1859.] Mr. Pratt's Second Letter on the Indian Arc. 23 



sure as I feel that Messrs. Adams and Le Verrier (to whom Mr. 

 Tenuant refers) never dreamt of ignoring the existence and attrac- 

 tion of that troublesome Planet, so clear does it appear to me that 

 the Hitnmalayan attraction must not be trifled with and passed 

 over. 



2. It was to calculate this that my paper of 1855 was written. 

 Other disturbing causes may exist, and should be estimated. But 

 this cannot do away with the importance of estimating the effect of 

 the Himmalayas. I have spared no pains to discover an antagonis- 

 tic cause which would nullify the influence of the Himmalayas, but 

 without effect. During the present year, I have forwarded to the Royal 

 Society two other papers ; one, estimating the effect of the deficiency 

 of matter in the Ocean, which extends down from Hindostan to the 

 South Pole ; the other, the effect of any slight deficiency or excess 

 in the density of mass of the earth prevailing over large spaces — 

 such variations in density from the density of a fluid mass, under 

 the same circumstances, as are not at all unlikely to have taken 

 place in the crust of the earth in its becoming solid, or by expansions 

 and contractions since that change occurred. The first of these 

 disturbing causes we know exists, because the Ocean exists and is 

 less than half as dense as rock. The amount of the effect is, how- 

 ever, uncertain because the depth of the Ocean is unknown. The 

 result of the paper, therefore, shows the tendency and the nature of 

 the effect, but not the exact amount.* The other calculation, viz. 

 that of the effect of slight but wide-spread departures from the law 

 of density in the interior mass, required by the fluid theory, was 

 suggested by the hypothesis of Mr. Airy, that there might be a 

 deficiency of matter below the Himmalayas which would, in a large 

 degree, counteract their effect on stations on the Indian arc from 

 Kaliana southwards. The result of this calculation is unfortu- 

 nate ; for it shows that such departures from the fluid-density as 

 I have alluded to, and which may not improbably exist, will have a 

 sensible and important effect on the plumb-line; hut we have no 

 possible means of becoming certain whether these variations of 



* The attraction of the mountains and the deficiency of attraction of the Ocean 

 are shown also to have a marked effect upon the sea-level, raising the level at 

 Karachee many feet above the level at Cape Comorin. 



