with an Equatorial Micrometer. 337 
is found more convenient, on account of a ready and certain 
adjustment, to place one of the wires AA or BB parallel to 
the apparent track of the Star wherewith the planetary body is 
to be compared: in confequence, when the Star * , fig. 1. is 
lifted up to L, it will not Strictly purfue the line LP ; but 
being lefs and lefs lifted up as it mounts higher, it will appa- 
rently fall more and more below the line LP as it afcends above 
the line HO, and will therefore take a courfe, fuppofe L p. 
The wire PLP being therefore adj ufted to agree with pLp ; by 
conftrudion of the instrument, the wire BLX will aStume the 
pofition qLx perpendicular to pL p. The Star, therefore, that 
ran along the parallel e + before it Suffered refraction, and at 
+ was fuppofed to be lifted up to /, there not meeting LX 
will take the courfe ly , nearly parallel to L p, and have fome 
difrance, as lz, to travel before it arrives at the new- placed wire 
Lx ; and it is now proper to examine what this quantity 
may be. 
Through the point % draw the line rzos parallel to HO, and 
cutting the vertical RLV in 0, and let L 0 be afiumed = 30 / ; 
then, lince the angle XLv is fuppofed to be minute, the grofs pro- 
portions of the fides of the triangles L yz and Lyl may be, for 
this purpofe, fuppofed the fame *, and the fame as Li be, dbL , 
fig. 2. to which the triangle L zo, fig. 1. will alfo be Similar ; 
as likewife the triangle yzo, and alfo the little triangle zlv : but 
making the Side Iv of the triangle zlv equal to the etteCl of 
refraCtion in perpendicular = 1 1 " . 8 ; then, to find the Side lz, 
* I 3m aware, that the fuppofition of the fides of the triangles L yz and Lyl 
being the fame cannot be itri&ly lb; nor can they have the fame proportions ; nor 
are any of the lines concerned right lines, that are fuppoled fuch ; but aflumptions 
near the truth are allowable for the correction of an error in the grcatcjl part , 
that if uncorreCted would fcarcely amount to a grofs error . 
C c c 2 
the 
