424 Afr. ramsden’s Defcription of 
a fpiral fpring within the part n, hy which means all 
fhake or play in the nut H, on which the meafure de- 
pends, is intirely prevented. 
From the difference of the threads on the fcrew at e 
and g it is evident, that the progreflive motion of the 
fcrew through the nut will be half the diftance of the 
reparation of the two halves of the mirror, and confe- 
quently the half mirrors will be moved equally in con- 
trary directions from the axis of the telefcope c. 
The wheel v fixed on the end of the double fcrew has 
its circumference divided into 100 equal parts, and num- 
bered at every fifth divifion with 5, 10, 8 cc. to 100, and 
the index I fliews the motion of the fcrew with the wheel 
round its axis, while the number of revolutions of the 
fcrew is fhewn by the divifions on the fame index. The 
fleel fcrew r may be turned by the key s, and ferves to 
incline the fmall mirror at right angles to the direction 
of its motion. By turning the finger head t (fig. 2.) the 
eye tube p is brought nearer or farther from the fmall 
mirror, to adjuft the telefcope to diftinCt vifion ; and the 
telefcope itfelf hath a motion round its axis for the conve- 
nieney of meafuring the diameter of a planet in any direc- 
tion. The inclination of the diameter meafured with the 
horizon is fliewn in degrees and minutes by a level and ver- 
nier on a graduated circle, at the breech of the telefcope. 
The 
