860 On the Resistance of Fluids. 



not then, nor have I now either leisure or inclination to engage in a 

 polemic discussion. Nor was it my principal object to develop the 

 truth in relation to the Resistance of Fluids ; but to exhibit the 

 necessity of using language, in the discussion of such subjects, with 

 greater precision than has been generally practiced. Of this ne- 

 cessity, 1 considered the communications of Professor K. and Mr. 

 Gibbes, as affording ample proof and an apt illustration ; and with 

 these views I referred to them with perfect freedom indeed, but 

 certainly with no intentional disrespect. 



In writing an article with this object in view, I made an effort, as 

 I was bound to do, to use language myself with entire precision, 

 and am therefore the more surprised that I should have been mis- 

 understood. I commenced by pointing out the ideas which I should 

 attach to several terms that are in common use, but without any 

 fixed meaning. Among these was the term force, which I defined 

 to be " simple pressure or effort at any point or indivisible instant of 

 time." I stated that " in this sense its magnitude is expressed sim- 

 ply in pounds." In recapitulating my definitions I varied the form 

 of this one, retaining the same idea. I said " force is simple pres- 

 sure or effort, irrespective of duration or motion." Now by all this 

 Prof. K. understands me to mean that force is a magnitude which 

 results from the product of simple pressure or effort by a unit or el- 

 ement of time ; a quantity totally different in its nature from that 

 which I intended to define, and a quantity too which surely is not 

 irrespective of duration. He not only misunderstands but mis-' 

 quotes my definition. He says, " Mr. B. defines force to be the 

 pressure " in an indivisible instant.'^ Not so by any means. I 

 said " AT any indivisible instant ;" — referring to time as an epoch or 

 date, and not as a period or duration.* The pressure at any in- 

 stant differs from the pressure in an instant as widely as a linear 

 inch differs from a square inch. As a linear inch has magnitude with- 

 out breadth, but a square inch has not, so the pressure at any in- 

 stant has magnitude independently of duration, but the pressure in 

 an instant has none. 



I am bound to presume, and am happy to do so, that this misquo- 

 tation of my definition was made through inadvertency. And since 



* It was necessary in a comprehensive definition to refer to time, because forces 

 are sometimes variable ; and if the magnitude of such a force be expressed sim- 

 ply in pounds, some particular instant must be pointed out at which its magnitude 

 is estimated. 



