[ 527 ] 
from o to 128 on each fide of the middle or point 
marked o upon the arch.' By a mean of all thefe mea- 
/ it 
fures, one divihon of the arch or 3 21,562 came out 
equal to four revolutions and 34,8272 parts of the mi- 
crometer, 41 of which make one revolution; and there- 
a 
fore one part is equal to 1,0137 545, and 41 or one re- 
volution is equal to 41,5639345. Hence the value of any 
number of revolutions and parts of the micrometer may- 
be eafily found. At all obfervations of the fame ftar, 
whether on the North or South fide of the hill, I brought 
the fame point of the arch, namely, that which agreed 
neareft with the zenith diftance of the flar, under the 
plumb-line, fo that the difference of the apparent zenith 
diflances of the fame flar on contrary f des of the hill is 
given in parts of the micrometer, and has no reference 
to the divif ons of the inflrument, whether they be equal 
or unequal ; and, the parts of the micrometer fcrew being 
perfe6lly equal, as I had formerly fatisfied myfelf by mea- 
furing the interval of two given points on the arch with 
different parts of the fcrew, that difference of apparent 
zenith diflances may be perfectly relied on, as far as de- 
pends upon the inflrument, provided the bife6lion of the 
liar by the wire in the telefcope, and that of the point 
on the arch by the plumb-line, were accurately per- 
formed. As the plane of the inflrument was placed both 
Eafl and Well, at both flations of the obfervatory, the 
difference of the latitude of the two flations maybe found 
VoL. LXV. 4 A as 
