sbct. ii.] DISSERTATION SECOND. 45 



SECTION II. 

 MECHANICS, GENERAL PHYSICS, &c. 



The discoveries of Galileo, Descartes, and other mathe- 

 maticians of the seventeenth century, had made known some 

 of the most general and important laws which regulate the 

 phenomena of moving bodies. The inertia, or the ten- 

 dency of body, when left to itself, to preserve unchanged its 

 condition either of motion or of rest ; the effect of an impulse 

 communicated to a body, or of two simultaneous impulses, 

 had been carefully examined, and had led to the discovery of 

 the composition of motion. The law of equilibrium, not in 

 the lever alone, but in all the mechanical powers, had been 

 determined, and the equality of action to reaction, or of the 

 motion lost to the motion acquired, had not only been esta- 

 blished by reasoning, but confirmed by experiment. The 

 fuller elucidation and farther extension of these principles 

 were reserved for the period now treated of. 



The developement of truth is often so gradual, that it is 

 impossible to assign the time when certain principles have 

 been first introduced into science. Thus, the principle of 

 Virtual Velocities, as it is termed, which is now recognized as 

 regulating the equilibrium of all machines whatsoever, was 

 perceived to hold in particular cases long before its full ex- 

 tent, or its perfect universality, was understood. Galileo 

 made a great step toward the establishment of this principle 

 when he generalized the property of the lever, and showed, 

 that an equilibrium takes place whenever the sums of the op- 

 posite momenta are equal, meaning by momentum the product 

 of the force into the velocity of the point at which it is appli- 

 ed. This was carried farther by Wallis, who appears to have 



