164 Mr WHEWELL, ON THE NATURE OF THE TRUTH 



municate an additional velocity, equal to the first, by the second law 

 of motion ; so that the whole velocity thus commvinicated will be the 

 double of the first. Therefore, if the velocity communicated be not 

 also the double of the first when the two pressures act together, the 

 difference must arise from this, that the effect of one force is modified 

 by the simultaneous action of the other. And when we find by expe- 

 rience (as we do find) that there is no such difference, but that the 

 velocity communicated in a given time is as the pressure which com- 

 mimicates it, this result shews that there is nothing in the circumstance 

 of a body being already acted on by one pressure, which modifies the 

 effect of an additional pressure acting along with the first. 



17- I have above asserted the law, of the direct action of bodies 

 only. But it is also true when the action is indirect, as when by 

 turning a winch we move a wheel, the main mass of which is farther 

 from the axis than the handle of the winch. In this case the pres- 

 sure we exert acts at a mechanical disadvantage on the main mass of 

 the wheel, and we may ask whether this circumstance introduces any 

 new law of motion. And to this we may reply, that we can conceive 

 pressure to produce different effects in moving bodies, according as it 

 is exerted directly or by the intervention of machines; but that we 

 find no reason to believe that such a difference exists. The relations 

 of the pressures in different parts of a machine are determined by con- 

 sidering the machine at rest. But if we suppose it to be put in 

 motion by such pressures, we see no reason to expect that these pres- 

 sures should have a different relation to the motions produced from 

 what they would have done if they were direct pressures. And as 

 we find in experiment a negation of all evidence of such a differ- 

 ence, we reject the supposition altogether. We assert, therefore, the 

 third law of motion to be true, whatever be the mechanism by 

 the intervention of which action and reaction are opposed to each 

 other. 



From this consideration it is easy to deduce the following rule, 

 which is known by the designation of D'Alembert's principle, and 

 may be considered as a fourth law of motion. 



