54 THE ATMOSPHERE. 



half in diameter. This is connected with one end of a 

 stick about four feet long, for the purpose of preserving 

 its direction. It will now be recollected what takes 

 place, when fire is applied to the material through the 

 ftisee at the lower end of the cylinder. Remembering our 

 description of the inflammation of gunpowder, we per- 

 ceive that the only exit for this newly formed air, is down- 

 ward, and consequently, as it rushes out, it meets with 

 the atmosphere, by which its escape is resisted. Hence 

 it must ascend if its weight be not greater than the 

 resistance, and continue its flight until its power is 

 consumed. 



fi? 



PRESSURE OF THE AIR. 



We are quite insensible to what principles of philoso- 

 we are indebted for our comforts, and how much 

 las been made subservient to our use by the great 

 artificer of nature. The first act performed by a human 

 being, the process by which the vital current from his 

 mother's breast is made to supply his wants, depends 

 on the pressure of the air. How few have any idea of 

 philosophical aid in what appears to be so simple. But the 

 same may be remarked of innumerable circumstances, 

 which appear to be produced with as little design. 

 Since the attention of philosophers has been directed 

 to these mechanical peculiarities, several phenomena 

 have been explained, which had depended for their so- 

 lution solely on conjecture. The awful thunder found 

 an explanation in the theory that the lightning in leap- 

 ing from cloud to cloud, or from a cloud to the earth, 

 drove back the air which opposed its progress, while the 

 atmosphere, rushing in from behind to fill the vacancy, 

 produced the sound which always succeeds the flash. 



To Sir Everard Home we are obliged for the expla- 

 nation of the cause by which flies are enabled to walk 

 on the lower side of a horizontal plane, or the perpen- 

 dicular surface of glass. He ascertained that their 

 feet have flat skins or flaps, like the feet of web-footed 

 animals, and that they have the power of drawing down 

 this web so closely upon the surface whereon they walk, 

 as effectually to exclude the air. The consequence of 



