Watermills are those kind of mills, the motion of which is derived from the flow of a stream of water against the lower part of a large wheel, provided with paddle-boards similarly to the paddle-wheels of steam-vessels; or else by the weight of a stream of water falling against the upper part of the wheel from a spout or trough; the former of these is called the under-shot, and the latter the over-shot mill. The former is used where there is a large body of water flowing at a sufficiently rapid rate, and the latter kind where there is but a small supply, the whole of which is often used for driving the mill; but other circumstances, of position, &c., may determine which shall be used. The large wheel being thus driven round, any kind of machinery may of course be attached, according to the nature of the work to be done. Like windmills these watermills are for the greater part superseded by steam power; the locality, &c., must determine which can be used with most advantage. WINDMILLS. Illustration: (‡ WINDMILLS.) These picturesque objects are buildings containing machinery, to be driven by the wind, for grinding corn, sawing wood, and any other purpose that may be required. They consist of a basement, generally of stone or brick, and a superstructure surmounted by a sort of dome capable of being turned round. From this dome projects the shaft of a wheel, and on this is fastened four fans or sails made of long bars of wood crossed by shorter ones; these being covered with canvass, form a surface to catch the wind. These sails are placed obliquely to the front of the cross, so that when the wind blows upon them right in front, they are at an angle with it, they are therefore turned round; for the wind which pushes them from the front, as they are oblique, tends also to push them on one side; when once in motion, being heavy, they form a sort of fly-wheel to the machinery. The dome has several small wheels attached to its lower border, to act as friction rollers and cause it to be easily turned round (which is often required), that the sails may be made to face the wind in whatever direction it may blow; this is sometimes done by ropes attached to the dome, but is more frequently effected by means of a small set of sails, shown in the cut, which are placed at right angles to the large set, so that when the wind acts on the large sails the small ones are not affected; but should the wind shift, these small ones begin to move, and they are connected with a toothed wheel acting upon a band which surrounds the dome; this is therefore caused to turn round whenever the small sails are turned, and as the dome turns, it brings with it the large sails until they are in the right position. These sails are generally fixed not quite upright, but inclined with their fronts looking a little upwards, which is found to be the best position to catch the wind. SYPHONS.