relating to the Mountains of the Moon. 509 
good micrometer let the diftance lm be taken by obfer- 
vation Draw lc perpendicular to lm ; draw alfo mc 
from the top of the lunar mountain to the center of the 
Moon: then in the triangle mlc, rediangled at l, we 
have given the fide lc, which is the Moon’s radius, and 
the fide lm taken by obfervation. Therefore, by trigo- 
nometry, we can find the hypothenufe mc from 
which, fubtradting the part p c or radius, there remains 
the perpendicular height of the mountain up. I have 
followed the fame method, as being the lead: liable to 
error. 
galileo takes the diftance of the top of a lunar 
mountain from the line that divides the illuminated part 
of the diik from that which is in the lhade to be equal 
to a aoth part of the Moon’s diameter; but hevelius 
affirms, that it is only the a 6th part of the fame. 
When we calculate from thence the height of fuch a ' 
mountain it will be found, in Englifh meafure, according 
to galileo, almoft miles; and, according to heve- 
lius, fomething more than 3-^ miles, admitting the 
Moon’s diameter to be 2180 miles. 
(a) I do not recoiled! that hevelius mentions in what manner he took the 
(diftance lm ; but I am apt to believe it was by a micrometer. 
(b) ✓ lcV 1 -p lm) s =: mc. 
X x x a 
He 
