igoS.] 



THE PHYSICS OF THE EARTH. 259 



tion, as was done in California after the great earthquake of April 

 1 8, 1906. And as the disturbance may alter the direction and in- 

 tensity of local gravity, this possibility must be taken account of in 

 the repetition of the observations. In order to be entirely rigor- 

 ous the equations connecting the triangulation should include unde- 

 termined multipliers to take account of possible variations in the 

 local attraction at each point. If with this general condition im- 

 posed, the triangulation before and after the earthquake comes out 

 rigorously the same, within the limits of errors of observation, it may 

 be supposed that the surface effects of the disturbance are insen- 

 sible ; otherwise the dift'erence must be attributed to disturbances 

 due to the earthquake. 



With the refinement now possible in geodesy, it is not to be 

 doubted that these effects will occasionally prove to be sensible to 

 observation. The great earthquake in Assam-Bengal gave rise 

 to horizontal movements of the order of 20 or 30 feet, which may 

 aft'ect the latitude by o".2 or o".3, and are thus within the limits of 

 astronomical measurement. But apparent changes in latitude may 

 result from change in the direction of gravity as well as from actual 

 displacements of the crust, and both possibilities need to be taken 

 into account. 



§ 52. The Necessity of Further Study of the Contours and 

 Movements of the Sea Bottom. — In view of the results brought out 

 in this paper and those which have preceded it, but especially that 

 on " The New Theory ot Earthquakes and Mountain Formation as 

 Illustrated by Processes now at \^"ork in the Depths of the Sea," 

 it is scarcely necessary to point out the extreme importance of 

 further study of the contours and movements of the sea bottom. 

 Our present maps of the ocean depths are very incomplete, although 

 they aff"ord a good general idea of the sea basins. But one can 

 scarcely doubt that more exact surveys would bring to light addi- 

 tional mountain ranges and plateaus in regions heretofore but slightly 

 explored ; moreover certain places in the sea bottom would be 

 found to be covered with a great variety of peaks or submerged 

 islands which do not reach the surface. 



Where the water is deep the exact survey of the bottom pre- 

 sents considerable difficultv. As movements arising from earth- 



