[47 ] 



meter fervingfor the fame purpofe with grehterer-f- 

 actnefs, It was for this reaion indeed that the late: 

 alteration in the form of the micrometer was made;, 

 they being, before-: not fo convenient'- for making.; 

 fuch fort of observations, both hairs being ufually 

 moveable, and no provifion being, made for fetting ; 

 the hairs parallel to the diurnal motion of the ob- 

 jects to be obferved j both which - inconveniencies- 

 are avoided in the prefent micrometers, . 



The micrometer^ , as now contrived, is not only 

 of. ufe in: meafuring. f snail angles or diftances be- - 

 tween fuch objects as appear; within the aperture of 

 the.telefcope at the fame time?, but likewife in taking; 

 the difference of i right afcenfion and declination be- 

 tween fears and planets,- &g. which in their apparent, 

 diurnal motion follow one another through the tele- - 

 fcope if kept in the fame fituation. In making the 

 firfi kind of obfervations, turn^ the fhort tube which ■■ 

 carries the eye glafs and. micrometer, &c. till the 

 crofs thread (or thatwhich, cuts -the parallel threads 

 at right angles) lies parallel to- a line paflang- -through 

 the objects - whofe diftance is- to be meafured^ and 

 then by raifimg-ordeprefling the telefcope by help* 

 of the ftand bring the objects to appear upon or. 

 near the crofs thread, and one of them juff. to touch. .» 

 the fixt parallel thread : then turn the index of the, 

 micrometer till .'the moveable, thread touches the 

 other, object, . and. the number of revolutions' and- 

 parts -of a- revolution: fhewn.,by- the index,, turned. 

 into minutes and feconds by the table made as -. 

 hereafter directed, . will, be the apparent angular r 

 didance of thofe. objects. It- is here fuppofed, that' 

 tka. threads exactly dofe 3 . fo as to touch each; bthepr 



