•TRACT 26. MEAii DENSITY OP TktE EARTH. 65

a mere useless labour, as incompatible with the unknownquantities and positions of the various kinds of the rocks, andalso with respect to the degree of accuracy to be counted on,in the observations and measurements made by the first con-duetors of this important experiment.

To what useful purposes the knowledge of the mean denisity of the earth, as above found, may be applied, it is notnecessary here to show. I shall therefore coriclude this tractwith a reflection or two on the premises that have been de-livered. Sir Isaac Newton thought it probable, that the meandensity of the earth might be,5 or 6 times as great as the den-sity of water; and we have now found, by experiment, thatit is very little less than what he had thought it to be : somuch justness was there even in the surmises of this wonder-ful man ! Since then the mean density of the whole earth is.about double that of the general matter near the surface, andwithin our reach, it follows, that there must be somewhere•within the earth, toward the more central parts, great quan-tities of metals, or such like dense matter, to counterbalancethe lighter materials, and produce such a considerable iiieatt.density on the whole. If we suppose, for instance, the den-sity of metal to be lO, which is about a mean ambng the va-rious kinds Of it, the density of water being 1, it Would require16 parts out of 27, or considerably more than one-half of thematter in the whole earth, to be metal of this density, iii. order to compose a mass of subh mean density as we have.found the earth to possess by the experiment: or -£ r , or be-. tween and |. of the whole magnitude will be metal; andconsequently -§4, or neatly y of the diameter of thfe earth, isthe central or metalline part. But if the metalline matter bechiefly iron, which as far as we know is by much the mostpredominant metal, then the half of the w'hole terrestrialmagnitude would be the bulk of the ferruginous matter.

Another inference that readily occurs, is this: viz, thatthus knowing tire mean density of the earth in comparison.with water, and the densities of all the planets relatively to.the earth, we can nOw assign the proportions pf the densitiesvon. ir. f