There are some very important natural laws which are illustrated in the
punka. The first is that which governs the movement of the pendulum. The
number of swings it makes per minute depends on the length of the
suspending cords; a pendulum three feet long will swing 621/2 times per
minute, and a pendulum six feet long will swing 441/4 times per minute.
Whether the swings are long ones or short ones, the number per minute is
still the same. You cannot, therefore, alter the natural rate of movement
of a punka unless you pull it at both sides.
The next law is that which determines that the angles of incidence and of
reflection are equal. This in simple language means that it is useless to
expect a good downward current of air from a slow moving and heavy punka,
with long suspending cords which keep it nearly always in a vertical
position to its plane of movement. Striking the air squarely as it does
in its forward and backward movement, it throws almost as much air upward
as downward, and of course all the air that is propelled in any other
than a downward direction represents just so much power wasted.
One more law, and then we may proceed to demonstration.
As the air weighs 0.072 lb. per cubic foot at 82 deg. Fahrenheit, and as a
considerable quantity of air is put in motion, the power required to
drive a punka depends upon the quantity of air it puts in motion in a
given time.
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