22 Mr. Pratt's Second Letter on the Indian Arc. [No. 1. 



us put full faith in the law of gravity as not to be shaken by 

 dubious analogies. 



18. I trust I shall not be supposed to depreciate Mr. Pratt's 

 analytical conclusion. All knowledge is gain and Mr. Pratt's law of 

 dissection is an acquisition which will doubtless find its practical 

 application in due time. At present we can only look on the 

 results of the paper as an arithmetical illustration of the formula 

 and as not to be employed in the question of the earth's figure. 



Mr. Pratt's Second Letter on the Indian Arc. 



To the Secretary of the Asiatic Society. 



Sir, — In reply to Mr. Tennant's letter brought before the Society 

 at its last monthly meeting, it appears to me quite sufficient to ask 

 your readers to read again my letter in jour Journal No. III. 1858. 



The question appears to myself to be a very simple one, and I 

 wonder at Mr. Tennant's not seeing it in the same light.* I can 

 readily comprehend his being disappointed that the effect of the 

 Himmalayas should be so troublesome ; in this I can fully sympa- 

 thise with him. But I have too great a respect for the Law of 

 Universal Gravitation to leave out of consideration such a disturbing 

 cause. These Himmalayas are as great a tyrant in the delicate 

 problem of determining the curvature of the arc of meridian in 

 Hiudostan, as the planet Jupiter is in the Solar System. But as 



* I have not noticed his analogy drawn from planetary orbits, simply to avoid 

 being drawn into a discussion on Physical Astronomy. But I may say 

 thus much, that this supposed analogy does not help matters at all. In the 

 Survey in order to map the country, they use the Fundamental Ellipse. This 

 analogy would therefore require that the Fundamental Ellipse should be used to 

 find the place of a planet. But it is the " Instantaneous or Varying Ellipse" 

 which is used for this purpose. It is the corresponding Local or Varying 

 Ellipse, therefore, in going from place to place, that ought strictly to be used in 

 mapping the country, and not the Fundamental or mean Ellipse. As far as there 

 is any approach to analogy, Messrs. Adams and Le Verrier pursued precisely the 

 same course which I have followed. 



