

METEORIC STONES AND SHOOTING STARS. 



been scattered about the system, which, owing to their extreme minuteness, 

 may have been subject to disturbing causes that have occasionally brought them 

 so near the earth, that they have been drawn by its attraction within the limits 

 of the atmosphere, and have ultimately, by the resistance of that fluid, fallen 

 apon the earth. We shall call this the planetary hypothesis. 



Fourthly. It has been suggested by LAPLACE, that meteoric stones may be 

 substances ejected from lunar volcanoes, either now or formerly in active opera- 

 tion. He has proved that no very improbable amount of mechanical force would 

 be sufficient to produce such an effect, since there is no atmosphere around the 

 moon, or, at least, none that could be sufficient to offer a sensible resistance to 

 the motion of a solid body. The force, therefore, that would be required is 

 only that which would be sufficient to overcome the moon's attraction, which is 

 found by calculation to be about four times the force with which a ball is ex- 

 pelled from a cannon with the ordinary charge of gunpowder. A body pro- 

 jected toward the earth, with the velocity of about eight thousand feet per sec- 

 ond from the lunar surface, would rise to such a height that it would arrire at 

 a point between the earth and moon where the attraction of the earth would 

 predominate and prevent its return. It would, consequently, continue to move 

 toward the earth with accelerated speed, and, arriving within the limits of the 

 atmosphere, would necessarily reach the surface. We shall call this the lunar 

 hypothesis. 



Fifthly. It has been supposed that meteoric stones, showers of dust, and 

 other similar meteorological phenomena, proceed from chaotic matter which 

 prevails in the spaces within which the planets move, and which is generally 

 but irregularly diffused throughout the universe, producing in the heavens the 

 appearances called nebulae. This matter is supposed to lie irregularly in the 

 space through which the earth annually passes and its neighborhood ; that it 

 is occasionally brought by the attraction of the earth within the limits of the 

 atmosphere, and thus descends to the surface. This we shall call the nebular 

 hypothesis. 



Such are the various theories which have been offered to explain the phe- 

 nomena attending meteoric stones. The evolution of light which attends their 

 rapid progress through space has been accounted for in all of them in the same 

 manner. It is supposed that, in the rapid motion with which the body pro- 

 ceeds, the air which lies in its path is so extremely condensed, as either to be- 

 come itself luminous, or to acquire so intense a heat as to render the stone in- 

 candescent, or, perhaps, to produce upon it even- a superficial combustion, the 

 signs of which are exhibited in the blackness which marks the surface of these 

 bodies. This reasoning is attempted to be supported by the well-known ex- 

 periment of the fire-syringe. In that instrument a solid piston is fitted in a 

 cylinder, so as to be air-tight, carrying a piece of amadou or other easily com- 

 bustible matter, at its end. When the piston is suddenly forced down, so as to 

 produce an instantaneous and severe compression of the air under it, tru" 1 ama- 

 dou takes fire, and, if the cylinder be glass, a flash of light is visible through 

 it. It has therefore been contended, that in this experiment the air under 

 the piston has acquired, by compression, such a temperature as renders it lu- 

 minous. 



More recent experiments, however, made in France (an account of which 

 has fallen in my way), throw doubt upon the validity of this inference. It 

 is said that the unctuous matter commonly used to lubricate the piston in the 

 fire-syringe is, in fact, the source of the ignition ; for that, when experiments 

 were made with pistons not so lubricated, the flash of light was not produced. 

 It is, therefore, considered not to be satisfactorily proved, that air by mere me- 

 chanical compression can ever become luminous. Still, however, it might be con- 



