Remarks on the theory of the Resistance of Fluids. 273 



nify several different sorts of quantities; accompanied by a corres- 

 ponding vagueness of apprehension in regard to the existence and 

 nature of these differences. Whether the ambiguity of language is 

 the cause or the effect of the indistinct notions on the subject, 1 need 

 not inquire. It is certain that they tend mutually to perpetuate each 

 other; and I have no doubt that collectively, they have done more 

 to retard the progress of Mechanical Philosophy, and to bring it 

 into disrepute among practical men than all other causes united. 

 That there may be no misapprehension in regard to the precise mean- 

 ing I attach to the language I shall use in speaking of these differ- 

 ent quantities, 1 shall, before I proceed further, point out the dif- 

 ferences to which I allude, and the mode in which I shall distin- 

 guish them. 



Mechanical agency may be contemplated under several different 

 aspects; in each of which respectively, its magnitude is a different 

 species of quantity. My present purpose requires me to notice 

 three of these. 



1. It may be considered with reference solely to the simple pres- 

 sure or effort, exerted at any point or indivisible instant of time. 

 In this view its magnitude is expressed simply in pounds. This 

 quantity I cd.]\ force or force of resistance. 



2. It may be considered with reference not only to the force, or 

 pressure at any point of time, but also with reference to the distance 

 through which it is exerted, but without reference to the time occu- 

 pied in moving through that distance. In this view its magnitude 

 is expressed by the product of force and distance. The cost and 

 value of all mechanical effects, and of course the cost and value of 

 all mechanical power are proportional to this product. For this 

 reason I designate the quantity resulting from the product of force 

 and distance, by the name power. This product is the true meas- 

 ure of mechanical power in all cases, when contemplated as an agent, 

 producing a determinate amount of any of the various ultimate ef- 

 fects aimed at in practical mechanics ; for the amount of effect pro- 

 duced will always be proportional to this product in the moving 

 power. 



3. Mechanical agency may be contemplated as a quantity depend- 

 ing for its magnitude, not on the total amount of its effects, but on 

 the rapidity with which it produces given amounts of effect ; that is, 

 it may be considered not only with reference to the force and distance, 

 but also with reference to the shortness of the time occupied in pass- 



