7,744, and 7,744 / 64 = 121. A stone or other body falling from a height
of 121 feet would have a velocity of 88 per second at the earth. The
pressure against the fan blades will be equal to that of a column of air
of the height due to the velocity, or, in this case, 121 feet. We
have seen that in round numbers 13 cubic feet of air weigh one pound,
consequently a column of air one square foot in section and 121 feet
high, will weigh as many pounds as 13 will go times into 121. Now, 121
/ 13 = 9.3, and this will be the resistance in pounds per _square foot_
overcome by the fan. Let the aggregate area of all the blades be 2
square feet, and the velocity of the center of effort 90 feet per
second, then the power expended will bve (90 x 60 x 2 x 9.3) / 33,000
= 3.04 horse power. The quantity of air delivered ought to be equal in
volume to that of a column with a sectional area equal that of one fan
blade moving at 88 feet per second, or a mile a minute. The blade having
an area of 1 square foot, the delivery ought to be 5,280 feet per
minute, weighing 5,280 / 13 = 406.1 lb. In practice we need hardly say
that such an efficiency is never attained.

[Illustration: FIG. 4]

The number of recorded experiments with fans is very small, and a great
deal of ignorance exists as to their true efficiency. Mr. Buckle is one
of the very few authorities on the subject. He gives the accompanying
table of proportions as the best for pressures of from 3 to 6 ounces per
square inch:

--------------------------------------------------------------
| Vanes. | Diameter of inlet
Diameter of fans. |------------------------| openings.
| Width. | Length. |