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earth's movement. This we know from the following passage, first brought to light by Venturi.1
"Of a weight descending through the air, the entire revolution of the elements of the movement of circumvolution takes place in twentyfour hours. The moving object descending from the highest part of the fiery sphere, will move straight towards the earth, as all the elements are in a continual motion of circumvolution round the centre of the globe. It can be proved. Let b be the falling weight, moving from a in order to descend to m, the centre of the world; I say that
such a weight, even while it makes a descent curved in the fashion of a helical line, never deviates from its rectilinear descent under which (it) advances continually between the place it left and the centre of the world; because it left point a and has descended to b. During
the time it took in descending to b, it has been carried to d, the position of a has been changed to c, and so the moving body finds itself in the right line which extends between c and the centre of the world m. If the moving body descends from d to f, c, principle of the movement, moves in the same time from c to f, [e] and if f descends to h, it turns itself at g, and so in twenty-four hours the moving body descends to the earth (at a point) under that from which it started."
On the margin Leonardo has written: "If the moving body descends from the highest part of the elements to the lowest in twentyfour hours, its movement is composed of straight lines and curves. I say straight, because it never deviates from the very short line which extends between the place from which it started in the centre of the elements, and it will stop at the lowest extremity of such a straight line,
1 £ssai, p. 7—8.
which will always be found, with respect to the zenith, under the point from which the moving body separated itself. And this movement is curved in itself with all parts of the line. Hence it is that a stone cast from a tower does not strike against the side of the tower before it reaches the ground." 1
Elsewhere, Leonardo declares in so many words that the sun does not move: "II sole non si move."2
Mechanics owe a crowd of other discoveries and inventions to Leonardo. He introduced the study of friction, the effects of which he had calculated with the help of a series of experiments; he invented a dynamometer for determining the power of machines and the traction power of animals, by combining their weight with their muscular strength, etc. It is also said that he was the first in modern times to turn his attention to the centres of gravity of solid bodies, though he had but imperfectly solved the problem of the fall of weights.3
We shall look in vain for the name of Leonardo in the works of writers who have related the genesis of the steam-engine. He has been given no place with the forerunners of Watt, Denis Papin, 4 and Solomon de Caus. And yet his right to be there is indisputable. One of his manuscripts in the Library of the Institut de France gives a minute description of a copper machine "which throws iron balls with great fuss and fury. The third part of the instrument consists of a large charcoal fire ; when it is well heated by this, tighten the screw D, which is above a vessel of water a, I), c, and on tightening this upper screw it (the water) will come out below. Having fallen into the heated part of the machine, the water will transform itself into smoke with
1 Ravaisson-Mollien, MS. G, fol. 55.
2 Richter, vol. ii., p. 152.
3 Libri, Histoire des Sciences Mathematigues, vol. iii. p. 41-45.
4 On Papin, see M. Berthelot's study in the Revue des Deux Mondes for October 1,
VOL. II. L
noise and fury. This (machine) drove a ball weighing one talent over a distance of six stadia." l
The museum at Valenciennes possesses a drawing by Leonardo which represents a spit turned by steam, or, to be more exact, by the air rarefied by the heat of the fire.2
As a practical mechanic Leonardo was gifted with extraordinary powers. He played with marvellous facility and adaptability with such matters as beams, supports, cranes, escapements, etc., etc. At a time when the use of iron was still very restricted, he was lavish with his cogwheels, his pulleys, in fact with all those refinements which, in our century, have led to the substitution of machine labour for that of men.
Among instruments invented by his untiring brain we hear of a very ingenious pedometer, of several machines for laminating iron, for making cylinders, files, saws, and screws, for shaving cloth, for winding, for planing, a mechanical press, a gold beater's hammer, a machine for digging ditches, and one for tilling the soil with the help of the wind, more than one sounding apparatus, paddle-wheels for boats, lamps with double currents of air, etc.
