254 Literary and Philosophical Society. 



Robins, contended that the force was as the quantity of 

 matter multiplied simply by the velocity. 



To explain the reason of these opposite opinions, Dr. 

 Wollaston, in his lecture on the Force of Percussion (Phil. 

 Trans. 1806) proposes the following experiment: 



' Suppose a ball of clay to be suspended at rest, having 

 two similar and equal pegs slightly inserted into its opposite 

 sides ; and let two other bodies A and B, whose weights are 

 as 2 to i, strike at the same instant against the opposite 

 pegs, with velocities which are in the proportion of I to 2. 

 In this case the ball of clay would not be moved from its 

 place to either side ; nevertheless, the peg impelled by the 

 smaller body B, which has the double velocity, would be 

 found to have penetrated twice as far into the clay as the 

 peg impelled by the larger body A.' 



The results of this experiment were admitted by both 

 parties ; but they reasoned upon them differently. The 

 party termed Newtonians asserted that as the clay is not 

 moved, it is a 'proof that the forces of impact of the two 

 balls were equal ; as they would infer from the momenta 

 being equal. Their opponents, on the other hand, main- 

 tained with equal confidence that the unequal depths to 

 which the pegs had been driven was a proof that the causes 

 of these different effects were unequal ; as might be inferred 

 from considering the forces as proportional to the squares 

 of the velocities. One party drew their conclusions from 

 the fact that equal momenta can resist equal pressures 

 during the time. The other party took into consideration 

 the spaces through which the same moving force was 

 exerted ; and as these were as 2 to I, or as the product 

 of the weight of each striking body and the square of its 

 velocity, they concluded that the vis viva, to which this is 



