the earth, is it not by the vibrations given to it by the sun that light appears to us; and may it not be, that every one of the infinitely small vibrations, striking common matter with a certain force, enters its substance, is held there by attraction, and augmented by succeeding vibrations, till the matter has received as much as their force can drive into it? Is it not thus, that the surface of this globe is continually heated by such repeated vibrations in the day, and cooled by the escape of the heat, when those vibrations are discontinued in the night, or intercepted and reflected by clouds? Is it not thus that fire is amassed, and makes the greatest part of the substance of combustible bodies? Perhaps, when this globe was first formed, and its original particles took their place at certain distances from the centre, in proportion to their greater or less gravity, the fluid fire, attracted towards that centre, might in great part be obliged, as lightest, to take place above the rest, and thus form the sphere of fire above supposed, which would afterwards be continually diminishing by the substance it afforded to organized bodies, and the quantity restored to it again by the burning or other separating of the parts of those bodies. Is not the natural heat of animals thus produced, by separating in digestion the parts of food, and setting their fire at liberty? Is it not this sphere of fire, which kindles the wandering globes that sometimes pass through it in our course round the sun, have their surface kindled by it, and burst when their included air is greatly rarefied by the heat on their burning surfaces? * *This paper was read before the American Philosophical Society, June 20th, 1788, as a letter to David Rittenhouse, with the addition only of the following sentence, viz. "May it not have been from such considerations, that the ancient philosophers supposed a sphere of fire to exist above the air of our atmosphere?"- Editor. Clock with three Wheels. Gravitation of Bodies. Passy, 29 April, 1785. I do not know that my contrivance of a clock with three wheels only, which showed hours, minutes, and seconds, has ever been published. I have seen several of them here at Paris, that were made by Mr. Whitehurst, and sent over, I believe, by Mr. Magellan. You are welcome to do what you please with it. Mr. Whitehurst's invention is very simple, and should be very effectual, provided the foot of the rod and the situation of the clock are invariably fixed, so as never to be at a greater or less distance from one another, which may be by fixing both in a straight-grained piece of wood of about four feet long; wood not changing its dimensions. the lengthwise of the grain, by any common degree of heat or cold. But this cannot be trusted to in the wood of a clock-case, because in sawing boards the grain is frequently crossed, and moisture and dryness will change their dimensions. You are at liberty also to publish, if you think fit, the experiment of the globe floating between two liquors. I suppose you remember to have seen it on my chimneypiece. Though it is a matter of no utility. Something of the same nature has been done more than a hundred years since by another person, I forget who. What I formerly mentioned to you of hanging a weight on a spiral spring, to discover if bodies gravitated differently to the earth during the conjunctions of the sun and moon, compared with other times, was this. We suppose, that, by the force of gravity in those luminaries, the water of the ocean, an immense weight, is * For a part of this letter, on Electricity, see Vol. V. p. 480.- EDITOR. 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. B. FRANKLIN. 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. SIR, At sea, on board the London Packet, 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,) AB represents the body of the vessel, CD 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 very considerable. is It is true, that, in the case stated, the resistance given |