With higher velocities of stream and wheel the efficiency will be from 2 to 3 per cent. less, although the horse power will increase nearly with the increase in velocity of the current. For details of application of various forms of current wheels for power purposes see illustrated description Yagn's and Roman's floating motors in SCIENTIFIC AMERICAN SUPPLEMENT, No. 463. A very good example of a floating motor of the propeller class is Nossian's fluviatile motor, illustrated and described in SCIENTIFIC AMERICAN SUPPLEMENT, No. 656. [Illustration: Fig. 24.] Fig. 24 represents a very complete floating motor, in which the floats are wedge shaped at the stem, for the purpose of increasing the current between them, the wheel being an ordinary current wheel, as shown in Fig. 23, with a curved shield or gate in front, which can be moved around the periphery of the wheel for the purpose of regulating its speed or stopping its motion by cutting off the stream from the buckets. The float, rising and falling with the stream, is held in position by a braced frame swinging on anchorages within the mill on shore, and parallel with a swiveled shaft. Tide wheels and tidal current wheels have been in use for more than 800 years, and were largely in use in Europe and the United States during the first half of the present century. No less than three were running in the immediate vicinity of New York, in 1840, for milling purposes. Their day seems to be past, except in some special localities. We will also pass them, and illustrate some of the SELF-ACTING WATER-RAISING DEVICES. The tympanum derives its name from its similarity to a drum as made by the Romans, but its origin was Egyptian. It is a current wheel with frame like Fig. 23, to the outside of which a set of chambers or tubes are fixed, radiating spirally, so as to lead the water to the shaft as the wheel revolves, as shown in Fig. 25. It has a lift of a little less than half its diameter, and answers an excellent purpose for the irrigation of rice and cranberry fields, or on streams running through low lands in arid districts. It is still one of the Nile irrigating wheels. [Illustration: Fig. 25] The building of these wheels is within the scope of the carpenter and the tinsmith. A short wooden shaft made square or octagonal, as convenient, with gudgeons in the ends and arms of wood bolted across each of the sides of the shaft, or as shown in the cut, will form a frame work upon which a rim may be fastened, to which the blades and tubular buckets can be attached. The directions in regard to the current wheel, Fig. 23, may be followed as to number and form of blades, which must be made in length and width proportional to the velocity of the stream and the quantity of water to be lifted by each tubular arm. The tubes may be made of galvanized sheet iron and attached to the outside of the wheel, as shown in Fig. 25. THE NORIA OR BUCKET WHEEL.