idea is that the fan blades when of this form push the air radially from
the center to the circumference. The fact is, however, that the air
flies outward under the influence of centrifugal force, and always tends
to move at a tangent to the fan blades, as in Fig. 3, where the circle
is the path of the tips of the fan blades, and the arrow is a tangent to
that path; and to impart this notion a radial blade, as at C, is perhaps
as good as any other, as far as efficiency is concerned. Concerning the
shape to be imparted to the blades, looked at back or front, opinions
widely differ; but it is certain that if a fan is to be silent the
blades must be narrower at the tips than at the center. Various forms
are adopted by different makers, the straight side and the curved sides,
as shown in Fig. 4, being most commonly used. The proportions as regards
length to breadth are also varied continually. In fact, no two makers of
fans use the same shapes.

[Illustration: FIG. 3]

As the work done by a fan consists in imparting motion at a stated
velocity to a given weight of air, it is very easy to calculate the
power which must be expended to do a certain amount of work. The
velocity at which the air leaves the fan cannot be greater than that of
the fan tips. In a good fan it may be about two-thirds of that speed.
The resistance to be overcome will be found by multiplying the area of
the fan blades by the pressure of the air and by the velocity of the
center of effort, which must be determined for every fan according to
the shape of its blades. The velocity imparted to the air by the fan
will be just the same as though the air fell in a mass from a given
height. This height can be found by the formula h = v squared / 64; that is to
say, if the velocity be multiplied by itself and divided by 64 we have
the height. Thus, let the velocity be 88 per second, then 88 x 88 =