_ PHYSICAL ann LITERARY. ot 
likewife, the impreffions are found to be e- 
qual, when the products of the quantities of 
"matter, by the fquares of the velocities, are 
equal. Thus, let a hollow ball, with a 
quantity of matter 1, move with a velocity 
20, and make a certain impreflion. If you 
afterwards inclofe as much lead within the 
fame ball, as to render it of double the mafs 
of matter, you muft give it a velocity fome- 
what exceeding 14, before it make an im- 
_ preffion equal to the former one. If the bo- 
dy 2, receive only a velocity 10, the impref- 
fion will be remarkably lefs. In the former 
method, the gravity or weight of the balls, 
continuing to act after they touch the clay, 
may contribute fomewhat in making the im- 
7 preffions ; but, in the other way of proceed- 
4 ing, the impreflions muft be more exadtly 
' owing to the vis znfita, or force acquired by 
_ the bodies, in falling from the determined 
heights. This experiment can eafily be re- 
conciled with the old meafure of forces, ac- 
: ‘cording to which the forces of equal bodies 
are to be eftimated from the velocities fimply. | 
‘But, in order to do this, we ought carefully 
| 0 diftinguifh betwixt two very different kinds 
tefiftance, viz. that from the inertia of 
the 
