■"fO 



LECTURE VIl. 



ON PRESSURE AND EQUILIBRIUM. 



We have now examined the principal cases in whicli a simple force is em- 

 ployed in the production of motion; it is pf equal consequence to attend to 

 the opposition of forces, where they prevent each other's action. A force 

 counteracted by another force, so that no motion is produced, becomes a 

 pressure: thus we continually exert a pressure, by means of our weight, up- 

 on the ground on which we stand, the seat on which we sit, and the bed on 

 which we sleep; but at the instant when we are falling or leaping, we neither 

 exert nor experience a pressure on any part. 



It was very truly asserted by the antients, that pressure and motion are ab- 

 solutely incommensurable as effects; for according to' the definition of pres- 

 sure, the force appears to he what is called in logic a potential cause, M'hich 

 is not in a state of activity: and since an interval of time must elapse after 

 the removal of the opposite force, before the first force can have caused any 

 actual motion, this effect of a finite time cannot with justice be conceived to 

 bear any proportion to the pressure, which is as it were a nascent effect only. 

 It is true that a large weight, pressing on a spring, may keep it bent, in ex- 

 actly the same place, into which a smaller weight, falling on it with a certain 

 velocity, would inflect it: but, to retain a spring in a certain position, and to 

 bend it into that position, are effects absolutely incommensurable; the one 

 being a measure of the constant repulsive force of the spring, bent to a certain 

 point, the other of the sum of the effects of the same spring, in various degrees 

 of flexure, for a certain time. Plencc the smallest possible momentum is said 

 to be more than equivalent to the greatest possible pressure: a very light 

 weight, falling from a very minute distance, will force back a very strong 

 spring, although often through an imperceptible space only. But the impulse 

 of a stream of infinitely small particles, like those of which a fluid is supposed 



