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

By Benjamin Franklin

Page 121




_Containing sundry Maritime Observations._

At Sea, on board the London Packet, Capt. Truxton.

_Aug. 1785._


Your learned writings on the navigation of the antients, which
contain a great deal of curious information, and your very ingenious
contrivances for improving the modern sails (_voilure_) of which
I saw with great pleasure a successful trial on the river Seine,
have induced me to submit to your consideration and judgment, some
thoughts I have had on the latter subject.

Those mathematicians, who have endeavoured to improve the swiftness
of vessels, by calculating to find the form of least resistance, seem
to have considered a ship as a body moving through one fluid only,
the water; and to have given little attention to the circumstance
of her moving through another fluid, the air. It is true that when
a vessel sails right before the wind, this circumstance is of no
importance, because the wind goes with her; but in every deviation
from that course, the resistance of the air is something, and becomes
greater in proportion as that deviation increases. I wave at present
the consideration of those different degrees of resistance given by
the air to that part of the hull which is above water, and confine
myself to that given to the sails; for their motion through the air
is resisted by the air, as the motion of the hull through the
water is resisted by the water, though with less force as the air is
a lighter fluid. And to simplify the discussion as much as possible,
I would state one situation only, to wit, that of the wind upon the
beam, the ship's course being directly across the wind; and I would
suppose the sail set in an angle of 45 degrees with the keel, as in
the following figure; wherein (Plate VI, Fig. 1.)


_Plate VI._ MARITIME OBSERVATIONS. _Vol. II. page 163._

_Published as the Act directs, April 1, 1806, by Longman, Hurst, Rees
& Orme, Paternoster Row._]

A B represents the body of the vessel, C D the position of the sail,
EEE the direction of the wind, MM the line of motion. In observing
this figure it will appear, that so much of the body of the vessel
as is immersed in the water must, to go forward, remove out of its
way what water it meets with between the pricked lines FF. And the
sail, to go forward, must

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