38 PNEUMATICS. 



darkness comes on slowly and gradually, instead of ra- 

 pidly and at once. The reflective power of the atmos- 

 phere has also its benefits, for without this quality the 

 appearance of things would be much altered. We should 

 most probably be only able to see that side of anything 

 on which the sun shone, which would be attended with 

 the greatest inconvenience. We should also have a most 

 sudden transition from daylight to darkness, on the rising 

 and setting of the sun. The light of the sun itself would 

 be also too fierce for our comfort, if our eyes were di- 

 rected towards it ; while at that part of the heavens op- 

 posite to the sun it would be dark and dismal, and the 

 planets and stars would be visible if there were no clouds 

 to prevent it. 



The principal instruments used to exhibit the weight, 

 elasticity, and other phenomena of the atmosphere, are 

 the Air-Pump, the Air-Gun, the Condensing Syringe, and 

 the Barometer ; a brief description of which is here an- 

 nexed. 



The AiR-PuMP is more properly a machine than an 

 instrument ; and is so constructed as to exhaust the air 

 out of a proper receiver, or rather to rarefy it, and the 

 principle of its action depends on the elasticity of the 

 air. The air-pump was first invented by Otto Guericke, 

 of Magdeburg, in the middle of the seventeenth century. 

 A single-barrelled air-pump consists of a plate of brass, 

 having a small hole in its centre, and connected by a 

 tube to a cylindrical barrel, in which a piston with a 

 proper valve is made to work, similar to a common suck- 

 ing-pump. If a receiver be placed on the brass plate, 

 and rendered air-light, by raising the piston a quantity 

 of air will be exhausted from the receiver, when the re- 

 maining air will, by means of its elasticity, become rare- 

 fied, and occupy the whole of the receiver : by repeatedly 

 working the piston upwards and downwards, the air will 

 become proportionately rarefied, when, by a series of in- 

 teresting experiments, the nature and properties of the 

 air are beautifully exemplified. The single-barrelled air- 

 pump is the most simple of its kind. Air-pumps are 

 now generally made with double barrels to work with a 

 winch, but the principle is the same. 



When the air is exhausted or more properly, consi- 



