40 



ON THE PROPERTIES OP MATTER, 



well-known theorem in projectiles, that the action of the powder on a 

 bullet ceases as soon as the bullet is out of the piece. In like manner a 

 billiard-ball at rest will continue so till put into motion by a bilhard-ball in 

 motion, for it can never commence motion of its own accord. While a 

 billiard-ball in motion would persevere in motion, and in the same velocity 

 of motion, for ever, if it met with no resistance. But it does meet with 

 resistance from a variety of causes, as the friction of the atmosphere, the 

 friction of the green cloth, and at last a contact with one of the sides of 

 the table, or with the ball against which it is directed. 



In this last case either ball will receive conversely the same precise pro- 

 portion of rest or motion which it communicates. Thus, if the ball in 

 motion strike the ball at rest obliquely, the latter will be put into a certain 

 degree of activity, and the former will, in the very same degree, be impeded 

 in its progress, and receive an equal tendency to a state of rest. If the 

 latter, on the contrary, by what is significantly called a dead stroke, receive 

 the whole charge of motion which .belongs to the former, it will give to the 

 former, in like manner, the whole possession of its quiescence, and the state 

 of each will be completely reversed ; the ball hitherto at rest proceeding 

 with all the velocity of that hitherto in motion, and the ball hitherto in mo- 

 tion exhibiting the dead stand of that hitherto at rest. 



So, if it were possible to place an orb quietly in some particular part of 

 space, where it would be equally free from the attractive influence of every 

 one of the celestial systems, it would, from the same tendency to inertitude, 

 remain quiescent, and at rest for ever. While, on the contrary, if a body 

 were to be thrown from any one of the planets by the projectile force of a 

 volcano, or of any other agency, beyond the range of the attractive or 

 centripetal power of such planet, it would continue the same velocity of 

 motion for ever which it possessed at the moment of quitting the extreme 

 limit of the planet's influence ; unless in its progress it should encounter the 

 influence of some other planet ; and in this last case it would be either 

 drawn directly into contact with the planet it thus casually approached, or 

 would have its path inflected into a circle, and revolve around it as a satel- 

 hte, according to its velocity, and the relative direction of its course at the 

 moment the planetary influence began to take effect. Thus a body pro- 

 jected horizontally to the distance of about 4.35 miles from the earth's 

 surface, provided there were no resistance in the atmosphere, would not 

 fall back again, but become a satellite to the earth, and perpetually revolve 

 around it at this distance. The moon is supposed to have no atmosphere, 

 or, at the utmost, one rarer than we can produce with our best air-pumps^ 

 she is also supposed to possess larger and more active volcanos than any^ 

 which are known to exist on the earth. And hence it requires no great 

 stretch of imagination to conceive that bodies may occasionally be thrown 

 from the inoon, by the projectile power of such volcanos, to such a distance 

 as that they should never return to her surface : for if the momentum be 

 only sufficient to cause the mass ejected to proceed at the rate of about 

 8,200 feet in the first second of time,* and in a line passing through the 

 moon and the earth, such effect would necessarily be produced ; since, in 

 this case, the propelled mass would quit the centripetal power of the former, 

 and be drawn into that of the latter, and would either become a satellite 

 to the earth, or be precipit9.ted to its surface, according as the rectilinear 



* Laplace, Exposition du Systeme du Monde. 



