VOL. XXXIJ.] PHILOSOPHICAL TRANSACTIONS. 639 



the body on which they fell ; as for example, making use of clay, or of soft 

 wax : and the effects were constantly the same ; which made me easily con- 

 clude, that there was always the same reason in nature for this phenomenon.'* 



Thus far Poleni, whose mistake lies in this ; that he estimates the force of 

 the stroke of the falling balls, by the depth of the impression made in the 

 tallow, clay, wax, or any yielding substance. But we must consider, that when 

 two bodies move with equal forces, but different velocities, that which moves 

 the swiftest, must make the deepest impression, while the slowest body com- 

 municates its motion to the clay round about, and therefore does not strike in 

 so deep as the swifter body, which puts in motion few parts of the clay, besides 

 those that are before it, and which parts have so much less time to oppose this 

 body's motion, as its velocity is greater than the other's. To make this plainer, 

 let us suppose a door half open, and moving very freely on its hinges ; if a 

 pistol be fired against it, the ball will go through the door without moving it 

 out of its place ; but if we take a large weight of lead, and throw it against the 

 same door, with the same force as the pistol bullet moved, the door will be re- 

 moved from its position, and carried out of the place on its hinges by the stroke; 

 because in the first case, the motion of the ball is communicated but to a few 

 parts of the door, and in the last it is diffused all over it. Nay, the door will 

 be moved by the stroke, even though there should be a prominent part in the 

 lead, that should be no larger than a pistol-bullet, in order to strike the door 

 on no more of its surface than the bullet had done. 



For illustrating this further I contrived the following Experiment. — I caused a 

 machine to be made, as represented in fig. 21, pi. 14, consisting of a wooden 

 base AB, which could be set horizontal by means of 3 screws, as ss. On this 

 board, or base, stood upright 1 parallel boards, about 4 inches wide, and 4 

 inches asunder, with the elbow-piece ep sliding behind one of them, so as to 

 raise its upper end p to any height desired. Between these boards, square 

 frames of wood gg &c. with paper extended on them, could slide in, to the 

 number of 6, in a horizontal position. These paper diaphragms being 

 thus placed, I suspended an ivory ball of about one inch and a half diameter, 

 weighing something more than an ounce and a half, by a short thread, under p, 

 so that its centre of gravity hung 4 feet over the first diaphragm ; then cutting 

 the thread, the ball fell on the paper, and by its perpendicular stroke broke 

 through that diaphragm, and the 3 next under it. Then putting so much lead 

 into the ball, which was made hollow for that purpose, as to make it weigh 

 twice as much as it did before; and bringing down f, to let it fall but from one 

 foot, it broke through only 2 diaphragms by its fall. Making the experiment 

 several times with different heights, but still keeping the proportion in height 



