Mr. R. Moon on the Law of Gaseous Pressure. 103 



supposed to exist at the time / are precise and definite. That 

 any thing indefinite should arise out of that state of circumstances 

 is simply impossible. In the case supposed, acceleration may, 

 and indeed will occur at the time / ; but the breadth, great or 

 small, over which it prevails must be a definite breadth ; and 

 whether great or small, if it have magnitude, if it have existence, 

 throughout that breadth Boyle's law does not hold. Mr. Strata's 

 apology for the law, therefore, in this case of its manifest failure, 

 is, in effect, simply an admission of its failure, coupled with an 

 wholly unfounded assertion that the failure will be confined 

 within extremely narrow limits. 



The next case treated of by Mr. Strutt is where a vertical cy- 

 linder closed at its lower end has an air-tight piston, capable of 

 working freely in the upper part of it, which is exactly supported 

 by the air beneath. I contend that if the received law of pres- 

 sure were true, the placing of an additional weight at the time t 

 upon the piston under the above circumstances would not de- 

 stroy the equilibrium ; for at the time t, when the weight is 

 upon the piston, the density is unaltered ; therefore, according 

 to the received law the pressure of the air upon the piston will 

 be unaltered, and the pressure of the piston upon the air (which 

 must be exactly equal to the latter) will also be unaltered ; i. e. 

 the introduction of the additional weight leaves the actions be- 

 tween the different parts of the system precisely what they were 

 during equilibrium*. 



Mr. Strutt, on the other hand, maintains that "precisely the 

 same argument may be used to prove that a body cannot begin 

 to fall under the influence of gravity ; for a body cannot leave 

 its initial position without acquiring velocity, and (by the law 

 of energy) cannot possess a velocity without having already fallen. 



Now the fallacy of this argument, to prove that a body cannot 

 fall from rest under the influence of gravity, may be exposed in a 

 moment. If by the allegation that "a body cannot leave its 

 initial position without acquiring velocity" is meant that acqui- 

 sition of velocity is the necessary result of its leaving the initial 

 position, the argument involves a non sequitur. For, granting 

 what is here affirmed, and granting, with or without the aid of 

 the law of energy, that the body " cannot possess a velocity with- 



* Another way of expressing the argument is as follows : — Change of 

 density can only occur through motion of the particles ; but the particles 

 being originally at rest and the system in equilibrium, motion of the par- 

 ticles can only occur from change of pressure. In other words, change of 

 pressure (the cause) must precede the motion which it effects, and must 

 precede therefore the change of density which results from the latter. But 

 the received law of pressure asserts the coatrary, viz. that change of den- 

 sity must precede change of pressure, which is absurd. 



