The water, being raised through evaporation into the atmosphere by the heat of the sun and of the earth, and after precipitation as rain and snow being drawn into the valleys by the attraction of gravitation, is made available for motive power in the latter track of this circuit through the agency of wheels (i) by its impact acting against their projecting parts or paddles, so placed as to receive the force of the current; (2) by its weight being delivered in quantity in receptacles, called "buckets," formed in the rims of the wheels; and (3) by its reaction or counter-force, which becomes available when the water, after being delivered into the wheel, is permitted to escape in a direction opposite to that which it had when entering the wheel.
Classznca /ion. —Water-wheels may be chiefly classified into (1) those which turn on a horizontal axis and (2) those which turn on a vertical axis. To the first class belong the overshot-, undershot-, and breast wheels, and to the second class belong the reaction-wheels and the tur bines. As the turbines constitute the most efficient variety of water wheels and are probably the most numerous, hydro-dynamic motors will be separately considered under the subject-heads of (1) water-wheels and (2) turbines.
Current-wheels.--The application of the force of moving water as a motive power is of early Eastern origin. The current-wheel or noria (Arabic, na'ara) has for thousands of years been employed in Egypt, Arabia, and Syria for raising water for purposes of irrigation. The pe riphery of the wheel has radial floats, which are sufficiently submerged to be acted upon by the current of the water and to give rotation to the wheel secured upon its horizontal axis. (See p. zoo.) The first mentioned floating current-wheels for driving machinery were those used to turn the corn-mills which were devised by Belisarins when the Romans were besieged by Vitiges. (See p. 34.) In iSo2 there was patented by Hawkins of England a current-wheel which was substantially a reinvention of the current-wheel of Belisarius.
The Overshot-wheel, shown in Figure i (p1.62), is constructed entirely of iron, and is built upon a wrought-iron axle or shaft (A), which rests on each side in a journal (C) secured to and supported by the masonry. In immediate connection with the shaft are three equidistant hubs, to which radial arms (B, E, D, F) arc fastened and braced diagonally by stay-rods (11, G). On the periphery of the wheel numerous curved plate-iron buckets are formed and arranged for the reception of the water, and on one end is secured a spur-gearing ring which engages with the small cog wheel (ilf) of the transmitting shaft (N).
The High Breast-wheel (Jig. 4) is also constructed of iron, and in addition to the radial arms, as ill Figure i, it has " brace-rods," which are connected to the two large cast-iron hubs, and also to the cast-iron rims. The construction of the buckets, which is similar to that of Figure (p1. 62), is shown in enlarged section in Figure 5. The continuous cog-gear on the interior of the rim of the wheel engages with the gearing, shown at the left of the illustration. Above the water-channel or race is seen the self-acting ball-governor, which regulates the sluicegate, and hence also the water-supply and the speed of the wheel. The human figure represented in the illustration is intended to give a comparative idea of the size of the wheel, which was constructed to operate a cotton-mill in Greenock, Scotland. It has the enormous diameter of over 21 metres (69 feet) for a height of fall of T9Y, metres (64 feet), and is but little smaller than the largest known water-wheel. The overshot-wheel em ployed for working the pumps which drain the mines at Laxey, on the Isle of Man, is perhaps the largest water-wheel in the world. It is 72 feet in diameter, with a width of 6 feet, and exerts a force of about two hun dred horse-power. The crank-shaft is to feet long, and the pump has a capacity for raising two hundred and fifty gallons of water per minute from a depth of 1200 feet.