jhnple Micrometer, 2S9 
extenfion of fix inches, and obferve how many divifions of 
the micrometer are equal to it, and that fame number of divi- 
fions is equal to half a degree, or 30 ' ; and this is all that 
needs be done for the required determination ; the reafon of 
which is, becaufe an extenfion of fix inches fubtends an angle 
of 30' at the diftance of 57 feet 3I inches, as may be eafily 
calculated by the rules of plane trigonometry. 
In one of Dollond’s 14-inch pocket telefcopes, if the divi- 
fions of the micrometer be the. hundredths of an inch, Hi of 
thofe divifions will be found equal to 30', or 23 to a degree. 
When this value has been once afcertained, any other angle 
meafured by any other number of divifions is determined by 
the rule of three. Thus, fuppofe that the diameter of the 
fun, feen through the fame telefcope, be found equal to ii 
divifions, fay as nl divifions are to 30 minutes, fo are 12 
divifions to 31', 3, which is the required diameter 
of the fun. 
Notwithftanding the facility of this calculation, a fcale may 
be made anfwering to the divifions of a micrometer, which 
will (hew the angle correfponding to any number of divifions 
to mere iafpeftion. Thus, for the above-mentioned fmall 
telefcope the fcale is reprefented in fig. 3. AB is a line drawn 
at pleafure ; it is then divided into 23 equal parts, and thofe 
divifions, which reprefent the divifions of the micrometer that 
are equal to one degree, are marked on one fide of iti The 
line then is divided again into 60 equal parts, which are marked 
on the other fide of it ; and thefe divifions reprefent the mi- 
nutes which correfpond to the divifions of the micrometer : 
thus the figure (hews, that fix divifions of the micrometer are 
equal to 15! .minutes* ii§ divifions are nearly equal to 29 
Vol.LXXXL Qq minutes, 
