154 Gen* Roy’s Account oj 
fcopes, to permit the micrometer fcrevv to be formed of it ; 
and it is reprefented in the uppermoft figure attached to a 
watch fpring coiled up in the ufual manner. 
By the motion of the micrometer head, the Aide, and with 
it the wire, moves upwards or downwards in the field of the 
telefcope, a fpace equal to half the diftance of the extreme 
wires from each other. This motion above or below the cen- 
tral point, which was made to correfpond with the acute inter- 
fedtion of the wires placed in the axis of vifion of the tele- 
icope, is performed in ten revolutions of the head, as denoted 
oy the motion of the dart, ten divifions upwards or down- 
wards, in the narrow groove feen at the top of the figure. 
Now, by the means of this piece of mechamfm in the eye- 
end of the telefcope, it will appear fufficiently obvious, that 
final! angles of elevation or depreflion may be determined with 
great accuracy, when the value of a certain number of revo- 
lutions and parts (the circumference of the head being divided 
into 1 00) have been once afcertained by repeated obfervations 
of the altitude of any well-defined objedt taken by the femi- 
circle. Thus it was found, by experiment, that 7 revolutions 
of the micrometer head were equal to an angle of elevation or 
depreflion of io 7 59", or 659", on the femicircle. Whence 
it follows, that one revolution raifes ordeprefies the wire above 
or below the central point i 7 24 // .8 i^ 4> or a little more than 
B4 8r. And hence a motion of one divifion on the head railes 
or deprefles the wire nearly 4LJL ths of a fecond. 
In this manner were determined the reciprocal elevations or 
depreflion s of the feveral ftations of the feries of triangles with 
regard to each other. 
By obferving attentively the four fcrews reprefented in the 
outward end of the telefcope, a dotted groove will be feen 
under 
