-star and ot 



AUD be a part 01 the diurnal circle of a star whose polar dis- another more 

 tance is 46° 30', and N. meridian altitude 5o. distant from 



XT , • • , . . the pole; tUo 



JNow, suppose a transit instrument, whose axis is accu- R. A. being 



rately levelled, and of course in the meridian at Z, to point at Siven. 

 the horizon to K, instead of I, the true meridian; then, at 3 

 (the altitude of the pole-star under the pole), it will point at B, 

 and the arch 3 B will be to IK as the cosine of the altitude 

 3 I to radius ; but 3 B, measured on the diurnal circle of the 

 pole-star, will be the sine of its distance from the meridian to 

 the radius P 3 or P B; and as, in small arches, the arch of a 

 %^2 great 



