THE INTERIOR OF THE EARTH. 293 



met. This difficulty is felt particularly in the Russian and Indian tri- 

 angulations. "While Colonel Chodsko found in the Caucasus a deflec- 

 tion of fifty-four seconds, and Schweitzer, in an open plain in the envi- 

 rons of Moscow, deflections of eight and nine seconds, the Himalayan 

 chain appears to have had but an insignificant influence in place of the 

 considerable one which the theory required as if these mountains 

 were composed of less dense rocks than the soil of the plain. 



The operations referred to serve to indicate the form of the earth 

 by the angles which the verticals of a series of stations i. e., the direc- 

 tion of weight make with the earth's axis. Another mode consists in 

 measuring at numerous points the degree of the weight, and from this 

 the distance to the center of the earth, the rate of oscillation of the 

 pendulum being also noted. These oscillations are accelerated as the 

 attractive power of the earth increases that is, as the center is ap- 

 proached. We have seen that Richer remarked these variations of the 

 pendulum in his voyage to Cayenne, and that Newton furnished the 

 explanation of the phenomenon. At the commencement of the present 

 century Biot, Sabine, Kater, Liitke, Foster, and others, made numerous 

 experiments of this nature which have furnished a valuable verification 

 of the results of geodesy, properly so called. But it must not be for- 

 gotten that the degree of weight may be changed by the same causes 

 that change its direction. A local accumulation of very dense rocks 

 may increase the terrestrial attraction, and light ones may diminish it. 

 The de-leveling of the ocean of which we have spoken, by which the 

 waters near continents are elevated while the mass of the ocean at 

 large is lowered, results in making an ocean-valley, as it were, from 

 which the islands, that are thus nearer the earth's center than the 

 continents, project. This will explain the increased rate of oscillation 

 of the pendulum observed in many islands, which is otherwise inex- 

 plicable. 



The perturbations to which the direction as well as the degree of 

 weight is subject have enabled us to determine the earth's mean den- 

 sity. The principle of the method is easily comprehended. Let us 

 suppose that the deflection of the plummet has been measured near an 

 isolated mountain whose volume and weight it is possible to estimate 

 with some degree of precision. The amount of the deflection will fur- 

 nish a means of calculating the relation of the mass of the mountain 

 to that of the earth, and, the two masses being known, their relative 

 densities can then be determined. The oscillations of the plummet at 

 the summit and at the foot of the mountain afford the basis for a simi- 

 lar calculation. On carrying the plummet to the top some oscillations 

 per day will be lost, the distance from the earth's center being increased; 

 but the mountain's own attraction in part offsets the decrease in weight 

 attributable to altitude, and herein we have the means of comparing 

 its mass with that of the earth. 



These methods were not neglected by Bouguer in his voyage to 



