relation to the Constitution of the Earth's Crust. 25 



a ( •3(3 + n)A 7 / bh h V a 



_ v^3 (3 + k)A 2 + 2M \ 

 2 " 2a )' 



Adding this to the attraction of the plateau, since the second 

 term of the one cancels the first of the other, we have the 

 attraction at the station 



/o h f /0 , \y ((3 + n)hb \, a\ 



If we substitute the values indicated above, and introduce 



the factor -^— * 3 to express the effect in swings, the former 



of the above terms gives —2*73 swings in defect, due to the 

 attraction of the plateau and other masses. The second 

 term is not easily estimated, on account of the irregular out- 

 line of the further boundary of the plateau ; but it will cer- 

 tainly be quite small, but being positive will make the negative 

 effect less. Now there were observed relative to Punnse 

 — 2*28 swings in defect due to local causes at Kaliana. 



The estimate —2*73, having been obtained on the suppo- 

 sition that the slope of the plateau is a solid inclined plane, 

 will be lessened by the circumstance that it is much inter- 

 sected by valleys ; and this will be equivalent to assigning a 

 lower mean value to the density p. Thus, on the whole, we 

 may conclude that the calculated effect of local attraction at 

 Kaliana brings the negative variation of gravity very close to 

 the observed variation relative to Punnse. 



The above number has (as was also done in the case of 

 More) been estimated on the suppositions that the ellipticity 

 is correctly assumed, and that there would be no local attrac- 

 tion at the mean surface of the crust h considered as a land 

 surface. Could we suppose Punnse to be in such a situation, 

 the above comparison would be exact. But since it is on the 

 sea-coast, there is probably some local attraction there. 



In all but six instances the local attractions, given in the 

 ' Account &c.' as due to the visible masses, have been esti- 

 mated as if they were caused by an infinite plain of the height 

 of the station. The exceptions are Somtana, Ahmadpur, 

 Usira, Dehra, Mussoorie, and More, at which places allowance 

 has been made for inequality of surface. It might be thought 

 that we need not have estimated the attractions afresh, as has 

 * See pages 6 and 16 



