[ 47 3 
meter ferving for the fame purpofe with greater ex— 
adtnefs-. It was for this reafon indeed that the late. 1 
alteration in the form of the micrometer was made,, 
they being before not fo convenient for making’- 
fuch fort of observations,, both hairs being ulually' 
moveable, and no provifion being made for fetting 
the hairs parallel to the diurnal motion of the ob-- 
jeds to be obfervedj. both which inconveniencies 
are avoided in the prefent micrometers.. 
The micrometer, as now contrived, is not' only' 
of ufe in meafuring fmall angles or diftances be- 
tween fuch objeds as appear within the aperture of: 
the telefcope at the fame time, . but likewife in taking ; 
the difference of right afcenfion and declination be- 
tween ftars and planets, &c.- wliich in their apparent- 
diurnal motion follow one another through the tele-- 
fcope if kept in the fame, fituatiorr. In making the 
firfl kind of obfervations, turn the fhort tube which', 
carries the eye glafs and micrometer, &c. till the 
crofs thread (or that which cuts the parallel threads • 
at right angles) lies parallel to a line paffing through 
the objeds whole difiance is to be meafured, and 
then by railing or deprefling the telefcope by help; 
of the Stand bring the objeds -to appear upon or 
near the. crofs thread, and one of them juft 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 andi 
parts of a revolution Shewn by the index, turned t 
into minutes and feconds by the table made as-, 
hereafter direded, will be the apparent angular- 
diftance of thofe objcds. It is here fuppofed, that! 
the threads exudl.v dofe, .fo as to touch each other; 
* . J 
whem 
