3~6 Dr. Herschel <?/z the 
for the thlcknefs of the wires, the afcertaining of which is 
liable to feme difficulties ill other conftrudtions ; but here, as 
we can note the divifions on the firft appearance of light at 
either fide of the fixed wire, when the moveable one paffes 
over it backwards and forwards, we may very conveniently 
determine that part of the fcale to which the zero ought to 
anfwer in central meafures. The value of the fcale was afeer- 
tained by the tranfit of ftars over the two wires opened to a 
certain number of divifions, and a chronometer beating five 
times in two feconds of mean time *, and in a number of feve- 
ral fets of experiments, the mean of each feldom differed fo 
much as the 50odth part of a fecond of fpace for each divifion, 
and thefe are large enough to be fub-divided and read off, with 
good exadhiefs to tenths ; and yet the fpace anfwering to each 
part amounts only to 282 millefimals of a fecond. The mea- 
fures of the diftances alfo were as often repeated as the oppor- 
tunities would permit, and a mean of them has been ufed. 
The light of the fatellites of the Georgian planet is, as 
we may well expedt, on account of their great diftance, uncom- 
monly faint. The fecond is the brightefb of the two, but the 
difference is not confiderable ; befides, we muft allow for the 
effedt of the light of the planet, which is pretty ftrong within 
the fmall diftances at which they are revolving. I have feen 
fmall fixed ftars, as near the planets as the fatellites, and with 
no greater light, which, on removal of the planet, fhone with 
a confiderable luftre, fuch as I had by no means expedted of 
them. A fatellite of Jupiter, removed to the diftance of the 
Georgian planet, would fhine with lefs than the 1 80th part of 
its prefent light ; and may we not conclude, that our new 
fatellites would be of a very confiderable brightnefs if they 
were brought fo near as the orbit of Jupiter, and thus appeared 
180 
