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 th-e Farth's Crust" — By Archdeacon Pratt. 



[Received 4th August, 1864.] 



To the Secretary of the Asiatic Society. 



Sir, — 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 

 Su r v e y of tins 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. 



Tbe 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 

 CftUBed 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. 



