feet of air weigh one pound. In round numbers 30,000 cubic feet of air
weigh one ton; this is a useful figure to remember, and it is easily
carried in the mind. A hall 61 feet long, 30 feet wide, and 17 feet high
will contain one ton of air.

[Illustration: FIG. 1]

The work to be done by a fan consists in putting a weight--that of the
air--in motion. The resistances incurred are due to the inertia of the
air and various frictional influences; the nature and amount of these
last vary with the construction of the fan. As the air enters at the
center of the fan and escapes at the circumference, it will be seen that
its motion is changed while in the fan through a right angle. It may
also be taken for granted that within certain limits the air has no
motion in a radial direction when it first comes in contact with a fan
blade. It is well understood that, unless power is to be wasted, motion
should be gradually imparted to any body to be moved. Consequently, the
shape of the blades ought to be such as will impart motion at first
slowly and afterward in a rapidly increasing ratio to the air. It is
also clear that the change of motion should be effected as gradually as
possible. Fig. 1 shows how a fan should not be constructed; Fig. 2 will
serve to give an idea of how it should be made.

[Illustration: FIG. 2]

In Fig. 1 it will be seen that the air, as indicated by the bent arrows,
is violently deflected on entering the fan. In Fig. 2 it will be seen
that it follows gentle curves, and so is put gradually in motion. The
curved form of the blades shown in Fig. 2 does not appear to add much to
the efficiency of a fan; but it adds something and keeps down noise. The