2i 2Tr. P raffs Second Letter on the Indian Arc. [No. 1. 



density do exist or not. The uncertainty of the existence or not of 

 this invisible enemy, and the utter impossibility (with our present 

 knowledge) of ascertaining whether it does exist or not, and there- 

 fore whether our plumb-line is affected or not by some such invisible 

 cause, is very troublesome — very far more so than the Himmalayas ; 

 because in their case there is a definite mass which it is possible to 

 measure, and the attraction of which can be calculated. There is 

 this to be said, that, as far as my investigations help me to make a 

 comparison, the effect of the Himmalayas seems to be much more 

 important, while it is more manageable, than any other of the 

 probable causes of derangement. But, whether or not, the Himma- 

 layas are a certainly-existing and a definite mass, and their effect 

 ought to be calculated. The calculation is not so " voluminous" or 

 tedious as Mr. Tennant seems to suppose — not near so toilsome as 

 some of those in which his duties occupy him.* Were it not for the 

 peculiar law of dissection made use of, no doubt it would have been 

 an herculean work which any one might well shrink from. But 

 this law reduces it simply to determining from the Survey Maps 

 the average height of the neighbourhood of about ninety different 

 places; multiplying them by the cosine of the azimuth, reducing 

 the result to miles, and multiplying it by 1".139, which gives the 

 deflection. In the parts beyond the range of the Survey Maps, and 

 which have not so great an effect on the stations of the arc in 

 question, an average form is obtained from Humboldt's observations 

 and from other sources, and the calculation requires only the sum- 

 mation of a few simple arithmetical series. All the heights used are 

 noted down in the Six Tables pages 78 — 83 of the paper of 1855 ; and 

 upon a correction being given me of any one of the heights, I can 

 (and any one who will examine the method can) in five minutes 

 find how much the resulting deflection of the plumb-line must be 

 altered — such is the simplicity which this law of dissection introduces. 



* He observes that the effect of the mountains in Central and Southern India 

 have not been considered. If Mr. Tennant will draw the lunes and compact- 

 ments on a map as described in my paper and note down the heights and 

 depressions of the several parts he will see that the effect will be too trifling to be 

 taken notice of. This examination would not occupy h im half an hour. 



