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and cosines, and not in the least depending on real physics. 

 Admitting in the abstract that such a space as a perfect void 

 existed (which is not the case, according to astronomers them- 

 selves), a body placed in it would remain for ever at rest; let 

 us then suppose the body to be put in motion by an external 

 force ; not a vital principle, nor means of generating a force, but 

 one impulsive action : what would be the consequence according 

 to the established laws of nature founded on experience ? Ac- 

 cording to terrestrial physics, experience and analogy, all forces 

 depend more or less on the expansion of a power previously con- 

 centrated : for instance, a boiler full of steam (or any other gas) 

 is a power, but cannot produce any action without expansion ; 

 but immediately on allowing it to expand it will produce effects 

 proportionable to its density, the force of which, if not supplied 

 by some generating power, will diminish inversely as the cube of 

 the volume increases. The ultimate extent of the expanding 

 power need not be considered, but its variable force and velocity; 

 these are the questions at issue. 



We know, and it has been proved experimentally, that an 

 impulsive force, limited in its amount, has an evanescent quality, 

 and consequently any body or point carried by it commences 

 with a maximum force and velocity ; it cannot describe equal 

 spaces in equal times ; " like causes produce like effects, or, in 

 similar circumstances, similar consequences ensue ;" the force 

 must therefore diminish, and consequently the velocity, inversely 

 as the volume increases, until it finally arrives at a state of rest, 

 i.e. a state of equilibrium between the limited impulsive power 

 imparted and the actual effect produced. Indeed engineers are 

 so much accustomed to the above law of forces that it requires 

 but little comment. 



When a ball is discharged, it is not the mere resistance of the 

 air and the earth's attraction that destroy its rectilinear velocity 

 and brings it into a state of rest, but it is the evanescent qua- 

 lity of that power which had put it in motion : the resisting 

 medium and the attraction of the earth only diminish the extent 

 of its path. According to the doctrine of gravitation, neither 

 the radial attraction of the planets, nor a resisting medium, can 

 destroy the impulsive effect : these obstructions are considered to 

 exist in the planetary system, and indeed are applied in calcula- 



