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.
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