relating to the Mountains of the Moon. 513 
therefore, lo : lo : : Lr : lm, or — — — — - lm; but lo is the 
7 7 LO 7 
radius of the Moon, and l r, or on, is the obferved 
diftance of the mountain’s projection; and lo is the fine 
of the angle rol = o ls, which we may take to be the 
diftance of the Sun from the Moon without any material 
error, and which therefore we may find at any given 
time from an ephemeris. 
I will now give an account of my own obfervations 
relating to the mountains in the Moon ; but, perhaps, it 
may not be amifs to mention the inftrument they \Yere 
made with, and a few of the circum fiances, that it may 
appear how far their accuracy may be depended upon. 
The telefcope I ufed was a Newtonian refieCtor of fix 
feet eight inches focal length, to which a micrometer 
was adapted confifting of two parallel hairs, one of 
which was moveable by means of a fine fcrew. The 
value of the parts fhewn by the index was determined 
by a trigonometrical obfervation of a known objeft at a 
known diftance, and was verified by feveral trials. The 
power I always ufed, except when another is mentioned, 
was 222 times, alfo determined by experiment, which I 
have often found to differ fomewhat from theory, on 
account of fome little errors in the data , hardly to be 
avoided. The moon having fufficient light, I ufed no 
more 
