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.