C 99 3 
VI. Part of a Letter from Mr. Michael Topping, to Mr. 
Tiberius Cavallo, F.R.S. 
Read February 16, 1792. 
DEAR SIR, Madras, February 4, 1789. 
I inclose you an account of a base line I have measured for a 
series of triangles I am carrying down the coast of Coro- 
mandel. I have already extended them to about 300 miles 
from Madras, and am upon returning back to prosecute the 
work quite down to Cape Comorin. The angles are all taken 
with my Hadley's sextant, made by Stancliffe, by means of 
three tall signals I have constructed of bamboos, 80 feet high, 
60 of. which I mount upon steps, so as to see (over all trees, &c.) 
very distinctly my two other signals, at the distance of from 
8 to 13 miles. It is, I believe, the first time the Hadley was 
ever made use of for a purpose of such magnitude ; but it is 
fully equal to it — nay, it does’ more — the sun's bearing, or ob- 
lique distance, from my signals is also taken by it ; by which, 
and his azimuth (computed) I obtain the angles made by them 
with the meridian ; and by combining the whole, the difference 
of latitude, and meridional distance of every one of them in 
English fathoms. This is found so nicely, that a mean of my 
astronomical observations for the latitudes, never differs more 
than a few seconds from those given by the geometrical men- 
suration. In all the operations I have had no one to assist me, 
O 2 
