364 On the Resistance of Fluids. 



Prof. K. in another place remarks, " When Mr. Blake denies that 

 the force of a particle is as the velocity of the plane, he must mean 

 that momentum is not a measure of the moving force ; a truth so 

 obvious that if we are to give it up we give up the whole theory of 

 mechanics." It was not my intention either to affirm or deny the 

 proposition that " momentum is the measure of the moving force :" 

 nor did my argument bear at all on that point, for I affixed to the 

 term force a narrower meaning than it has in that proposition. My 

 views however on that point are fully settled, and indeed 1 have in 

 effect already partially expressed them in the preceding paragraph. 

 Believing that a further development of them may subserve the in- 

 terests of Mechanical Philosophy, I avail myself of this opportunity 

 to express in a few words the substance of what I had heretofore 

 intended to make the subject of a separate communication to the 

 Journal of Science. 



If in the proposition that momentum is the measure of the mo- 

 ving force, the term force means the power of a moving body to 

 effect changes of motion, that is, to generate or destroy momentum 

 in other bodies, then momentum is unquestionably the measure of 

 the moving force : but if it means power to penetrate or make im- 

 pressions on other bodies, measuring the magnitude of the effect by 

 the magnitude of the impression, or if it means power to grind grain, 

 saw lumber, drive a steam boat or rail road car, or to impede the 

 progress of a steamboat or car, measuring the magnitude of the re- 

 sistance by the amount and cost of the steam or other power neces- 

 sary to overcome it; or. in short, if it means the power either to aid 

 or to hinder the production of any of the effects for which mechan- 

 ical power is used in any of the various processes of engineering 

 and the arts, then momentum is not the measure of the moving 

 force ; and "the whole theory of mechanics," so far as it is based 

 on the supposition that it is so, "must be given up." The propo- 

 sition, as I understand it, means that the momentum of a moving body 

 is a universal measure of its power to produce mechanical effects. 

 If Prof. K. understanding it in this way, believes it a truth, he is 

 not alone in that opinion. So far as my reading and observation ex- 

 tend, it is generally admitted or taken for granted that momentum 

 is a universal measure of mechanical power. This idea pervades 

 the standard Treatises of Mechanics, and in some of them is laid 

 down in plain and unequivocal terms. Gregory, Vol. I. Article 477, 

 says, " the quantity of motion extinguished or produced, is the 



