Dr. Eottomley on Ewart. 2 5 t 



our notion of force appears to be derived from pressure as 

 it is perceived by the sense of touch. When a mass is 

 moved by pressure the pressure must also move — unless 

 the pressure follow and act upon the mass through some 

 portion of space no motion can be produced. If pressure 

 be increased in the same ratio that the space through 

 which it acts be diminished, or vice versa^ the same effect 

 will still be produced. The space therefore compensates 

 for the pressure and the pressure for the space, and when 

 taken together they constitute a determinate measurable 

 quantity of moving force, but in determinate quantities 

 which are always proportional to the moving force by 

 which they are produced. The word Force has been 

 ambiguously used. The force of a body in motion and a 

 quiescent pressure are different in kind. Leibnitz and his 

 followers adopted the distinct terms, vis and vis viva. 

 Wollaston prefers impetus to vis viva, but he sometimes 

 uses energy in the same sense. Smeaton uses the term 

 mechanical power to express the product of the pressure 

 into the space through which it acts, or the product of 

 the mass into the square of the velocity. Ewart thinks 

 that in this sense moving force would be a convenient term 

 for action of moving pressure. It need not be confounded 

 with motive force or the pressure uncombined with the 

 space or time through which it acts. Moving force might 

 be defined as ' moving pressure producing change of velo- 

 city or change of figure in masses of matter.' The dura 

 tion of a moving force cannot be taken generally as an 

 element in the estimation of its quantity. There appears 

 to be no more reason for taking duration as the general 

 measure of a moving force than for taking temperature 

 as the general measure of heat. Ewart then refers to 



