Note 13 — Chapter III

Three hypotheses may be used to account for this most curious phenomenon.

1st. The body shines by its own light, and then explodes like a sky-rocket, breaking into minute fragments too small to be any longer visible to the naked eye.

2nd. Such a body, having shone by its own light, suddenly ceases to be luminous. “The falling stars and other fiery meteors which are frequently seen at a considerable height in the atmosphere, and which have received different names according to the variety of their figure and size, arise from the fermentation of the effluvia of acid and alkaline bodies which float in the atmosphere. When the more subtile parts of the effluvia are burned away, the viscous and earthy parts become too heavy for the air to support, and by their gravity fall to the earth.”—Keith’s Use of the Globes. According to Sir Humphry Davy, in the Philosophical Transactions for 1847, “the luminous appearances of shooting stars and meteors cannot be owing to any inflammation of elastic fluids, but must depend upon the ignition of solid bodies.”

3. The body shines by the reflected light of the sun, and ceases to be visible by its passing into the earth’s shadow, or, in other words, is eclipsed. Upon the two former suppositions the fact of the star’s disappearance conveys to us no knowledge of its position, or of its distance from the earth; and all that can be said is, that if it be a satellite of the earth, the great rapidity of its motion involves the necessity of its being at no great distance from the earth’s surface—much nearer than the moon; while the resistance it would encounter in traversing the air would be so great that it is probably without the limits of our atmosphere. Sir J. W. Lubbock leans to the third hypothesis.—Sir J. W. Lubbock, On Shooting Stars: Phil. Mag. No. 213, p. 81.

Sir J. Lubbock also published a supplementary paper on the same subject, in No. 214, p. 170.

Mr. J. P. Joule entertains an hypothesis with respect to Shooting Stars similar to that advocated by Chladni to account for meteoric stones, and he reckons the ignition of these miniature planetary bodies by their violent collision with our atmosphere, to be a remarkable illustration of the doctrine of the equivalency of heat to mechanical power, or vis viva.

If we suppose a meteoric stone of the size of a six-inch cube to enter our atmosphere at the rate of eighteen miles per second of time, the atmosphere being 1/100 of its density at the earth’s surface, the resistance offered to the motion of the stone will in this case be at least 51,600 lbs.; and if the stone traverse twenty miles with this amount of resistance, sufficient heat will thereby be developed to give 1° Fahrenheit to 6,967,980 lbs. of water. Of course by far the largest portion of this heat will be given to the displaced air, every particle of which will sustain the shock, whilst only the surface of the stone will be in violent collision with the atmosphere. Hence the stone may be considered as placed in a blast of intensely heated air, the heat being communicated from the surface to the centre by conduction. Only a small portion of the heat evolved will therefore be received by the stone; but if we estimate it at only 1/100 it will still be equal to 1° Fahrenheit per 69,679 lbs. of water, a quantity quite equal to the melting and dissipation of any materials of which it may be composed.—Mr. J. P. Joule, On Shooting Stars: Phil. Mag. No. 216, p. 348.