re 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.

The _useful effect_ is a separate matter; it depends on the amount of air
thrown in a downward direction.

To summarize; all punkas of the same size or surface, and going at the
same speed, require the same amount of pulling. The best one is that
which will throw down more air than any other of the same size.

To obtain the greatest result from the power expended in driving it, the
punka should be placed as near as possible to the person to be cooled,
as the loss of effect, due to distance, increases not in direct ratio,
but in proportion to the square of the distance between punka and person.
If at two feet of distance he receives one eighth of the total effect, he
will at four feet of distance obtain only one thirty-second part.

In practice, the punka should just clear his head when standing, and the
weighting of the curtain should be of some yielding material, so as not
to damage any person who might stand in its course.

We shall now proceed to examine several forms of punka, all made to the
same size, and, for purposes of comparison, we shall drive them all at
the same speed. And in order that their effects may be visible to you, I
have prepared an indicator which resembles more than anything else the
keyboard of a piano. It consists of a series of balanced levers with
blades or keys attached, forming a keyboard four feet long. The levers,
each three feet long, are delicately hung on fine brass centers, and each
lever is counterbalanced by a weight hung in a vessel of water, which
acts as a hydraulic brake, and checks any spasmodic movement in the
apparatus.

On the end of each blade is fixed a disk of white Bristol board four
inches in diameter, forming a row which faces the audience.

This apparatus is so sensitive that a slight change in the humidity of
the atmosphere is sufficient to throw it out of balance.

The power required to drive a punka is nearly all due to the resistance
of the air; that part due to the force of gravity, and the friction of
the suspending joints, is scarcely worth counting. We may readily observe
the effect of the resistance of the air by swinging two pendulums of
equal length and having each a large ca