32 ON PERCUSSION. 



have already been appealed to by either party have re- 

 ceived diflferent interpretations from their opponents, al- 

 though the facts were admitted. 

 The Newtoni- My object in the present lecture is to consider which of 

 ^ *h h"^a*^° these opinions respecting the force exejted by moving bo- 

 law gf motion ^^^^ ^^ most conformable to tlie usual meaning of that 

 is not contrary word, and to shew that the cxi)lanation given by New- 

 of Lcibnii^'^* ^^'^^^^ the third law of motion is in no respect favour- 

 able to (hose who in their view of this question have been 

 called Newtonians. 



If bodies were made to act upon each other under the 



circumstances which I am about to describe, the leading 



pkaenomena would occur, which, afford the grounds of 



reasoning on either side. 



Statenrient of Let a ball of clay or of any other soft and wholly in- 



the leading elastic Substance be suspended at rest, but free to move in 



phenomenon. ,. ,. .it,., • / i i .i i 



any direction with the slightest impulse ; and let there be 



two pegs similar and equal in every respect inserted 

 slightly into its opposite sides. Let there be also two 

 other bodies, A and B, of any magnitude, which are to 

 each other in the proportion of 2 to 1 ; susjitndcd in such 

 a position, that when perfectly at rest they 'shall be in 

 contact with the jextremities of the opposite pegs without 

 pressing against them. Now if these bodies were made to 

 ^wing with motions so adapted that in falling from heights 

 in tlie proportion of 1 to 4 they might strike at the same 

 instant against the pegs opposite to them, the ball of clay 

 would not be moved from its place to either side ; never- 

 theless the peg impelled by the smaller body B, which has 

 the double velocity, would be found to have penetrated 

 twice as far as the peg impelled by A. 



It is unnecessary to make the experiment precisely as 



here stated, since the results are admitted as facts by both 



parties ; but upon these facts they reason differently. 



Inference* con- ^^^ side observing that the ball of clay remains un- 



cerning the moved, considers the proof indisputable that the action 



dklc'e" h^'JX ^^ *^^ ^^^y ^ ^^ ®^^^^ *^ that^of B, and tliat their forces 



party. are properly measured by their momenta, which are equal, 



because their velocities are in the simple inverse ratio of 



the bodies. Their opponents think it equally proved by 



the unequal depths to which the pegs have penetrated, that 



the 



