146 THE EEASON WHY. 



" And the seventh angel poured out his vial into the air ; and there came a 



great voice out of the temple of heaven, from the throne, saying, 



It is done." REY. xvi. 



accepts the invitation, and passes harmlessly to the earth. But this not all : 

 man learn&by observation that electricity resides in all matter ; that it may bo 

 collected or dispersed; that it travels along a good conductor at the rate of 

 half-a-million of miles in a second of time ; he constructs a battery, a kind of 

 scientific fortress, in which he encamps the great warrior of nature ; and then, 

 laying down a conducting wire, he liberates the mighty force : but its flight 

 must be on the path which man has defined, and its journey must cease at the 

 terminus which man has decreed, where, by a simple contrivance of his ingenuity 

 (the movements of a magnetic needle) , the electric current is made to deliver 

 whatever message of importance he desires to convey. Thus, the element 

 which in an instant might deprive man of life, is subdued by him, and made 

 the obedient messenger of his will 



CHAPTER XXX. 



638. What is the atmosphere ? 



The atmosphere is the transparent and elastic body of mixed 

 gases and vapours which envelopes our globe, and which derives its 

 name from Greek words, signifying- sphere of vapour. 



639. To ivhat height does the atmosphere extend? 



It is estimated to extend to from forty to fifty miles above the 

 surface of the earth. 



640. Why is it supposed that the atmosphere does not 

 extend beyond that height? 



Because it is found, by experiment and observation, that the air 

 becomes less dense in proportion to its altitude from the earth's 

 surface. The gradual decrease of atmospheric density observed in 

 ascending a mountain, or in a balloon, supplies sufficient data to 

 enable us to calculate the height at which the atmosphere would 

 probably altogether cease. 



At an altitude of 18,000 feet the air is indicated by the barometer to be only 

 Juilfas dense as at the surface of the earth. And as the densities of the atmo- 

 sphere decrease in a geometrical progression, the density will be reduced to one- 

 fourth at the height of 36,000 feet ; and to one-eighth at 54,000 feet. The effects 

 of the decreasing density of the atmosphere are, that the intensity of light and 

 sound are diminished, and the temperature is lowered. Persons who have 

 reached a very high elevation, state that the sky above them began to assume 

 the appearance of darkness ; and there can be no doubt that, if it were possible 

 to reach an altitude of some fifty to sixty miles, there would be perfect black- 



