I o HER OES OF INVENTION AND DISCO VER Y. 



this piston was drawn up from the bottom to the top of the 

 barrel, it would carry along with it all the air that had 

 previously filled the space through which it had passed. Now 

 were air, like water, possessed of little or no expansive force, 

 this space, after being thus deprived of its contents, would have 

 remained empty, and there would have been an end of the 

 experim'^nt. But in consequence of the extraordinary elasticity 

 of the element in question, no sooner would its original air be 

 lifted by the piston out of the barrel, than a portion of that in 

 the vessel beyond the piston would flow out to occupy its 

 place. -The vessel and the barrel together would now, there- 

 fore, be filled by the same quantity of air which had originally 

 been contained in the first alone, and which would consequently 

 be diminished in density just in proportion to the enlargement 

 of the space which it occupied. But although so much of the 

 air to be extracted had thus got again into the barrel, there 

 would still at this point have been an end of the experiment, ii 

 no way could have been found of pushing back the piston for 

 another draught, without forcing also the air beyond it into the 

 vessel again, and thus merely restoring matters to the state in 

 which they were at the commencement of the operation. But 

 here Guericke was provided with an ingenious contrivance — 

 that of the valve ; the idea of applying which he borrowed, no 

 doubt, from the common water-pump, in which it had been long 

 used. A valve, which, simple as it is, is one of the most useful 

 and indeed indispensable of mechanical contrivances, is, as most 

 persons know, merely a flap, or lid, moving on a hinge, which, 

 covering an orifice, closes it, of course, against whatever attempts 

 to pass through from behind itself (a force bearing upon it from 

 thence evidently only shutting it closer), while it gives way to 

 and permits the passage of whatever comes in the opposite 



