“The event, doubtless, will be altogether exceptional, but I do not believe either, that it will be of so tragical a nature as has been depicted, or that it can really bring about blood poison or universal asphyxia. It will rather present the appearance of a brilliant display of celestial fireworks, for the arrival in the atmosphere of these solid and gaseous bodies cannot occur without the conversion into heat of the mechanical motion thus destroyed; a magnificent illumination of the sky will doubtless be the first phenomenon.
“The heat evolved must necessarily be very great. Every shooting star, however small, entering the upper limits of our atmosphere with a cometary velocity, immediately becomes so hot that it takes fire and is consumed. You know, gentlemen, that the Earth’s atmosphere extends far into space about our planet; not without limit, as certain hypotheses declare, since the Earth turns on its axis and moves about the Sun: the mathematical limit is that height at which the centrifugal force engendered by the diurnal rotary motion becomes equal to the weight; this height is 6.64 times the equatorial radius of the Earth, the latter being 6,378,310 meters. The maximum height of the atmosphere, therefore, is 35,973 kilometers.
“I do not here wish to enter into a mathematical discussion. But the audience before me is too well informed not to know the mechanical equivalent of heat. Every body whose motion is arrested produces a quantity of heat expressed in caloric units by mv2 divided by 8338, in which m is the mass of the body in kilograms and v its velocity in meters per second. For example, a body weighing 8338 kilograms, moving with a velocity of one meter per second, would produce, if suddenly stopped, exactly one heat unit; that is to say, the quantity of heat necessary to raise one kilogram of water one degree in temperature.
“If the velocity of the body be 500 meters per second, it would produce 250,000 times as much heat, or enough to raise a quantity of water of equal mass from 0° to 30°.
“If the velocity were 5000 meters per second, the heat developed would be 5,000,000 times as great.
“Now, you know, gentlemen, that the velocity with which a comet may reach the Earth is 72,000 meters per second. At this figure the temperature becomes five milliards of degrees.
“This, indeed, is the maximum and, I should add, a number altogether inconceivable; but, gentlemen, let us take the minimum, if it be your pleasure, and let us admit that the impact is not direct, but more or less oblique, and that the mean velocity is not greater than 30,000 meters per second. Every kilogram of a bolide would develop in this case 107,946 heat units before its velocity would be destroyed by the resistance of the air; in other words, it would generate sufficient heat to raise the temperature of 1079 kilograms of water from 0° to 100°—that is, from the freezing to the boiling point. A uranolite weighing 2000 kilograms would thus, before reaching the Earth, develop enough heat to raise the temperature of a column of air, whose cross-section is thirty square meters and whose height is equal to that of our atmosphere, 3000°, or, to raise from 0° to 30° a column whose cross-section is 3000 square meters.
“These calculations, for the introduction of which I crave your pardon, are necessary to show that the immediate consequence of the collision will be the production of an enormous quantity of heat, and, therefore, a considerable rise in the temperature of the air. This is exactly what takes place on a small scale in the case of a single meteorite, which becomes melted and covered superficially by a thin layer of vitrified matter, resembling varnish. But its fall is so rapid that there is not sufficient time for it to become heated to the center; if broken, its interior is found to be absolutely cold. It is the surrounding air which has been heated.
“One of the most curious results of the analysis which I have just had the honor to lay before you, is that the solid masses which, it is believed, have been seen by the telescope in the nucleus of the comet, will meet with such resistance in traversing our atmosphere that, except in rare instances, they will not reach the Earth entire, but in small fragments. There will be a compression of the air in front of the bolide, a vacuum behind it, a superficial heating and incandescence of the moving body, a roar produced by the air rushing into the vacuum, the roll of thunder, explosions, the fall of the denser metallic portions and the evaporation of the remainder. A bolide of sulphur, of phosphorus, of tin or of zinc, would be consumed and dissipated long before reaching the lower strata of our atmosphere. As for the shooting stars, if, as seems probable, there is a veritable cloud of them, they will only produce the effect of a vast inverted display of fireworks.
“If, therefore, there is any reason for alarm, it is not, in my opinion, because we are to apprehend the penetration of the gaseous mass of carbonic-oxide into our atmosphere, but a rise in temperature, which cannot fail to result from the transformation of mechanical motion into heat. If this be so, safety may be perhaps attained by taking refuge on the side of the globe opposed to that which is to experience the direct shock of the comet, for the air is a very bad conductor of heat.”
The permanent secretary of the academy rose in his turn. A worthy successor to the Fontenelles and Aragos of the past, he was not only a man of profound knowledge, but also an elegant writer and a persuasive orator, rising
