OUR MOLTEN GLOBE 47 



The Argument from Variation of the Force of Gravity. 



It is well known that mountains attract the plumb-line, 

 and thus render latitudes determined by its means, or by 

 a spirit or mercurial level, inaccurate in their vicinity. 

 During the trigonometrical survey of India the amount of 

 this error was carefully determined in several localities 

 near mountains, but a discrepancy appeared. When the 

 mass of the Himalayas was estimated and its attraction 

 calculated, it was found to be more than the observed 

 attraction. The same thing had occurred in the original 

 experiment by Maskelyne at Schehallion in Scotland ; and 

 a similar deficiency in the error produced was noticed by 

 Petit in the case of the Pyrenees. Many attempts were 

 made to explain the discrepancy, but that which was ad- 

 vanced by the late Sir G. B. Airy seems best to account 

 for all the phenomena, and is that adopted by Mr. Fisher. 

 It is, that every mountain mass on a continent has a much 

 larger mass projecting beneath the crust into the liquid 

 substratum, exactly as an iceberg has a larger mass under 

 the water than above it. Sir G. B. Airy argued that, 

 whether the crust were ten miles or a hundred miles 

 thick, it could not bear the weight of such a mass as the 

 Himalayan and Tibetan plateaus without breaking from 

 bottom to top, and receiving support by partially sinking 

 into the liquid mass. The best experiments show that the 

 proportionate densities of most rocks in a solid and a liquid 

 state are approximately as ice is to water, and thus no 

 mountain masses can be formed, whether by lateral pres- 

 sure or other agency, without a corresponding protuberance 

 forming below to keep the crust in equilibrium. It is this 

 displacement of the denser substratum by the less dense 

 " roots of the mountains " that leads to the total attraction 

 of such mountains being less than they otherwise would 

 be. In our author's words — " the roots of the mountains 

 can be felt by means of the plumb-line." 



Still more important and interesting are the revelations 

 afforded by the pendulum, since they not only support the 

 interpretation of the plumb-line experiments above given, 



