68 Experimental Philosophy. [Lecture 6, 



barometer the quicksilver in the ball below is 

 left open to the pressure of the atmosphere, which, 

 according as it increases in weight or density, 

 presses on the surface of the quicksilver, and 

 forces it into the vacuum in the glass above. 

 When the air is dense or heavy it supports the 

 clouds and vapours ; when it is rarefied and thin 

 it is unable to support them, and they fall in the 

 form of mists, rain, hail, or snow. When, there- 

 fore, the quicksilver rises in the glass, we say it 

 is a sign of fair weather, when it falls it prognos- 

 ticates foul. 



That the air is elastic is easily seen from various 

 experiments, particularly when it is confined in a 

 bladder or any flexible substance, we then find it 

 may be compressed by force into a narrower com- 

 pass, and that it will expand again when that 

 force is removed. But of all instruments for 

 showing the elasticity as well as all the other 

 properties of the air, the air-pump is the most 

 complete. It was invented nearly simultaneously 

 by our illustrious countryman, Mr. Boyle, and a 

 celebrated German, Otto Guericke. 



Whoever is acquainted with the construction 

 of a common water-pump, can have no difficulty 

 in comprehending the nature and action of the 

 air-pump ; the principle is exactly the same, and 

 we may therefore, without further preface, refer 

 immediately to the Plate VII. fig. 28, to explain 

 its operation. 



Having put a wet leather on the plate L L of 



