[ 49 3 



peated trials, which may fometimes take up a great 

 deal or" time. 



If therefore a liar is made to move on the parallel 

 thread jufl at the crofs, and (the telefcope continuing 

 fixt in the fame pofition) it is afterwards near its 

 going out of the aperture found not to be upon the 

 thread, that inuft then be brought to the flar by 

 the help of the worm, and then the thread will lie 

 parallel to the diurnal motion of the flar in that 

 part of the heavens, and confequently the crofs 

 thread will reprefent a meridian, and the others 

 parallels of declinations, and the difference of time 

 between the paffage of the ffar at the crofs wire 

 (which was made to move^ along the thread), and 

 the tranfit of any other ffar, &c. over the crofs 

 thread which reprefents a meridian, turned into degrees 

 and minutes, will give the difference of right afceniion 

 And, ii the moveable parallel thread be brought, by 

 turning the index, to touch the otfier ftar about the 

 time of its paffage over the crofs thread, then the 

 number of revolutions and parts (hewn. by the index 

 (turned into minutes and feconds of a degree by the 

 table) will be the difference of declination between the 

 two ftars. If the flar is made to pafs along the fixed 

 thread fo as to feem perfectly bifeclied, there mufl 

 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 j 

 but it may, if it is found convenient, be adjuifed to 

 the middle of the threads, or die correction may be 

 made in the obferved diflance. 



In taking any angle, it is convenient that each of 

 the parallel threads be about the fame difknee frcm 

 5 Vol. LXIL H the 



