122 SEVENTEENTH CENTURY. ft. hi. 



upon it, and so the piston will force 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 

 the air can be entirely drawn out and a perfect vacuum 

 left ; but ye 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 directions. 

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



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