relating to the Mountains of the Moon. 523 
fcribe them all, which I fhould have done in the paper 
delivered by Dr. watson, had I not feared to be too par- 
ticular. 
The firft method I ufed was to fet the immoveable 
hair bb (fig. 4.) of my micrometer parallel to a line ab, 
joining the cufps of the Moon; then, by opening the 
moveable parallel hair till it included the projection on y 
intended to be taken, I marked that down as the meafure 
of on. As this method required fome attention (that 
part of the ellipfis of illumination avb which is the ver- 
tex v of the lefler axis may ferve as a direction) and took 
up fome time, on account of the imall field of view of 
my telefcope, I ufed occafionally thefe two following 
ways. 
When there was any remarkable figure on the diflt of 
the Moon near the line of illumination, I put on a com- 
pound eye-piece whofe magnified field of view is full 
40% and power about 90 times, fo that it takes in the 
l 
greateft part of the whole Moon; by this means I was 
enabled to view the projection intended for meafuring 
at the fame time with the reft of the Moon, and to fix 
upon fome mark in the difk very near to its edge towards 
which I judged the line on fhould be directed; then put- 
ting on the eye -piece which carries the micrometer I 
Vol. LXX. Z z z took 
