of form, and are not, in some cases, even round; in winding yarn on to bobbins, where a motion is required of a constantly varying rate, two elliptical wheels are made to act on each other, the end of one being approximated to the centre of the long axis of the other, (fig. 10). Illustration: FIG. 9. (‡ ECCENTRIC-WHEEL.) Illustration: FIG. 10. (‡ TWO ECCENTRIC-WHEEL GEARS.) Wheels for carriages are used to diminish friction, by causing the “tire” or smooth outer edge to roll upon the surface instead of being rubbed; all the friction in wheels is in the centre or axle, which being turned smooth, and greased or oiled, works very easily. Carriage wheels are made to revolve upon a fixed axle, and each wheel revolves independently of the other, but in railway-carriages and engines, the wheels are united in pairs, and the axle revolves with them, the weight being borne outside of the wheel on a small part of the axle which projects. The various parts of a wheel are the box or “nave” which is the centre part, the “spokes” or those bars which connect it with the centre edge or felloes, and the “tire,” an iron band binding the whole together. Wheels for gun-carriages are made at Woolwich Arsenal by machinery. The following is a description of them, taken from the “Times” newspaper:-- “Here a few unskilled labourers superintending the machines produce forty complete gun-carriage wheels a day, though all their component parts are made of the hardest woods--viz., elm for the naves, oak for the spokes, and ash for the felloes. The novelty here was the new mode in which a wheel is fitted together. Instead of by hand, as formerly, the pieces are all laid together on the ground, and of course in a circle, around the outside of which are six small hydraulic rams, with the head of the piston of each curved so as to form a segment of a circle touching the outside portion of the wheel. One small steam-engine pumps the water into all these with an equal pressure, which, as it increases, forces the felloes into the spokes and the spokes into the nave of the wheel, with such force as to compress the whole, by a strain of 250 tons, into the solidity of one piece.” Paddle-wheels are made to revolve with their lower part in water, and are furnished with a series of short boards fixed to the tire of the wheel, which is generally double, that they may be better held on; these boards or paddles take a great hold on the water and cause the resistance which is necessary to move the vessel. The wheel of a watermill is constructed in the same way. WATERMILLS. Illustration: (‡ WATERMILL.)