The Complete Works in Philosophy, Politics and Morals of the late Dr. Benjamin Franklin, Vol. 2 [of 3]

By Benjamin Franklin

Page 122

move out of its way all the air its whole
dimension meets with between the pricked lines CG and DG. Thus both
the fluids give resistance to the motion, each in proportion to the
quantity of matter contained in the dimension to be removed. And
though the air is vastly lighter than the water, and therefore more
easily removed, yet the dimension being much greater its effect is
very considerable.

It is true that in the case stated, the resistance given by the air
between those lines to the motion of the sail is not apparent to the
eye, because the greater force of the wind, which strikes it in the
direction EEE, overpowers its effect, and keeps the sail full in the
curve a, a, a, a, a. But suppose the wind to cease, and the vessel in
a calm to be impelled with the same swiftness by oars, the sail would
then appear filled in the contrary curve b, b, b, b, b, when prudent
men would immediately perceive, that the air resisted its motion,
and would order it to be taken in.

Is there any possible means of diminishing this resistance, while
the same quantity of sail is exposed to the action of the wind, and
therefore the same force obtained from it? I think there is, and
that it may be done by dividing the sail into a number of parts, and
placing those parts in a line one behind the other; thus instead of
one sail extending from C to D, figure 2, if four sails containing
together the same quantity of canvas, were placed as in figure 3,
each having one quarter of the dimensions of the great sail, and
exposing a quarter of its surface to the wind, would give a quarter
of the force; so that the whole force obtained from the wind would be
the same, while the resistance from the air would be nearly reduced
to the space between the pricked lines _a b_ and _c d_, before the
foremost sail.

It may perhaps be doubted whether the resistance from the air would
be so diminished; since possibly each of the following small sails
having also air before it, which must be removed, the resistance on
the whole would be the same.

This is then a matter to be determined by experiment. I will mention
one that I many years since made with success for another purpose;
and I will propose another small one easily made. If that too
succeeds, I should think it worth while to make a larger, though at
some expense, on a river

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