The ordinary paddle-wheel had the floats fixed upon the radial arms, but it was soon found that an improvement could be made by causing the floats to assume a position vertical, or nearly so, at the moment of contact with the surface of the water, and to retain that position until the float had left the water. To effect this the floats are not bolted to the arms but pivoted, and are retained in the required position by means of levers operated by an eccentric pin. By this means a much greater propulsive force was exerted. The old style of paddle-wheel with fixed floats is now very seldom employed. These wheels are now only to be found in vessels in which the expense of construction has to be cut down to a minimum, or in a certain type of steamer plying in shallow rivers, where the wheel is rather large, and the dip of the float slight; but here again economy of construction may count for more with the proprietor of the boat than the increased speed he could obtain with the more expensive feathering wheels. Many of the modern wheeled vessels have floats of steel, but in the great majority of cases wood is employed, elm being largely used for this purpose. The floats are usually about four times as long as they are broad. Various forms are used, some being left square at the corners, others are rounded, others again have the outer edge elliptical in shape, and the experiment has also been tried with a fair measure of success of inclining the floats to the axis of the wheel, instead of having them parallel to it. The advantages claimed for this last method are that the stream of water formed by the rotatory motion of the paddles is driven slightly away from the sides of the vessel, instead of in a direction parallel with her length. Wheels of this type, however, lose much of their effectiveness when the engines are reversed. Radial wheels are sometimes made with the floats adjusted so that they enter the water almost perpendicularly, but they are much more oblique under this arrangement when leaving the water.