vieW] itt'^elr'i^ riitttiiier 'as^bfufii^'i^iuftd thte c^htre'M 'every direcq 

 lion, and to make -with the declination circle any given angle. The 

 author then proceeds to show, that if r°, t', and T, be the times of 

 passage across the wire of three stars (the first two being known, 

 aWd.the third being unknown or its position required), whose right 

 ascensions are a°, a', and A, and declihations ^°, 8', and D, for any 

 given position of the wire ; and if 6°, 9', and 9, be the correspond- 

 ing times of transit of the same three objects for any other positioit 

 transverse to the former, then ••■'-•^- -■ '■• ^ ..-."-.■ io 



-too ,doj(Iw .nodi^oq odi tol D-^08 ""^XL : (' " ^ )-\^ —^.hmcno 

 or ^''"%^^.''^^^^fyj;^^CT_2^^i2M:] ^ (T-r°)^(9 --<f ) ^ boiooi 

 ,t>8-fii^8f ilooqvl ;(> .w) 01 -^^S 30iuii>i^ i"^' —r°)—{^''r^%aiiho'J 



Silftiabigfioo ;tiii3t>^.r fe : ^i H ^^ , (^ ~ ^p ~ ^T ~ "^Pr '^"^-^'-^"^ ^f^T" 

 -do sdi no isJyraoiorxif ^ (^ ~T.r /r'/lTdTnTlj*^ ,iol .oanabBnoo 

 ijyiJjBSsn b djiv/ bIe<Jsfj'i('5' + J°) + i'f bov/yiv vtibuaJg aRw Ji ,8lo9r 



-Bpa£L ?rfi,t jtO .,':;■■ ^ ■:v/' Joffg^ir;; •.M•f^,:o vhc'utjiaiviollol 



Qh/Wnen the unknown object is a planet, since its position is deter- 

 mined by observations made at two different times, T and 0, a cor*- 

 rection will be necessary for its motion during the interval of time 

 © — T; and the author determines the corrections to be applied to 

 the times 6 and T, to reduce thena to the same epoch. 



'The author considers this micrometer, both from its simplicity 

 and from the precision of its results, to be exceedingly well adapted 

 for observations of Mars and neighbouring stars, for the purpose of 

 determining its parallax ; and he gives the following formulae for 

 tj^^ value of the parallcix : — 



,f J^l^et \|/vv and \J/e be the latitudes of two observatories favourably 

 situated for determining the parallax by observations of right ascen- 

 sions, or differing considerably in longitude, t|/w being the latitude 

 of the western and ^^ of the eastern station. Let also A^ and 

 He be the corresponding hour-angles of Mars ; A^ and Aj the 

 observed right ascensions, reduced to the same instant; aadJDitiie 

 approximate decimation ; then . ■> rfj-onsi 



^„ Jhe horizontal paraUax of Mars . ^^^^^ ^^^^^ ^^^^^ .fBooLq 



liofi ^ (Ae — Aw)cosBiI \o J ioiai'/ib iijupa gdt 



-9VII03 ■^iamijwza ,^ cos ^^ sin A^ — cos ^t, sin hj./ 1 aioiaiad;} fanis) 



'" Again, ^ if Dn ancl Dg denote the declinations bi)served at "two 



stations differing very considerably in latitude (N denoting the 



northerlv and S the southerly station, and, the rest of the notation 

 , . -..Ill j\ ■jO'iJ-.'.b :* . 



being suitably changed), 



1 he horizontal parallax- , . , ^ • i t r -i r. , , ■ - 



,90fi : si"^ ^n ~ sin 4'3 + tan D cos ^|/s cos hs — tan D cos v|/n cos hj, i^ 

 ■f '-This communication is accompanied by a list of stars selected Hy 

 -t^ author as favourably situated for observation at the ensuing op- 



