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.
Pages:
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74