On Machines in GoneraL 157 



•PART FIRST. 



General Pri?icipk'S^ 



When one body acts upon another, it is always imme- 

 diately, or by the agency of sISme intermediate body: This 

 intermediate body is generally what is called a machine : the 

 movement lost every instant by bodies applied to this ma- 

 chine is partly absorbed by the machitVeitseU*, and partly re-. 

 ceived by the other bodies in the system ; but as it may hap- 

 pen that the object of the question is simply to find the re- 

 ciprobal action of bodies applied to intermediate bodies, 

 without having arty occasion to know the effect of it upon 

 the intermediate body itself, it has been thought, in order 

 to simplify the question, to make an abstraction of the very 

 mass of this body* preserving to it on the other hand all the 

 other properties of matter. Hence the science of machines 

 has become in some measure an isolated branch of me- 

 chanics, in which it is required to consider the reciprocal 

 action of the different parts of a system of bodies ; among 

 which there are found things which, when deprived of the 

 inertness common to all parts of matter such as exists in na- 

 ture, have retained the name of machhies. 



IX. This abstraction may simplify in certain particular 

 cases, where circumstances indicate those of bodies, the 

 mass of which it is convenient to neglect, in order more easily 

 to attain our object; but we conceive that the theory of ma- 

 chines in general has really become more complicated than 

 formerly : for this theory was once contained in that of the 

 movement of bodies, such as nature presents them to us ; 

 but at present we must consider at once two kinds of bodies;, 

 the one as they really exist, and' the other as deprived in part 

 of their natural properties. Now, it is clear, that the first of 

 these problems is a particular case of the latter; therefore the 

 latter is more complicated : further, although we easily suc- 

 ceed by similar hypotheses, in finding the laws of equili- 

 brium and of movement in each particular machine, such aj» 

 the lever, the axle, and the vice, there results an assemblacre 

 of facts, the connection of which is perceived with diffi- 

 culty, and solely by a kind of analogy ; which should ne- 

 cessarily 