Aerial navigation gave him many a sleepless night. He contrived wings, flying cars, winged chairs, etc. One of his papers, running to about twenty pages, is devoted to the flight of birds.3
Gilberto Govi has shown that Leonardo was the first inventor of screw propulsion, from which modern navigation derives such enormous advantages. As early as the end of the fifteenth century Leonardo applied it, of course on a very small scale. One of the Institut manuscripts (B., fol. 83 verso) contains a drawing of a large screw meant to turn round a vertical axis. Not only, says Govi, did Leonardo invent the screw, he even thought of applying it to aerial locomotion. He constructed small paper models, set them in motion by means of thin bands of steel twisted into spirals, and then left them to themselves. He made a study, moreover, of the power which might be derived from the air by beating it with thin boards of a certain size, and he invented the parachute, which he thus describes: "If a man has a pavilion (a tent) of starched linen, of which each face measures twelve cubits square, he may throw himself from any height whatever without fear of danger."1
l Charles Ravaisson-Mollicn, vol. ii., fol. 33. Cf. S&iilles, p. 346-7.
2 Sdailles, p. 347-8.
s Sabachnikoff, Piumati and Ravaisson-Mollien, Codice sul Volo degli Uccdli. Paris, Rouveyre, 1873. Cf. De Geymiiller: Gazette des Beaux Arts, 1894.
If we brought together all the separate fragments Leonardo wrote on these subjects, fragments to be numbered in thousands, we should have a vast treatise on physics; for he touched on gravity, on equilibrium, on compressibility, on elasticity, on the action of heat, on fusion. on dilatation, on the radiation of heat, on optics and acoustics, on magnetism, lavishing on each the most striking definitions and the most luminous generalisations. He anticipated Pascal in noting that any liquid in communicating vessels, however different in form those vessels might be, remained at the same level in each (Codex Atlanticus, fol. 314). This was the principle of the hydraulic press invented in 1653. He forestalled Chevreul, in laying down with perfect clearness the laws governing complementary colours, showing, for instance, that red gained in intensity when placed in juxtaposition with green.
As for heat, how clearly he defines its effect upon liquids when they hold foreign bodies in suspension !" If," he says, "you heat muddy water it promptly becomes quite clear. This is because as the water becomes hotter it increases (in volume), and, in increasing, it becomes rarefied, and when rarefied it can no longer support the heavy materials it contains." Turning to the radiation of heat, he notes that the rays reflected from the surface of a glass ball filled with cold water are warmer than the fire from which they originate, and that it is the same with those reflected from a concave mirror.
His researches into the nature, weight, and movement of water inspired Leonardo with one of his most curious treatises. He studied the laws governing liquids in repose, as well as those which govern gases. We may say that he laid the foundation for those laws of the movements and equilibrium of fluids which were afterwards completed by S. Stevin, Galileo, Torricelli, Pascal, Boyle, Mariotti, Bernoullli, d'Alembert, and others.1
1 ComfU-rendus de f Academic des Sciences, August 29, 1881.
The resistance, the condensation, and the weight of air were studied by Leonardo, who was thereby enabled to explain the suspension of bodies in the atmosphere, and the formation of clouds.2
In the Codex Atlanticus (fol. 245) Leonardo describes an instrument for showing the constitution and density of the air, and for warning us of the approach of rain. A drawing of this contrivance has been preserved. It consists of a graduated dial, in the centre of which is fixed a movable needle with a ball at each extremity, one ball being covered with cotton, the other with wax.3
Leonardo was the first to note that the sun's light is more brilliant on the tops of mountains than at their bases. He attributes the difference to the fact that the layer of air between the summit of a mountain and the sun is not so thick as that between the sun and its base. This observation—with which Deluc has been wrongly credited —has been confirmed by Saussure and Humboldt.4
He had a glimpse of the telescope when he wrote," Fa ochiali da vedere la luna grande." But, as I have said on a previous page, to assert and to prove are different things, and it is a long way from a simple wish to realisation.
Libri credits Leonardo with the discovery of diffraction; but the passage on which he founds his assertion really has to do with the phenomena of the penumbra, and of the confused images to which it gives rise.5
Experiments concerned with the properties of mirrors complete this branch of his researches These experiments were chiefly carried on during his stay at Rome.
Optics excited his interest more than any other branch of physics; he revivified and enriched it in every direction. The study he devoted to the "camera obscura" shows how long it takes poor humanity, I
1 Govi, Saggio, p. 18.
2 Libri, vol. iii., p. 42-43.
3 See Venturi, £ssai, p. 28; Grote, Leonardo da Vinci ah Ingenieur und Philosoph.
4 Uzielli, Leonardo da Vinci e fe Alpi, p. 17.
6 Figuier, Savants du Moyen Age, p. 199-204.