148 THE BEASON TVHT. 



* I therefore so run, not as uncertainly ; so fight I, not as one that beateth th 

 air." COHIXTH. ix. 



arising out of the changes in the state of the atmosphere, may 

 amount to as much as a ton and a-Jialf. 



646. Wlnj does not man feel this pressure ? 



Because the diffusion of air which, surrounding him in every 

 direction, and acting upon the internal as well as the external 

 surfaces of his body, and probably surrounding every atom of his 

 frame, establishes an equilibrium, in which every degree of pressure 

 counteracts and sustains itself. 



647. What is the weight of air relative to that of water ? 



A cubic foot of air weighs only 523 grains, a little more than an. 

 ounce ; a cubic foot of water weighs one thousand ounces. 



648. Wliat is the greatest height in the atmosphere which 

 any human being has ever reached? 



M. Gay Lussac, in the year 180-i, ascended to the height of 

 23,000 feet. 



649. What is a vacuum ? 



A vacuum is a space devoid of matter. The term is generally 

 applied to those instances in which air is drawn from within an 

 air-tight vessel. 



650. Is it possible to form a perfect vacuum ? 



It is probably impossible to do so, even with the most powerful 

 instruments some portion of air would remain, but in so thin a 

 a form that it would be imperceptible. 



651. Why does the depression of a pump-handle cause the 

 water to flow ? 



Because the putting down of the handle lifts up the piston with 

 its valve closed, thereby tending to produce a vacuum ; but th 

 pressure of the air upon the water not contained in the pump, 

 forces more water up into the part where a vacuum would 

 otherwise be formed. Then, when the handle is raised, and the 

 piston forced downwards, the valve opens, and the water rushes 

 through. 



There is a second valve, below the piston, which closes with the 



