276 Remarks on the theory of the Resistance of fluids. 



ing through that distance. In this viewi ts magnitude is expressed 

 by the product of force and distance divided by time, or which is 

 the same thing, the product of force and velocity. This product 

 of force and velocity is not a measure of power, but of a ratio which 

 power bears to time, or in other words, it is a" measure of the rapidi- 

 ty with which power flows out and is brought into action. This 

 quantity, therefore, I call fluent poiver. 



I will recapitulate these distinctions. 



Force is the pressure in pounds. 



Power is the product of force and distance. 



Fluent power is the product of force and velocity. 



Again. — Force is simple pressure, irrespective of duration or 

 motion. 



Power becomes developed as this force moves, in proportion to the 

 distance moved through. 



Fluent power is the ratio of this developement to the time in 

 which it takes place. 



These are three quantities which are totally different in their na- 

 ture, and between which it is of the utmost importance clearly to 

 distinguish; in every branch of mechanical philosophy in which they 

 are introduced, and in none more so, than in that which treats of 

 the resistance of fluids. 



It may seem incredible to some that distinctions so obvious and 

 simple, and the propriety and even necessity of which is so mani- 

 fest, can need to be laid down and insisted upon in the nineteenth 

 century, especially in a branch of science which has been cultiva- 

 ted ever since the days of Archimedes. But if any one to whom 

 this sentiment may occur will first clear up his own views in regard 

 to these distinctions, and will then in the light of them examine the 

 books, he will be astonished to find what a medley of confusion and 

 error they contain in every branch of mechanical philosophy to which 

 these distinctions are applicable. Even the treatise by Olinthus 

 Gregory, who perhaps brought as high a degree of mental and math- 

 ematical accumen to bear upon the subject, as any who have pre- 

 ceeded or followed him, should not be excepted from this remark. 



It was an oversight or misapprehension of these distinctions that 

 embarrassed the views of your two correspondents to whose com- 

 munications I have already referred, and which has led them to sup- 

 pose that the different results at which they have arrived, in estima- 

 ting the perpendicular action of a fluid on a plane oblique to the 



