128 PNEUMATICS. 



same level. The water is, therefore, prevented 

 from rising within the tumbler, by some other sub- 

 stance which already occupies the inside j which 

 substance is the air that filled the tumbler when it 

 was inverted, and which could not escape, on ac- 

 count of the superior pressure of the water. 



In like manner, having opened a pair of com- 

 mon bellows, stop up the nozzle securely, and you 

 will find that you cannot shut the bellows, which 

 seems to be filled with something that yields a 

 little, like wool; but if you unstop the nozzle, the 

 air will be expelled, and may be felt against the 

 hand. 



When the air is at rest, we can move in it with 

 the utmost facility ; nor does it offer to us a sen- 

 sible resistance, except the motion be quick, or the 

 surface opposed to it considerable ; but when that 

 is the case, its resistance is very sensible, as may 

 be easily perceived by the motion of a fan. 



When air is in motion, it constitutes wind, which 

 is nothing more than a current or stream of air, 

 varying in its force, according to the velocity with 

 which it flows. 



The invisibility of air, therefore, is only the 

 consequence of its transparency ; but it is possessed 

 of all the common properties of matter. When a 

 vessel is empty, in the usual way of speaking, it is 

 in fact still filled with air. 



But it is possible to empty a vessel, even of the 

 air which it contains, by which means we shall be 

 able to discover several properties of this fluid. 



Galileo first discovered, in lGOO, that air had 

 weight like other bodies. And his pupil, Torricelli, 

 applied this discovery of the pressure of the atmos- 

 phere, to the explanation of the riseof water in pumps, 

 which it had been formerly imagined was owing to 



