394 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



amount of compression which the continents have suffered. While 

 we know that continents have been squeezed, it is not known that 

 ocean beds have been similarly affected. The broad flat ocean bot- 

 toms have rather the form of surface of a mass which flattens and 

 spreads under its own weight. The writer has suggested that the 

 spreading masses below the oceans may squeeze the masses beneath 

 tlie continents, and finds a cause therefor in the fact that a cubic 

 mile of the former is heavier than one of the latter. This brings us 

 to the idea of differences of density in the earth's crust. 



As far back as 1880 an English physicist, Airy, entertained the 

 idea that some parts of the earth's crust might be heavier, some 

 lighter, and in 1855 he contributed the suggestion to a discussion by 

 Pratt of the attraction exerted by the Himalaya Mountains. In 

 course of surveys in India it had been found that the great mass of 

 that mountain range exerts an attraction, which was, however, much 

 less than it should be according to calculation, if the mass beneath 

 the Himalayas were of the same density as that beneath the penin- 

 sula of India. Hence Airy and Pratt suggested that the mountains 

 must be lighter.^ 



When Pratt wrote in 1885 no one doubted but that the earth had 

 cooled from a molten condition, become covered with a rigid crust, 

 and finally assumed its present configuration with all the detail of 

 ocean basins, continents, and mountains. Although Pratt and Airy 

 did not wholly agree, they both explained the lightness of the moun- 

 tains by reasoning based on the processes of cooling and floatation 

 of the crust on the still fluid interior. Now that it is known that 

 the earth has the rigidity of steel and can not possibly be liquid 

 within, the basis of their reasoning has disappeared and their theories 

 are no longer entertained; but the inference as to the lightness of the 

 mountains has been confirmed not only in regard to the Himalayas, 

 but for many other mountain ranges. It has also been shown that 

 continental masses are relatively light as compared with those be- 

 neath the oceans. And it follows that if we thinlv of a column be- 

 neath the continent and one beneath the ocean extending down to a 

 common level, the taller column of lighter material can be of the 

 same weight as the shorter column of heavier material. The two 

 columns might then balance each Other or be in equilibrium. 



It seemed probable to Button and Gilbert 20 years ago that this 

 relation of equilibrium was characteristic of the masses that make 

 up the outer earth. Dutton discussed the problem in tlie following 

 terms : 



If tbe earth were composed of homogeneous matter, its normal figure of 

 equilibrium, without strain, would be a true spheroid of revolution ; but if 



1 Pratt, J. H. A treatise on Attractions, LaplaQe's Functions, and the Figure of me 

 Earth, 4th ed., pp. 93-94, 1871. 



