1822.] on Mr, Herapath's Theory, 359 



the other is equal to the momentum of the moving body. In 

 proof of this, Mr. H. argues that " the fixture being at rest, the 

 part of the intermediate body which is against it cannot be 

 urged any way by the fixture ; and, therefore, the force with 

 which the moving body comes in contact with the other side ; 

 that is, the momentum of the body, is the force of constipation." 

 But C. says, " the two surfaces of the intermediate body will be 

 urged towards its centre," in consequence of the reaction of the 

 fixture, *' with a force exactly as great as if each side had been 

 struck with a momentum equal to that of the moving body." 

 Thus instead of the centre being urged towards the fixed plane, 

 which merely opposes a passive resistance, this quiescent plane, 

 according to C. drives the side it is in contact with towards the 

 centre, will C. have the goodness to tell us how this takes 

 place ? Will he be kind enough to explain to us how and in what 

 direction a quiescent and a fixed body can actively urge another 

 without elasticity ? But he informs us the thing can be proved 

 by experiment. No doubt C. has made this experiment, and 

 will immediately favour the world with it. A great treat I am 

 persuaded it will be to our men of science. As an humble 

 admirer of scientific truth, I shall myself feel highly gratified and 

 obliged. In the interim, however, I cannot help saying, that 

 had I seen an experiment producing such a result, I should have 

 much questioned the fidelity of my senses. 



'' If," says Mr. Herapath, " two perfectly hard bodies, mov- 

 ing in the same right line, but towards opposite parts, come in 

 contact, the sum of their momenta being the motion with which 

 the two bodies approach, is, therefore, the motion or force with 

 which their surfaces come in contact." This, C. has *Hhe dig- 

 nified condescension" to admit. "But" continues Mr. H. 

 '' the force with which the surfaces come in contact is the force 

 with which each surface, or body, is acted on at the time of the 

 contact in a direction opposite to that in which the body was 

 moving." Nothing surely can be more evident than this ; and, 

 therefore, to have attempted to explain or illustrate so obvious a 

 thing would have been to offer an insult to the understanding of 

 his readers ; particularly when we consider that these readers 

 were to be the members of the Royal Society, who are reputed 

 to be men of talent and ability. Nevertheless, C. says " he is 

 at a loss to discover how it can be proved," notwithstanding he 

 allows that the intensity of the stroke is equal to the sum of the 

 momenta. What, I would ask C. is meant by the intensity of 

 the stroke but the violence of the contact ? And what is this 

 violence of contact but the force with which each surface is 

 struck? For example, if I strike a nail with a hammer, the 

 momentum with which the hammer comes in contact with the 

 nail measures, and is just equivalent to the violence or intensity 

 of the blow on the nail; supposing both bodies absolutely hard. 



