new Nebula and Clujlen cf Stars . 463 
ferve as an hygrometer to (hew the variations of the ropes that 
fufpend the teiefcope. If a fhower of rain, for in fiance, fhould 
(horten them fo as to elevate the teiefcope 2, 4, or 6 minutes, 
which has happened fometimes, notwithftanding they have all 
been well faturated with oil, the index cord will immediately 
make the polar-diftance-clock fnew this effeCt of the rain, by 
pointing out an equal change on - the dial. As to the varia- 
tions of the cord itfelf, they are in the firft place very trifling, 
fince it confifts merely of a few threads of hemp, very loofely 
twifted, well oiled, and always equally ftretched ; but efpe- 
daily thefe variations are of no confequence, as they are fo 
eafily to be difcovered by the check of the quadrant of altitude 
affixed to the teiefcope, or the fucceftive tranfits of known 
ftars, and may either be immediately corrected by the adj tilla- 
ble hand of the polar diftance dial, or be left to be accounted 
for afterwards. 
The improvement of the right afcenfion has not been 
Ids attended to; and the Royal Society having kindly in- 
truded me with an excellent time-piece, I fucceeded at laft by 
means of the addition of the following apparatus. -Again/! the 
fide of the tube is fixed a vertical iron plate, and the point of 
fufpenfion of the teiefcope is difpofed fo as to permit this 
plate to be juft in contaCt with a roller which remains fixed 
during the time of a fweep. There is alfo a confiderable fpring 
applied on the oppofite fide, in fuch a manner as, by always 
exerting a preflure nearly uniform, to caule the iron plate to 
rub againft the fixed roller as the teiefcope fweeps up and 
down. By this means I have frequently, in very flormy wea- 
ther, obferved many hours without finding my time materially 
affeSed, and the corrections will feldom, in accurate obfer- 
vations, exceed a few feconds. 
To 
