MEASURE OF MOVING FORCE. 145 
represent definite physical properties ; with the 
suggestion of a new Term in mechanics.” 
Mr, Gilbert observes it has been remarked that 
neither impetus nor momentum are usually correct 
measures of the effective action of machines. The 
criterion of this is the force exerted multiplied by 
the space through which it acts; and to this func- 
tion he proposes to give the name Efficiency, 
retaining the word Duty for a similar function, 
indicative of the work performed; and by a com- 
parison of these two functions, viz. the Efficiency 
expended on, and the Duty performed by any 
machine, an exact measure of its intrinsic work 
will be obtained (see Abstracts of Phil. Trans. 
1827). 
The Rev. Dr. Whewell in his “ First Principles 
of Mechanics,” published in 1832, gives a chapter 
‘on the work done by machines,” the first section 
of which is headed Measure of Efficiency, and he 
adopts the suggestions of Mr. Gilbert, as men- 
tioned above. 
Since the vis viva expresses, dynamically, twice 
the product of any pressure by the space through 
Xx 
