“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 fire-works, 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 mv^2 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.