34 On Local Attraction. [No. 1, 
“ On the degree of uncertainty which Local Attraction, if not allowed 
for, occasions in the Map of a Country, and in the Mean Figure of 
the Earth as determined by Geodesy ; a Method of obtaining the 
Mean Figure free from ambiguity by a comparison of the Anglo- 
Gallic, Russian, and Indian Arcs; and Speculations on the 
_ Constitution of the Earth's Crust.”—By Arcuvgeacon Pratt. 
[Received 4th August, 1864. | 
To the Secretary of the Asiatic Society. 
Srr,—I beg to forward to you a copy of a Paper lately printed in the 
Proceedings of the Royal Society (No. 64) on the topics notified at 
the head of this letter. Two years ago you accepted from me a 
“Series of Papers on Mountain and other Local Attraction in India,” 
and published in your Journal a memorandum, regarding the effect 
of local attraction upon the operations of the Great Trigonometrical 
Survey of this country. The present Paper is not confined to India ; 
but appertains to the globe in general. But as the results of the 
Indian Survey occupy an important position in the calculations, you 
may deem it to be not irrelevant to the objects of your Journal to 
publish some account of it. 
The state in which the question of local attraction was left in my 
former communications to the Royal Society was this :—That in India 
the deviation of instruments of observation from the true vertical — 
caused by the Mountains and by the Ocean is very great, far greater 
than had ever been supposed; that this deviation might be much 
increased or diminished by the effect of variations of density in the 
solid crust of the earth, but that of the amount of this we have no 
means of judging, as we are entirely ignorant of the constitution of the 
crust: and that the effect of local attraction on the Map of India 
constructed from the Survey would fortunately disappear as far as 
regards.the relative position of places laid down, but that the precise 
position of the Map on the terrestrial spheroid could not be discovered, 
as it would depend upon the unknown total resultant local attraction 
arising from all causes at the station from which the Survey operations 
commence. 


