PHYSICAL AND LITERARY. 103 ■ 



dy 2, receive only a velocity 10, the im* 

 preffioii will be remarkably lefs. In the 

 former method, the gravity or weight of 

 the balls, continuing to adl after they 

 touch the clay, may contribute fomewhat 

 in making the impreffions ; but, in the 

 other way of proceeding, the impreffions 

 muft be more exadly owing to the vis in^ 

 fita, or force acquired by the bodies in 

 falling from the determined heights. 

 This experiment can eafily be reconciled 

 with the old meafure of forces, accord- 

 ing to which the forces of equal bodies 

 are to be eftimated from the velocities 

 limply. But, in order to do this, we 

 ought carefully to diltinguifh betwixt 

 two very different kinds of refiftance, 'uiz, 

 that from the inertia of the matter, and 

 the other from the force of cohefion. 

 When an impreffion is made by a ftroke, 

 upon the furface of any yielding fubftance, 

 fuch as foft clay, the refiftance to be o- 

 vercome in making that impreffion pro- 

 ceeds almoft entirely from the tenacity or 

 cohefion : Any refiftance arifing from the 



inertia 



