364 ON THE MEASURE OF THE FORCES OF BODIES 
and decisive kind to prove that the forces must be as the velocities, or to show the rela- 
tion that one must bear to the other. For while we can thus unite two distinct masses, 
making them to coexist under a common velocity, we cannot unite two distinct 
velocities, making them to coexist in a common mass. With the hope, then, of aiding 
to form a clearer conception of this subject, let me propose the following experiment. 
Take a spiral spring, formed of eleven turns of wire, coiled so as to leave an open 
space between each turn of the coil (as in Fig. 2). Let this coil be then pressed 
together as in Fig. 1, and place against its ends respectively the weights A and 
Fig. 1. 
yi 
B, suspended by cords, like pendulums. Let the weight of A be 10 pounds and 
the weight of B be 1 pound. On releasing the spring from its constrained con- 
dition it recovers, under the operation of its elasticity, its original form as shown 
in Fig. 2, while the masses A and B are thrown, one to the right and one to the 
left, with velocities inversely as their masses, namely, A with a velocity of 1, and .B 
with a velocity of 10. All the force which had been applied to compress the spring, 
Fig. 2. 
a A 
from whatever source it was derived, and which existed in the spring’s elasticity, has 
now passed away from the spring, and been exhausted in giving motion to A and 
B. It has overcome, in those bodies, to a certain extent, their state of rest, and 
the force, vis inertie, with which they resisted all change of that state; and those 
bodies together now possess a force of motion, or inertia of motion, equal to the 
force of elasticity given out from the spring. We may therefore conceive that the 
force of elasticity in the spring has been transferred to A and B in the form of 
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