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 Gravity. | Substance. | Specific Gravity. | Substance. | Specific 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.
Fig. 75.—Irrigation wheel in Egypt.
The Water Ram is a machine used for raising water to a great height by means of the momentum of falling water.
The Hydraulic Lift is familiar to us all, as it acts in our hotels, and we need only mention these appliances here; full descriptions will be found in Cyclopædias.
We have by no means exhausted the subject of Water in this chapter. Far from it. But when we come to Chemistry and Physical Geography we shall have more to tell, and our remarks as to the application of science to Domestic Economy, in accordance with our plan, will also lead us up to some of the uses of water. So for the present we will take our leave of water in a liquid form, and meet it again under the application of Heat, which subject will take us to Ice and Steam,—two very different conditions of water.