On Machines in General. 1 29 



What is a body? It is, most people will tell us, an im- 

 penetrable extent ; i. e. which cannot be reduced to a less 

 space. But is this property not common to the body and 

 tp the empty space ? Can a cubic foot of vacuum occupy a 

 less space ? It is clear that it cannot. Suppose a cubic foot 

 of water, for example, is contained in a vessel capable o£ 

 containing two cubic feet and closed on all sides: suppose wa 

 shake or roll this vessel as much as we please, there will 

 always remain a cubic foot of water and a cubic foot of va- 

 cuum : here are two spaces, of different nature indeed, but 

 both equally irreducible : — it is not in this therefore that the 

 characteristic property of bodies consists. Other people tell 

 us that this property consists in<mobility. The indefinite and 

 empty space, say they, is immoveable, while bodies may be 

 transported from one place of this space to another : but 

 when the body A passes into B for example, Has not the 

 empty space which was in B, passed into A ? There is not, 

 io my opinion, more reason for attributing the movement 

 to the plenum which was in A, than to the vacuum which 

 was in B ; the movement consists in one of these spaces 

 supplanting the other ; and this supplanting being recipro- 

 cal, the mobility is a property which belongs no more to 

 one than to another. Without departing from our first 

 supposition, When I shake the vessel half empty and half 

 full, is not the vacuum moved as well as the fluids ? I dip 

 a hollow ball of metal into a bottle; the ball goes to the 

 bottom, Have we not here a vacuum which is moved in a 

 plenum, in the same way as bodies are moved in a vacuum ? 

 The full space does not therefore differ from the empty space, 

 eiiher in mobility or in irreducibility ; the impenetrability 

 which distinguishes the first from the second, therefore, is 

 not the same with this irreducibility; it is something which 

 we cannot define, because it is a primitive idea ! 



The two fundamental laws from which I set out (XI.) 

 are therefore truths purely experimental, and I have pro- 

 posed them as such. A detailed explanation of these prin- 

 ciples would not enter into the plan of this work, and per- 

 haps would have only darkened the subject : the sciences are 



Vol. 32. No. 126. Nov. 1808, I like 



