114 On the Resistance of Fluids. 



aginary, that is called force. Among the infinite number of laws 

 mathematically possible, these are the only ones that are physically 

 true. They can be proved by experiment, and are obtained by in- 

 duction from observed facts, and unless they are applied to cases of 

 the same nature, they prove nothing. I knov^^ that some writers 

 have neglected to observe these principles. Professor Farrar, for 

 example, in his Mechanics, has, under the head of statics, given 

 a demonstration, not of the parallelogram of forces, which is no 

 where in his work proved, but of the parallelogram of motions, 

 or velocities.* But Mr. Blake has left far behind him all pre- 

 cedents. He applies these laws of motion to the determination 

 of an instantaneous impulsive force, a thing which has no existence 

 in nature, and of which I can form no conception. How can the 

 laws of motion, got by induction of facts, be applied to determine 

 such a force, or such a force be applied as Mr. Blake applies it, to 

 determine an actually existing force ? I say then, as before, that 

 the logic of that reasoning is unsound, and that Mr. Blake " setting 

 out to determine the ' force of resistance' has unconsciously deter- 

 mined a quantity of a very different nature." 



I have no time, nor inclination, nor need for remark on Mr. Blake's 

 curious suggestions respecting the Leibnitzian controversy, and the 

 possibility of my confounding the vis motrix and vis mechanica. 



* I know how difficult it is to give a simple elementary demonstration of that 

 important proposition, but it were better to give a proof, though unintelligible to 

 beginners, than to leave that whole grand division of Mechanics without founda- 

 tion. Unfortunately the foundations of both Statics and Dynamics, in the work 

 refeired to, are assumed. 



