[ 82hved 
LIL. On Measuring the Depths of Cavities seen on the Sur- 
face of the Moon. By A CoRRESPONDENT. 
To Mr. Tilloch. 
Sir, — Tue attention of astronomers having for years past 
been directed to measuring the altitudes of lunar mountains, I 
have frequently been surprised that no attempts have yet been 
made to ascertain the depths of those cavities which are so con- 
spicuous on the surface of the moon. 
Under a conviction that determining the depths of such ca- 
vities will not be considered unworthy of notice, after the first 
astronomers of Europe have endeavoured to ascertain the heights 
of mountains on the moon’s surface, J beg leave to submit to 
the examination of your readers the inclosed method of mea- 
suring the depths of lunar cavities. 
I remain, sir, yours, &c. 
Islington, Sept. 26, 1818. H. M. 
<‘ Having given the apparent length of a shadow projected by 
the side of a lunar éavity on the bottom of it, and the angle be- 
tween the edge of the cavity and the boundary of vision or moon’s 
limb, to find the depth from the edge to the bottom of the ca- 
vity. 
B 
wv 
EK 
Let bd be the direction of a ray of light touching the edge J 
of the cavity de and falling on the bottom d; then will d re- 
Vol. 52, No, 247. Nov. 1818. xX present 
