1821.] Ph(cnomena of Heat y Gases, Gramtatiorij S^c. 293 



momentmn of the moving body before the contact ; so that the 

 effect of a very large body moving with a less velocity, may be 

 equal to the effect of a very small body moving with a greater 

 velocity. Hence the whole difficulty of this case turns upon the 

 abstraction of the ideas of magnitude and momentum ; and, 

 therefore, if we admit a reciprocal change of state when the 

 balls are about equal, we cannot refuse it in any other case. 



Again, it has generally been admitted that the relative motions 

 of bodies, included in a given space, and their mutual actions on 

 one another, are the same, whether that space be at rest, or move 

 uniformly forward in a straight line. This is true with elastic 

 and soft bodies, and also with hard ones when they are equal ; 

 because then their relative velocity is the same before and after 

 the stroke ; but when they are unequal, it is very different. Ill 

 cases where the masses are very unequal, the difference in the 

 two results will, in general, be very great. Let a body whose 

 mass is 8 ^nd velocity 6 strike another at rest whose mass is 2 ; 

 then the velocity with which these bodies separate after the 

 stroke is 24. But if we estimate this relatively to a space mov- 

 ing wdth a velocity of 4 in the same direction, it becomes 9, 

 which is but httle more than a third of the velocity, with which 

 the two bodies do really separate. And the same anomalies 

 might be shown to exist in other cases of tliis theory with respect 

 to the collisions of unequal bodies under similar circumstances. 

 It is, therefore, by no means immaterial, as it has generally been 

 imagined (Newton, cor. 5, of the third Law of Motion), whether 

 we calculate the effects of collision according to absolute or 

 relative rest ; the substitution of the one for the other might 

 produce very erroneous results. These considerations, however^ 

 will not at all affect the vaUdity of our deductions in the laws of 

 gaseous bodies. For the particles moving and striking in aU 

 directions, whatever force is gained by relative motion in the 

 one is lost in the opposite direction; so that the mean force, 

 which is all that we consider, will be the same in both cases 



{To be continued.) 



Article IX. 



On the Comparative Temperature of Penzance, By Dr. Forbes. 



(To the Editor of the Annals of Philosophy/.) 



SIR, Penzance, Feb. 3, 1821, 



In the small tract lately published by me on the Climate of 

 Penzance, I have pointed out the relative temperature of a 



