ON PERCUSSION. 35 



could have I?een no occasion to refer to the second law, 

 with a view thence to deduce the equality of momenta 

 produced. 



Some authors however have interpreted the third law A more coin- 

 diftbrently, and accordingly have expressed a difficulty in J]J,^^.^f"thl^^^' 

 comprehending the simple illustration given by Newton, third law. 

 When they say that action is equal to reaction, they mean 

 not .only that the instantaneous intensity of the moving 

 forces, or pressures opposed to each other, are necessa*-. 

 rily equal, but conceive also a species of accumulated 

 force residing in a moving body, which is capable of re- 

 sisting pressure during a time that is proportional to its 

 momentum or quantitas motus. 



If it be of any real utility to give the name of force to 

 this complex idea of vis matrix extended through time, 

 as^ well as that of momentum to its elfects when unre- 

 sisted, it would be requisite to distinguish this force 

 always by some such appellation as w?ow6'«/rt/ force ; for 

 it is to be apprehended that for want of this distinction 

 many writers themselves, and it is certain that many 

 readers of disquisitions on this subject have confounded 

 and compared together vis matrix^ tnomentum^ and vis 

 mechanical quantities, that are all of them totally dissi- 

 milar, and bear no more comparison to each other, than 

 lines to surfaces, or surfaces to solids. 



In practical mechanics, however, it is at least very The momen- 



rarety that the momentum of bodies is in any degree an ^*"^, 5^^ bodies 



. . T . 1 /. 1 . '^ seldom to be 



object of consideration: the strength ol machinery being considered fa 



in every case to be adapted to the quantitas matrix^ and practical me- 

 the extent and value of the effect to be produced depend- 

 ing upon the quantitas mcchanica of the force applied, or 

 in other words to the space through which a given vis 

 motrix is exerted. 



The comparative velocities given by different quantities Smcaton has 

 of mechanic force to bodies of equal or unequal magni- mechanic force. 

 tude, have been so distinctly treated of by Smeatox, in a 

 series of most direct experiments^^, that it would be a 

 needless waste of time to reconsider them in this place. 

 So also, on the contrary, the quantities of extended me- 



* PhU. Trans. Vol. LXVI. 4.50. 



^ ^ chanic 



