MECHANICAL PARADOXES. 



It is, therefore, in the horizontal direction, 

 instead of upwardly against the disc, that the 

 rapidly flying particles of steam can deliver a 

 blow and exercise force. In this direction the 

 steam's motion is strong enough to drive out 

 and keep out the air from between the two 

 discs, and to do this by virtue of its flying 

 energy only, without itself exercising, except 

 just at the centre, much fluid pressure vertically 

 between the two discs. 



Now, when the air and the air-pressure are 

 thus removed from between the two discs, there 

 is nothing to force them apart against the 

 atmospheric pressure on their other sides. 

 Upon the upper surface of the upper disc and 

 beneath the lower surface of the lower disc, 

 the air exerts its full pressure in the ordinary 

 way. This amounts to almost fifteen pounds 

 against every square inch of surface, either up- 

 wards or downwards. If the upper disc be a 

 little more than two and a quarter inches across, 

 its area will be four square inches, and the 

 weight of the atmosphere pressing downwards 

 upon it will be about sixty pounds. With 

 similar force it is pressing upwards beneath 

 the under surface of the lower disc, so that the 

 two discs are being forced together by this very 

 considerable pressure on their outer surfaces. 



In between the discs, as we have seen, the 

 air-pressure is kept back toward the circum- 

 ference by the impact of the expanded steam. 



102 



