[Footnote: Extract from a lecture recently delivered at Bombay.]
By J. WALLACE, C.E.
The function of a punka is to cause a current of air to pass the human
body so that the animal heat may escape more rapidly. This has nothing to
do with ventilation; for if the punka were used in a closed room, it
would still produce a cooling effect on the skin.
Let us for a moment examine into what takes place in this operation, for
a clear idea of the cause of our sensations of heat is absolutely
necessary to enable us to go directly to the simplest and best form of
remedy. The heat we feel, and which sometimes renders us uncomfortable,
is produced _within us_ by the slow combustion of the food we eat.
This heat continues to escape from the whole surface of the body during
the whole lifetime, and if anything occurs to arrest it to any great
extent, the result is fatal.
In cold weather, and especially when there is much wind, the animal heat
escapes very rapidly from the body, and extra clothing is used, not for
any heat it imparts, but simply because it interrupts the escape of the
heat, and thus maintains the temperature of the skin--that part of us
which is most sensible of change of temperature. It is a wonderful fact
that the heat of the interior of the body varies very little in a healthy
man between India and Greenland.
The skin may bear a good many degrees of change of temperature with
impunity, but the blood will only suffer a very small variation from the
normal temperature of 98-4/10° Fahrenheit without serious consequences.
Well, to keep the skin at an agreeable temperature in India we generally
wear a minimum of clothing, and when there is no breeze, we try to
produce one with the punka.
The escape of animal heat from the body forms a subject which is much
more complicated, and much more important, than the one we have met to
consider, but it is impossible within the limits of our time to refer to
it, except in the measure that is strictly necessary to elucidate the
principles that should control the construction of the punka.
It has often been said that every engineer on his arrival in India sets
about improving this useful apparatus; but if we may judge from the
endless variety of forms which may be seen in shops and offices, in
public and in private buildings, no general principle of construction has
been recognized, and the punka, as we see it, seems to depend, for its
form, more upon the taste of the workman who makes it than on anything
else.
We shall begin by directing our attention to the suspended punka, which
is usually hung from the ceiling, and put in movement by a cord. The
object of this class of punka is to produce a downward current of air by
swinging to and fro, and the best punka is the one which throws downward
the greatest quantity of air with the smallest applied force.
The swinging punka is one of the simplest forms of mechanism; it can be
fitted up with the most primitive materials, and however badly made, it
will always have _some_ effect. This fact has its good and its bad
aspects; it brings a certain comfort within the reach of all, but it
removes a great part of that _necessity_ which, as we all know, is the
mother of invention.
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 62½ times per
minute, and a pendulum six feet long will swing 44¼ 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° 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 cardboard disk attached. One of the
disks shall present its edge to the line of movement, and the other its
face.
_Exp. 1._--They are now swinging, and being both of the same gravity
length, they should swing together and for an equal length of time. This
they would do in a vacuum, but you have already observed that one of them
is lagging, and will evidently soon come to a standstill. It is the one
_facing_ the air.
If punkas were pulled from both sides, they might be made very much
lighter than they are at present, but for the sake of simplicity a single
pull is preferred. They must, therefore, be made of such a weight that
they will swing nearly as far on the opposite side as they are pulled on
the near side; any greater weight is useless and only serves to wear out
the suspending cords, which, by the way, are nearly always too numerous
and too thick for their purpose.
_Exp. 2._--Here is a panel punka which we shall try to use without the
customary swing bar. It is of calico stretched on a light wooden frame,
and you will be able to judge if it swings equally on each side of the
post which supports it. The irregularity of its movement shows that it is
too light, so we shall add, by way of swing bar, a bar of round iron one
and a quarter inch thick.
_Exp. 3._--It is now swinging regularly, and experiments have already
proved that the swing bar should not be lighter than this one, which
weighs four and a sixth lb. per foot of length. Iron is the best material
for this purpose, as it offers the smallest surface to the resistance of
the air. The length of the suspending cords is usually a matter of
accident in the construction of a punka, but a little attention to the
subject will soon convince us that it is one of the most important
considerations.
The limit of movement of a punka is to be found in the man who pulls it.
Twenty-four pulls a minute of a length of 36 inches give in practice a
speed of 168 linear feet to the punka curtain. This speed is found to
produce a current sufficiently rapid for practical purposes, and
twenty-four pulls or beats per minute correspond to a length of
suspending cord of fifty inches.
* * * * *