Connected with the Earth's Field of Force. 145 



This very sensitiveness of the instrument makes it avail- 

 able for the study of discontinuities in density concealed 

 just below the surface; these discontinuities may often 

 indicate the existence of conditions of geological or of 

 commercial interest. It has been proposed to use the 

 Eotvos balance in certain geologic investigations and I am 

 informed that a business concern is considering a cam- 

 paign with the Eotvos balance in order to locate salt 

 deposits in Poland; another campaign with the balance 

 is, or was recently, under way to locate lignite deposits 

 in Austria.^^ The balance is not, of course, a divining 

 rod; its usefulness for geological or commercial purposes 

 depends on our ability to interpret irregularities in the 

 second derivatives in terms of irregularities in the distri- 

 bution of mass. 



The second derivatives, erratic though they may be at 

 individual stations, will be found to have about the theo- 

 retical average value when the results for considerable 

 areas are combined. If this were not so, we should 

 find that the discrepancies between the theoretical values 

 of gravity and the observed values would be very consid- 

 erable, while, as a matter of fact, they are nearly always 

 small. The same might be said of the differences between 

 the astronomical values of the latitude, longitude and azi- 

 muth and the values obtained by geodetic methods. Irreg- 

 ularities in the second derivatives imply irregularities in 

 the curvature. The form of a level surfaoe near the 

 surface of the earth has been appropriately compared to 

 that of a withered apple, which, considered as a whole, has 

 a regular curvature, but which, taken in detail, is charac- 

 terized by minute elevations and depressions. The argu- 

 ment for the existence of forces tending to cause motion 

 toward the equator, although based on the theoretical 

 form of the level surfaces, still holds good when we con- 

 sider average conditions, though at any particular point 

 the assumption previously made may be greatly at fault. 



This brings us to the end of our list of mechanical 

 curiosities connected with the earth's field of force. 

 These may be summarized as follows : 



(1) The limiting level surface with the sharp edge. 



^^ Akademie der Wissenschaften in Wien, math-naturw. Klasse, session of 

 Jan: 8, 1920 ; reported in the Akademischer Anzeiger no. 1. 



Am. Jour. Sci.— Fifth Seeies, Vol. II, No. 9.— September, 1921. 

 10 



