TO MR. ALPHONSUS LE ROY, MEMBER OF SEVERAL ACADEMIES AT PARIS.[30]
[Containing sundry Maritime Observations.]
At Sea, on board the London Packet, Capt. Truxton.
Aug. 1785.
Sir,
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.