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elevated so as to form the tides; if that be so, might we not expect that an iron ball of a pound weight, suspended by a fine spiral spring, should, when the sun and moon are together both above it, be a little attracted upwards or rendered lighter, so as to be drawn up a little by the spring on which it depends, and the contrary when they are both below it. [See Plate XI. Fig. 2.] The quantity, though very small, might perhaps be rendered visible by a contrivance like the above. It is not difficult to make this experiment, but I have never made it. With regard to the tides, I doubt the opinion of there being but two high waters and two low waters existing at the same time on the globe. I rather think there are many, and those at the distance of about one hundred leagues from each other. The tides found in the River Amazons seem to favor this opinion. Observations hereafter in the isles of the Pacific Ocean may confirm or refute it.
If I were in a situation where I could be a little more master of my time, I would, as you desire, write my ideas on the subject of chimneys. They might, I think, be useful; for by what I see everywhere the subject seems too little understood, which occasions much inconvenience and fruitless expense. But, besides being harassed by too much business, I am exposed to numberless visits, some of kindness and civility, many of mere idle curiosity, from strangers of America and of different parts of Europe, as well as the inhabitants of the provinces who come to Paris. These devour my hours, and break my attention, and at night I often find myself fatigued without having done any thing. Celebrity may for a while flatters one's vanity, but its effects are troublesome. I have begun to write two or three things, which I wish to finish before I die; but I sometimes doubt the possibility.
TO DAVID LE ROY, AT PARIS.*
Improvements in Navigation. Observations on the Sails and Cables of Vessels; the Models for their Construction; Means of preserving them from Accidents at Sea; Methods of giving Motion to Boats. Gulf Stream.Precautions to those who are about to take a Voyage at Sea. - General Reflections on Navigation.
READ AT A MEETING OF THE AMERICAN PHILOSOPHICAL SOCIETY, DECEMBER 2D, 1785.
At sea, on board the London Packet,
SIR, Your learned writings on the navigation of the ancients, 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
• This letter was translated into French, and published at Paris in the year 1787, entitled, "Lettre de Monsieur Benjamin Franklin à Monsieur David Le Roy, Membre de Plusieurs Académies, &c." The following note is prefixed by the French editor. "Cette lettre a été lue à la Société Philosophique Américaine de Philadelphia, le 2 Décembre, 1785. Elle est imprimée dans les Mémoires de cette Société. On lit dans le titre, à M. Alphonse Le Roy. Comme cet Académicien ne se nomme pas Alphonse, nous y avons substitué l'un de ses noms de baptême. Il est de l'Académie des Belles-Lettres, de celle de Marine, de la Société des Antiquaires de Londres, de la Société Philosophique Américaine, &c." Hitherto the letter has been printed as addressed to Alphonsus Le Roy. The address is now changed to David Le Roy, in conformity with the French edition. - EDITOR.
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 forty-five degrees with the keel, as in the following figure; wherein (Plate XII. Fig. 1,) 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 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. α, 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 canvass, 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 boat; and perhaps
time, and the improvements experience will afford, may make it applicable with advantage to larger vessels.
Having near my kitchen chimney a round hole of eight inches diameter, through which was a constant steady current of air, increasing or diminishing only as the fire increased or diminished, I contrived to place my jack so as to receive that current; and, taking off the fliers, I fixed in their stead on the same pivot a round tin plate of nearly the same diameter with the hole; and having cut it in radial lines almost to the centre, so as to have six equal vanes, I gave to each of them the obliquity of forty-five degrees. They moved round, without the weight, by the impression only of the current of air, but too slowly for the purpose of roasting. I suspected that the air struck by the back of each vane might possibly by its resistance retard the motion; and to try this, I cut each of them into two, and I placed the twelve, each having the same obliquity, in a line behind each other, when I perceived a great augmentation in its velocity, which encouraged me to divide them once more, and, continuing the same obliquity, I placed the twenty-four behind each other in a line, when the force of the wind being the same, and the surface of vane the same, they moved round with much greater rapidity, and perfectly answered my purpose.
The second experiment that I propose, is, to take two playing cards, of the same dimensions, and cut one of them transversely into eight equal pieces; then with a needle string them upon two threads, one near each end, and place them so upon the threads that, when hung up, they may be one exactly over the other, at a distance equal to their breadth, each in a horizonta position; and let a small weight, such as a bird-shot, be hung under them, to make them fall in a straight line when let loose. Suspend also the whole card by threads