igoS.] THE PHYSICS OF THE EARTH. 265 



where the plateaus are highest, as in Himalayas and Tibet, where 

 the deficiency in the attraction of these elevated masses long ago 

 attracted attention. In his " Account of the Operations of the 

 Great Trigonometric Survey of India," Calcutta, 1879, General 

 J. T. Walker says : 



" There appears to be no escape form the conclusion that there is a 

 more or less marked negative variation of gravity over the whole of the 

 Indian continent, and that the magnitude of this variation is somehow con- 

 nected with the height. 



" Pratt's calculations had reference only to the visible mountain and 

 oceanic masses and their attractive influences — the former positive, the latter 

 negative — in a horizontal direction ; he had no data for investigating the 

 density of the crust of the earth below either the mountains on the one 

 hand, or the bed of ocean on the other. The pendulum observations fur- 

 nished the first direct measures of the vertical forces of gravity in different 

 localities which were obtained, and these measures revealed two broad facts 

 regarding the disposition of the invisible matter below; first, that the force 

 of gravity diminishes as the mountains are approached, and is very much 

 less on the summit of the highly elevated Himalayan table-lands than can 

 be accounted for otherwise than by a deficiency of matter below; secondly, 

 that it increases as the ocean is approached, and is greater on islands than 

 can be accounted for otherwise than by an excess of matter below. As- 

 suming gravity to be normal (in amount) on coast lines, the mean observed 

 increase at the islands stations was such as to cause a seconds' pendulum to 

 gain three seconds daily, and the mean observed decrease in the interior of 

 the continent would have caused the pendulum to lose 2^/^ seconds daily at 

 stations averaging 1,200 feet above the sea level, 5 seconds at 3.800 feet, 

 and about 22 seconds at 15,400 feet — the highest elevation reached— in excess 

 of the normal loss of rate due to the height above the sea." 



The facts here mentioned by General Walker are recognized in 

 geodesy as applying in different degrees to all the elevated table- 

 lands and mountainous regions of the globe. The physical cause 

 of this deficiency in attraction is now established beyond all doubt, 

 and the intumescence of the land, first suggested by Sir John 

 Herschel, is shown to have arisen from the expulsion of lava from 

 beneath the sea. Thus arises the physical condition which 

 secures the equilibrium of the earth between the land and water 

 hemispheres. This must be regarded as not the least remarkable 

 among several interesting results on the physics of the earth deduced 

 from the principle of the secular leakage of the oceans. Earth- 

 quakes, volcanoes, mountain formation, the uplift of islands, plat- 



