120 SEVENTEENTH CENTURY. PT. in. 



drive the air between B and B out of the top of the 

 cylinder. If the valve e remained also shut there would now 

 be a vacuum, or space without air, in the cylinder B ; but 

 this will not be so, because the air in the jar below, being 

 no longer kept down by air above it, will expand, and 

 forcing up the valve e will fill the whole of the jar and the 

 cylinder with expanded air. 



Now bring down the piston c c again and observe what 

 will happen. The thin air in the cylinder will be pressed 

 down upon the valve e and will shut it, and then, not being 

 able to get down into the jar, it will force up the valve d 

 again, and escape out at the top. The piston will now be 

 resting once more upon the valve e but the glass jar will 

 have much less air in it than it had at first, because it will 

 have lost all that which went up into the cylinder and was 

 pressed out at the top. You have only to repeat this 

 process and more air still will be drawn out, and thus by 

 moving the piston up and down you gradually empty the 

 glass jar. You cannot get quite all the air out, because 

 there must be enough left to push open the valve e when 

 you pull the piston up, but you can go on till there is very 

 little indeed. Air-pumps are now constructed, by which all 

 but an infinitesimal quantity of air can be drawn out and 

 a vacuum left which is almost perfect ; but we are speaking 

 of the one Guericke made, which was like the one I have 

 described, only more complicated, and he worked it under 

 water to make quite sure that no air should creep in at the 

 cracks. 



The Experiment of the Magdeburg Hemispheres. 

 The first experiment which Guericke made with his air- 

 pump was to prove that the atmosphere round our earth is 

 pressing down upon us heavily and equally in all d/'rections. 

 To do this he took two hollow metal hemispheres, like the 



