$44 GV//. Roy’s Account of 
feftion ia Plate IV. ufed for reading off the divifions of the 
inftrument, under the vertical microfcopes. The fron r of one 
of thefe is (hewn, and the back, or that to which the handle 
is fixed, of the other. Their narrow fides are prefented 
towards the microfcopes, there being in each a filvered re- 
flector of copper at FF ; and oppofite to it, at GG, a fcreen of 
talc or tranfparent oiled paper. The light from a wax candle 
being thrown on the reflectors, and thence back again through 
the fcreens, on the divifions of the inftrument under the mi- 
crofcopes, thefe could be very diftinCtly read off and regiftered : 
for the light communicated in this way was very ftrong, at 
the fame time that the glare of it, which otherwife would 
have been difagreeable to the fight, was removed by paffing 
through the fcreen. 
Art. XII. Arms projecting from the bell-metal plate under the 
plane of the infrument. 
By referring to Plates III. and IV. but more particularly the 
latter, it will be perceived, that there are three flat arms, 
fir on gly fixed by fcrews to the edge of the circular bell-metal 
( i 
plate, forming, as has been already mentioned, the bafts of the 
interior vertical axis. Thefe arms, which are alfo firmly braced 
to the feet of the inftrument, rife gradually as they project 
outwards towards the circumference of the circle, whofe radius 
they exceed about an inch and a quarter, and their extremities 
are about an inch lower than its upper furface. One arm, 
lying direCily over one of the feet, is that to which are 
attached the wheels and fcrew moved by the HookVjoint, and 
alfo the clamp of the circle, as reprefented in Plate V. The 
other two arms, whereof one lies alfo over a foot, and the 
other diredliy oppofite to it, become thereby a diameter to the 
circle, 
