with reference to the Measure and Transfer of Force. 333 



their common centre of gravity being at rest, so that if we 

 "suppose an obstacle interposed to hinder their congress"*, 

 that obstacle must be as much pressed on one side as the other ; 

 nevertheless, when after being set free to obtain motion from 

 the impulse of the mutual gravitation, and after a certain time 

 their condition is examined, we find that the lesser has gained 

 the power of performing the work 10, while the greater has only 

 obtained the power of performing the work 1. 



The development of mechanical force, during the same interval 

 of time, in bodies set in motion by the same pressure is in- 

 versly proportional to their mass, if the pressure acts with the 

 same intensity when a body is in motion as when it is at rest. 



Suppose a long horizontal cylinder with its transverse section 

 equal to one square inch. Let one end be stopped with a piston 

 weighing 15 lbs., and the other end with another piston weighing 

 150 lbs. Let the cylinder be now exhausted of air, and suppose 

 the pistons to be moveable air-tight without friction. If they 

 are supposed to press on each end of a rod passing through the 

 axis of the cylinder, they would be m equilibrio, they would press 

 equally in opposite directions. Take away the rod, and allow 

 them to move freely by the atmospheric pressure. At the end 

 of one second the 150 lb. piston acquires a velocity of 3"2 feet 

 per second, and the 15 lb. ])ound piston a velocity of 32 feet per 

 second. With these velocities, suppose them to be projected up- 

 wards in a vertical, the lighter would ascend 16 feet and the 

 heavier yjpo*^*^ ^^ ^ ^"°*'* •"•'^ descending, the lighter would thus 

 be enabled to exert ten times the mechanical power of the heavier. 



Strictly, the pressure on the smaller piston in this example is 

 not quite equal to that on the larger after they have begun to 

 move, in consequence (if the expression may be allowed) of the 

 vis a tergo not being infinitely prompt, 



§6. 

 Equal pressure does not imply equal capacity of generating 



motion. 



This position, inferred from the examples given in §§ 4, 5, 

 shows the necessity of guarding against being Jed away by the 

 words action (which implies states of acting or moving) and 

 reaction, to conceive that an equal impressed tendency to move 

 implies an equal capacity of generating motion. 



A motive cause may act equally on two bodies at rest, but 

 unequally on bodies moving with unequal velocities, and the effect 

 may be in the proportion of these velocities respectively. . 

 * Principia. 



