FOUNDATIONS OF THE GREAT DEEP T] 



GEOID WITH SURFACE GEOID WITH 



ISOSTASY (a) /- ' -~-_ NO ISOSTASY (b) 



SPHEROID 



figure 12. diagrammatic section to illustrate departure of the 

 geoid from the standard spheroid, when: (a) continental crust is 

 in balance with the sub-oceanic crust (condition called isostasy); 

 and (b) continent represents excess matter on the earth's body 



(no isostasy). 



region. But it has been proved beyond doubt that the actual 

 geoid shows no large-scale warping of the kind described. 

 There is only one possible explanation: the relative lack of 

 attracting matter in the ocean water must be compensated for 

 by relative excess of attracting matter in the rock under the 

 ocean; in other words, the density of the sub-oceanic rock must 

 be greater than the density of the rock under the continental 

 surfaces. 



That conclusion comes clearly and even more directly from 

 the second method of diagnosis, based on measurements of the 

 intensity of gravity at many land and sea stations. The instru- 

 ments used are called gravimeters, and the method is called the 

 gravimetric. Most of the measurements have been made with 

 special pendulums, one type designed for land stations and 

 another type for use at sea. In each case the pendulum's period 

 of swing gives, after proper precautions and calculations, the 

 intensity of gravity. Manifestly the sea-going pendulum must 

 operate in the absence of strong disturbance of the instrument 

 by the waves. The requirement is met by setting up the pen- 

 dulum in a submarine vessel, capable of submergence to the 



