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 know 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 [ 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 isto give a simple elementary demonstration of that 
on. Unfortunately the foundations of both Statics and Dynamics, in the work 
red to, are assume 
