5V8 On Machines in General, 



in the same way as those which would become so also, re- 

 garding as freely perTneable to each other, those of the 

 bodies among which no pressure is exercised, although they 

 Are adjacent. l>ie,iof]owing, however, shows the utility of 

 this observation : 



If, when we undertake the solution of any piobleni, we 

 know beforehand that a certain part of the machine does 

 not exercise any actioa upon the other parts of the system, 

 we shall be able to suppose that this part of the machine is 

 totally annihilated, and ascertain the movement of the sy* 

 ^tem according to this hypothesis, i. e. by treating as geo- 

 metrical all the movenw.nts which would really become so 

 by this supposiiion ; and in the same way, if one of the given 

 conditit)ns of the problem is, that certain adjacent bodies d« 

 not exercise any p e^^sure upon each other, we shall express 

 this condition by regarc^ng these two bodies as permeable 

 to each other, i, e. by treating as geometrical the move- 

 ments which would in fact become so by this supposition. 



But if it happens that we arc ignorant whether this pres- 

 sure be real or null, wc must ascertain the movement of 

 the system, by first supposing the one or the other at plea- 

 6ure : we shall suppose therefore, for example, that this' 

 pressure is real : then, if on inquiring, according to this 

 hypothesis, the value of this pressure, we find it real and 

 positive, we shall conclude that the hypothesis is legitimate, 

 and the exact result; or else we shall be assured that the 

 pressure in question is null, and that we may consequent- 

 Iv treat as geometrical^ motions which would become so 

 in fact, iF the two bodies in question were freely permeable 

 to each other. 



Further, if there was a machine in the system, a wire for 

 example, and that we were ignorant if the tension of this 

 wire is null or real, we might make the calculation by at 

 first supposing that there really is tension ; then, if we find 

 for the value of this tension a real and positive quantity, we 

 shall conclude that the supposition is legitimate, and that 

 the result is exact; or else, we must recommence the cal- 

 culation, setting out from the conh-ary supposiiion, i. e, 

 guppGsing that the tension of the wirc is equal to zero; 



. which 



