[Illustration: Fig. 73.—Hydrometer.] But to return to SPECIFIC GRAVITY, which means the “Comparative density of any substance relatively to water,” or as Professor Huxley says, “The weight of a volume of any liquid or solid in proportion to the weight of the same volume of water, at a known temperature and pressure.” Water, therefore, is taken as the unit; so anything whose equal volume under the same circumstances is twice as heavy as the water, is declared to have its specific gravity 2; if three-and-a-half times it is 3·5, and so on. We append a few examples; so we see that things which possess a higher specific gravity than water sink, which comes to the same thing as saying they are heavier than water, and _vice versâ_. To find the specific gravity of any solid body proceed as above, in the experiment of the lead. By weighing the substance in and out of water we find the weight of the water displaced; that is, the first weight less the second. Divide the weight in air by the remainder, and we shall find the specific gravity of the substance. [Illustration: Fig. 74.—Over-shot wheel of mill.] The following is a table of specific gravities of some very different substances, taking water as the unit. +----------+----------++----------+----------++------------+----------+ |Substance.| Specific ||Substance.| Specific ||Substance. | Specific | | | Gravity. || | Gravity. || | Gravity. | +----------+----------++----------+----------++------------+----------+ | Platinum | 21·5 || Iron | 7·79 || Water | 1·000 | | Gold | 19·5 || Tin | 7·29 || Sea Water | 1·026 | | Mercury | 13·59 || Granite | 2·62 || Rain Water | 1·001 | | Lead | 11·45 || Oak Wood | 0·77 || Ice | ·916 | | Silver | 10·50 || Cork | 0·24 || Ether | 0·723 | | Copper | 8·96 || Milk | 1·032 || Alcohol | 0·793 | +----------+----------++----------+----------++------------+----------+ But we have by no means exhausted the uses of water. Hydrodynamics, which is the alternative term for hydraulics, includes the consideration of many forms of water-wheels, most of which, as mill-wheels, are under-shot, or over-shot accordingly as the water passes horizontally over the floats, or acts beneath them. These wheels are used in relation to the fall of water. If there is plenty of water and a slight fall, the under-shot wheel is used. If there is a good fall less water will suffice, as the weight and momentum of the falling liquid upon the paddles will turn the wheel. Here is the Persian water-wheel, used for irrigation (fig. 75). The Archimedian Screw, called after its inventor, was one of the earliest modes of raising water. It consists of a cylinder somewhat inclined, and a tube bent like a screw within it. By turning the handle of the screw the water is drawn up and flows out from the top.