11. And in the first place, we may remark that the Earth is really much less heavy than we should expect, from what we know of the materials of which it consists. For, measuring the density, or specific gravity, of materials, (that is their comparative weight in the same bulk,) by their proportion to water, which is the usual way, the density of iron is 8, that of lead 11, that of gold 19: the ordinary rocks at the Earth's surface have a density of 3 or 4. Moreover, all the substances with which we are acquainted, contract into a smaller space, and have their density increased, by being subjected to pressure. Air does this, in an obvious manner; and hence it is, that the lower parts of our atmosphere are denser than the upper parts; being pressed by a greater superincumbent weight, the weight of the superior parts of the atmosphere itself. Air is thus obviously and eminently elastic. But all substances, though less obviously and eminently, are still, really, and in some degree, elastic. They all contract by compression. Water for instance, if pressed by a column of water 100000 feet high, would be reduced to a bulk one-tenth less than before. In the same manner iron, compressed by a column of iron 90000 feet high, loses one-tenth of its bulk, and of course gains so much in density. And the like takes place, in different amounts, with all material whatever. This is the rate at which compression produces its effect of increasing the density, in bodies which are in the condition of those which lie around us. But if this law were to go on at the same rate, when the compression is greatly increased, the density of bodies deep down towards the centre of the Earth must be immense. The Earth's radius is above 20 million feet. At a million feet depth we should have matter subjected to the pressure of a column of a million feet of superincumbent matter, heavier than water; and hence we should have a compression of water 10 times as great as we have mentioned; and, therefore, the bulk of the water would be reduced almost to nothing, its density increased almost indefinitely: and the same would be the case with other materials, as metals and stones. If, therefore, this law of compression were to hold for these great pressures, all materials whatever, contained in the depths of the Earth's mass, must be immensely denser, and immensely specifically heavier, than they are at the surface. And thus, the Earth consisting of these far denser materials towards the centre, but, nearer the surface, of lighter materials, such as rock, and metals, in their ordinary state, must, we should expect, be, on the whole, much heavier than if it consisted of the heaviest ordinary materials; heavier than iron, or than lead; hundreds of times perhaps heavier than stone.