THE INTERIOR OF THE EARTH. 241 



load applied over an area only about 30 miles wide are probably con- 

 fined almost entirely to the first 100 miles of depth. The magnitude 

 of changes of inclination produced at an observing station on the 

 shore by the changing water load would, therefore, be dependent pri- 

 marily on the modulus of elasticity of the material below and around 

 the bay to a depth of less than 100 miles. The observations might 

 serve, therefore, to determine a modulus of elasticity of the surface 

 portion of the earth rather than of the whole earth. 



Turn now to the third of the physical constants, which it was pro- 

 posed to examine, namely, the strength. 



Among the forces which we may consider as furnishing tests of 

 strength are: (1) The forces involved in earthquakes, (2) the 

 weight of continents, and (3) the weight of mountains. 



The forces which produce the more intense earthquakes evidently 

 cause stress differences locally, which are beyond the breaking 

 strengih of the material. However, from earthquakes we may obtain 

 but little information as to the strength of the earth material, be- 

 cause the intensity of the stress differences can not be reliably de- 

 termined. We know simply that the intensity exceeds the breaking 

 strength of the material at the points of rupture. 



It is uncertain how great are the maximum stress differences pro- 

 duced by the weight of continents. One great difficulty in computing 

 these stress differences arises from the fact that the isostatic com- 

 pensation of continents, now loiown to exist, reduces the stress differ- 

 ences much below what they would otherwise be. Love computed the 

 maximum stress differences thus reduced as 0.07 ton per square inch. 

 Darwin computed the greatest stress difference due to the weight of 

 the continents, without isostatic compensation, as 4 tons per square 

 inch. If each of these computations were based upon assumptions, 

 which correspond closely with the facts, one should be warranted in 

 drawing the conclusion that the maximum stress difference caused by 

 the actual continents, supported in part by the actual isostatic com- 

 pensation, is between 0.07 and 4 tons per square inch, and that it is 

 much nearer to the smaller than to the larger value. But a close 

 examination of either of these computations shows that it is based 

 upon assumptions made to simplify and shorten the computations, 

 which a&sumptions depart widely from the facts and tend strongly to 

 make the computed stress differences much smaller than the actual. 

 For example, both Darwin and Love used in their computations 

 hypothetical continents, represented by regular mathematical forms, 

 in the place of the actual continents with their many irregularities. 

 The maximum stress difference caused by the actual continents is 

 necessarily much greater than would be produced by the assumed 

 smoothed out, regular, symmetrical continents. 



