102 



On the Indian Pendulum-observations. 



[Nov. 24, 



compartments by concentric circles and radii ; they have then been reduced 

 to the sea-level by Dr. Young's formula. The stations thus corrected are 

 Masoori, Usira, Ehmadpur, and Somtana; at Masoori, curvature was taken 

 into account, and the calculations were extended to a distance of 100 miles 

 all round ; but at the three other stations curvature was not allowed for, as 

 the calculations were only carried to a distance of one mile. 



(13.) The preceding details will suffice to explain all that is necessary 

 regarding the observations, and the preliminary results which have been 

 derived therefrom which accompany this note. I will therefore now pro- 

 ceed to indicate the principles by which we have been guided in selecting 

 the positions of the pendulum-stations. 



(14.) In the first instance, the original programme of observing at a 

 certain number of stations of the Great Arc was duly carried out ; the 

 pendulums were swung at eighteen stations between Cape Comorin and the 

 Siwalik Hills at the base of the southern slopes of the Himalayas, and at 

 two stations north of the Siwaliks. 



(15.) As yet no observations have been taken on the higher ranges, 

 or on the tablelands, of the Himalayas, and thus the full influence of 

 these ranges in producing local variations of gravity has not yet been 

 ascertained. But the observations at the five northernmost stations indi- 

 cate that there is much probability that the density of the strata of the 

 earth's crust under and in the vicinity of the Himalayan mountains is less 

 than that under the plains to the south, the deficiency increasing as the 

 stations approach the Himalayas, and being greatest when they are north 

 of the Siwaliks. On the other hand, the observations at the five southern- 

 most stations show an increase of density in proceeding from the interior 

 of the Peninsula to the coast of Cape Comorin. Thus both groups of 

 observations tend to confirm the hypothesis that there is a diminution of 

 density in the strata of the earth's crust under mountains and continents, 

 and an increase of density under the bed of the ocean. 



(16.) In order to test this hypothesis still further, as soon as the 

 observations at the stations of the Meridional Arc were completed, the 

 pendulums were taken to an ocean station — the Island of Minicoy, in the 

 same latitude as Punnae, and about 250 miles from the mainland ; and 

 afterwards to five stations on the east and west coasts, each in nearly the 

 same latitude as one of the stations in the Meridional Arc. Thus the 

 comparisons between the local variations of gravity under the continental, 

 the coast, and the ocean stations are independent of an exact knowledge of 

 the normal variation of gravity in proceeding from the poles to the equator. 

 It will be seen that, without a single exception, gravity at a coast station is 

 in excess of gravity at the corresponding inland station, and that at the 

 ocean station it is greater than at the corresponding coast station; thus: 



