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.

This is a simple current wheel with pot buckets, rigid or swinging, arranged on the rim of the wheel, to carry up and discharge the water nearly at the top of the wheel, and through the long ages that it has been in use for irrigation, village water supply, and even for private establishments, has assumed a variety of forms in detail of construction ranging from the bamboo wheels of the Chinese to the light iron wheels of modern construction.

We illustrate the most simple of these forms in Figs. 26 and 27, in which the first is a series of boxes or chambers in the rim of the wheel with side openings in the forward part of the box as the wheel revolves, and a lip extending from the inner edge of the opening to direct the outflow into the trough.

[Illustration:  Fig. 26.]

Another form, Fig. 27, is arranged with swing buckets or pots, pivoted just above their centers, and with the catch trough so fixed as to tip the buckets at the highest point, thus giving this wheel the greatest possible advantage as to height of discharge for a given diameter.

[Illustration:  Fig. 27.]

The power value of these wheels for raising water is a matter of computation as nearly reliable as for other devices for the same purpose, when the velocity of the current is known at the point of contact with the blades.