The gravity of a portion of the earth was calculated by Dr. alaskelyne in the years 1774 and 1775, at the mountain Schehallien, and its attraction on a plum met ascertained on each side ; besides which, he computed the quantity of mat ter contained in it by a considerable num. ber of sections in various directions ; and the result being afterwards compared with the acknowledged magnitude and attrac tion of the earth, he found that the den.. city was as 9 to 5 of common stone, and as 9 to 2 water ; whence it was inferred that large quantities of metal lay conceal ed within it. Before this period little was known of the gravity or density of the earth, though the relative densities of that and others of the planets had been ascertained with tolerable preci sion. From the data thus acquired, the quantity of matter in the earth is known to be equal to the product of its density by its magnitude ; the force of attraction on the surface has been proved by expe riments, that substances fall feet in one second of time ; and hence, that if any place within or beyond it may be known, as in the former case the force is at its distance from the centre, and in the latter as the square of its distance from the same point.
The earth has three motions : that which gradually occasions the precession of the equinoxes ; that round its axis, which causes the succession of day and night, accomplished in 24 hours ; and that of the whole mass round the sun in a wide orbit, of which the luminary of day is the centre : in the latter its axis is constantly parallel to itself, and inclined in the same angle to its path ; thus pro ducing the visible and perceptible alte ration from spring to summer, and from that period to winter. This annual mo tion of the earth round the sun is per formed between the orbits of' Venus and Mars with the former, and that of Mer cury within its own, or between it and the sun in the centre, and those of Mars, Jupiter, Saturn, &c. above it or without,
which are called from this circumstance superior planets, and the rest inferior. The time occupied in performing this re volution is 365 days, 6 hours, and 49 mi nutes, or a tropical year, calculated from an equinox or solstice to the same again ; the sideral year, as computed from any tired star to the same point, and seen from the sun, makes the revolving of the earth to occupy 365 days, 6 hours, and 9 minutes : the figure of the orbit is el liptical, with the sun in one focus : the supposed distance is 95 millions of miles, adulating the sun's parallax to be 8r, or the angle beneath which the semidiame ter of the earth would appear from the sun ; "and the eccentricity of the orbit, or distance of the sun in the focus from the centre of this elliptic orbit, is about as of the mean distance." There are nine descriptions of earth, which are, barytes, strontian, lime, mag nesia, alumina, ytti•ia, glucina, zirconia, and silica. Every substance is an earth which is insoluble in water, or becomes so when combined with carbonic acid, I with little taste or scent under the above circumstances, and incapable of altera tion by fire when in a pure state, then capable of being turned to white pow der, and not exceeding 4.9 in specific gravity.
The word earth is generally applied to the whole mass of the globe, and always to the mould which supports the growth of vegetables ; the latter has been atten tively examined by various chemists, and found to consist of several substances, without any regularity in their arrange ment.