TRACT 26.

MEAN DENSITY OF THE EARTH.

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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-ductors of this important experiment.

To what useful purposes the knowledge of the mean den-sity of the earth, as above found, may be applied, it is notnecessary here to show. I shall therefore conclude 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 tViis wonder-ful man ! Since then the mean density of the whole earth isabout double that of the general matter near the surface, andwithin our reach, it follows, that there must be somewherewithin 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 meandensity on the whole. If we suppose, for instance, the den-sity of metal to be 10, which is about a mean among 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, inorder to compose a mass of such mean density as we havefound the earth to possess by the experiment: or T 4 r , or be-tween ^ and i of the whole magnitude will be metal; and ~consequently or nearly •§. of the diameter of the 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 whole terrestrialmagnitude would be the bulk of the ferruginous matter.

Another inference that readily occurs, is this: viz, thatthus knowing the mean density of the earth in comparisonwith water, and the densities of all the planets relatively tothe earth, we can now assign the .proportions of the densitiesVOE. II. B