484« Royal Society. 



2. " Researches in the Theory of Machines." By the Rev. H. 

 Moseley, M.A., F.R.S., Professor of Natural Philosophy and Astro- 

 nomy in King's College, London. 



Of the various names, such as " useful effect," " dynamical effect," 

 •* efficiency," " work done," " labouring force," " work," which have 

 been given to that operation of force in machinery which consists in 

 the union of a continued pressure with a continued motion, the au» 

 thor gives the preference to the term work, as being that which con- 

 veys, under its most intelligible form, this idea of the operation of 

 force, and as being the literal translation of the word " travail," 

 which among French writers on mechanics has taken the place of 

 every other. 



The single unit, in terms of which this operation of force is with 

 us measured, viz. the work of overcoming a pressure of one pound 

 through one foot, he considers to be distinguished sufficiently, and 

 expressed concisely enough, by the term unit of work, rejecting as 

 unnecessary, and as less likely to pass into general use, the terms 

 " dynamical unit," and " dynam," which it has been proposed to 

 apply to it. 



Having thus defined the terms work and unit of work, and paid a 

 tribute of respect to the valuable labours of M. Poncelet in the theory 

 of machines, and expressed admiration of the skill with which he has 

 applied to it the well-known principle of vis viva under a new and 

 more general form, the author proceeds to remark, that the inter- 

 pretation which M. Poncelet has given to that function of the velo- 

 city of a moving body which is taken as the measure of its vis viva, 

 associates with it the definitive idea of a force opposed to all change 

 in the state of the body's rest or motion, and known as its " vis in~ 

 ertiee," " vis insita," &c. The author conceives that the introduction 

 of the definitive idea of such a force into questions of elementary and 

 practical mechanics is liable to many and grave objections ; and he 

 proposes a new interpretation of it, viz. " that one half of this func- 

 tion represents the number of units of work accumulated in the moving 

 body, and which it is capable of reproducing upon any resistance 

 opposed to its progress." This interpretation he establishes by me- 

 chanical considerations of an elementary kind. Taking, then, this 

 new interpretation of the function representing one half the vis viva, 

 and dividing the parts of a machine into those which receive the 

 operation of the moving power (the moving points) and those which 

 apply it (the working points), he presents the principle of vis viva in 

 its application to machines under the following form : — "The number 

 of units of work done by the moving power upon the moving points 

 of the machine is equal to the number yielded at the working points, 

 plus the number expended upon the prejudicial resistances, plus the 

 number accumulated in the various parts of the machine which are 

 in motion." So that the whole number of units of work done by 

 the moving power, or upon the moving points, is expended, partly 

 in that work done at the working points, whence results immediately 

 the useful product of the machine, and partly upon the prejudicial 

 resistances of friction, &c. opposed to the motion of the machine in 

 its transmission from the moving to the working points ; and all the 



