^2 Dr. Herschei/s Account oj &c. 
mountains they represent. Then, from the known angular 
magnitude of the moon, calculate its diameter, at the distance 
of your situation ; this, multiplied by the power of the tele- 
scope, gives the diameter of a circle, to the circumference of 
which belongs the line, upon which are placed the marks 
above described. Now, measure the elevation of these marks, 
above that line, and you will obtain the proportion they bear 
to the diameter of the circle. 
In my experiment, I found that I could plainly see some 
small protuberances at 9 feet distance, which were no higher 
than the 50th part of an inch. Then putting the diameter of 
the moon at 30', we have the sum of the logarithms of the 
tangent of 30' ; of the power 287; and of the 50ths of an inch 
contained in 9 feet ; which, taken from the logarithm of the 
diameter of the moon in miles, gives the logarithm of ,16. 
By which we find, that so small a mountain as the T ^dth, or 
not much more than the sixth part of a mile, may be per- 
ceived and estimated, by the telescope and power that was 
used upon this occasion ; and that, consequently, the estima- 
tion of mountains, near a mile and an half high, must become 
a very easy task. 
Slough, near Windsor,, 
Dec. 30, 1793. 
\VM. HERSCHEL, 
