REPULSION AND ATTRACTION. 



down to atmospheric pressure just as it leaves 

 the outer end of the hole. 



In the second case, the outer end of the 

 small hole is so nearly closed by the disc placed 

 over it that the boiler may be considered as 

 including the small hole itself, and boiler-pres- 

 sure extends to its outer end, just beneath the 

 disc at D. It might seem then, so far as that 

 goes, that the upward pressure beneath the 

 disc should be greater in the second case, 

 instead of less, being, in fact, the boiler-pressure 

 itself, instead of the mere flight-energy obtained 

 by steam particles through expansion from 

 boiler-pressure to a lower pressure. 



However, the boiler-pressure beneath the 

 disc is limited to a very small area just at its 

 centre, where it is almost closing the small 

 hole ; while the expansion from that area gives 

 rise to a very different state of things in the 

 rest of the space between the two discs. 



For, just as the steam from A, expanding 

 through B, gains a rapid forward movement 

 towards C, so the steam just beneath D, ex- 

 panding laterally, gains a rapid horizontal 

 movement outwards between the two discs 

 from the centre towards every part of the cir- 

 cumference. At the centre the way upwards 

 is blocked by the upper disc, so that the only 

 direction in which the particles of steam can 

 start moving as the steam expands, and gain 

 rapid movement, is horizontally. 



101 



