PNEUMATICS. 169 



"We do not know, moreover, that any vacuum exists in nature, although there 



is no conclusive evidence that the spaces between the planets are filled with 



any material substance. 



If we dip a pail into a pond, and fill it with water, a hole (or vacuum) is 



made in the pond aa big as the pail; but the moment the pail is drawn out, 



the hole is filled up by the water around it. In the same manner air rushes 



in, or rather is pressed in by its weight, to fill up an empty space. 



„ , When we place one end of a straw, or tube in the mouth. 



How does ' ' ' 



water rise in a and the other end in a liquid, we can cause the liquid to riso 

 tionT ^^ ""''" ^ *^^® straw, or tube by sucking it up, as it is called. We, 

 however, do no such thing ; we merely draw into the mouth 

 the portion of air confined in the tube, and the pressure of the external air 

 which is exerted on the surface of the liquid into which the tube dips, being 

 no longer balanced by the elasticity of the air in the tube, forces the hquid up 

 into the mouth. If, however, the straw were gradually increased in length, 

 we should find that above a certain length we should not be able to raise 

 water into the mouth at all, no matter how small the tube might be in diam- 

 eter ; or, in other words, if we made the tube 34 feet long, we should find 

 that no power of suction, even by the most powerful machinery instead of 

 the mouth, could raise the water to that height. The water rises in the com- 

 mon pump in the same way that it docs in the straw ; but not above a height 

 of 33 or 34: feet above the level of the reservoir. 



„ ^. 378. The reason why water thus rises in a straw, or pump, 



How was the ■' > i ri 



ascent of water remained a mystery until explamed and demonstrated by Tor- 



Hon'^'firit^Tx" ™^^'' » pupil of Galileo. It is clear that the water is sus- 

 pluined uiid de- taincd in the tube by some force, and Torricelli argued that 

 monstrate whatever it might be, the weight of the column of water sus- 



tained must be the measure of the power thus manifested ; consequently, if 

 another liquid be used, heavier or lighter, bulk for bulk, than water, then the 

 same force must sustain a lesser or greater column of such liquid. By using a 

 much heavier liquid, the column sustained would necessarily be much shorter, 

 and the experiment in every way more manageable. 



Torricelli verified his conclusions in the following manner: — He selected 

 for his experiment mercury, the heaviest known UquitL As this is 13^ 

 times heavier than water, bulk for bulk, it followed that if the force imputed 

 to a vacuum could sustain 33 feet of water, it would necessarily sustain 

 13^ times less, or about 30 inches of mercury. Torricelli therefore made the 

 following experiment, which has since become memorable in the history of 

 ecience : — / 



He procured a glass tube (Pig. 158) more than 30 inches long, open at ond 

 end, and closed at the other. Filling this tube with mercury, and applying 

 his finger to the open end, so as to prevent its escape, he inverted it, plung- 

 ing the end into mercury contained in a cistern. On removing the finger, he 

 observed that the mercury in tlie tube fell, but did not fall altogether into the 

 cistern ; it only subsided until its surface was at a height of about 30 inches 

 above the surface of the mercury in the cistern. The result was ^vhat Tor- 



8 



