E 49 3 
peated trials, which may fometim.es take up a great 
deal of time. 
If therefore a ftar is made to move on the parallel 
thread juft at the crofs, and (the telefcope continuing 
ffxt in the fame pofition) it is afterwards near its 
going out of the aperture found not to be upon the 
thread, that muft then be brought to the ftar by 
the help of the worm, and then the thread will lie 
parallel to the diurnal motion of the ftar in that 
part of the heavens, and confequently the crols 
thread will reprefent a meridian, and die others 
parallels of declinations, and the difference of time 
between the paffage of the ftar at the crofs wire 
(which was made to move along the thread), and 
the tranfit of any other ftar, &c. over the crofs 
thread which reprefents a meridian, turned into degrees 
and minutes, will give the difference of right afcenffon 
And, if the moveable parallel thread be brought, by 
turning the index, to touch the other ftar about the 
time of its paffage over the cfofs thread, then the 
number of revolutions and parts fhewn by the index 
(turned into minutes and feconds of a degree by the 
table) will be the difference of declination between the 
two ftars. |f the ftar is made to pafs along the fixed 
thread fo as to feem perfedtly biiected, there muft 
be an allowance made for the femidiameter of the 
thread or wire, becaufe I fuppofe the index to be 
adjufted as before to the inner edges of the wires ; 
but it may, if it is found convenient, be adjufted to 
the middle of the threads, or elfe correction may be 
made in the obferved diftance. 
In taking any angle, it is convenient that each of 
|he parallel threads be about the fame diftance frcnj 
Vol. LXIIo H the 
