THE BEASOff WHT. 



' For he looketh to the ends of the earth, and seeth under the whole heaven; 

 To make the weight for the winds." JOB xxvm. 



ness, although the sun's rays might be pouring through the darkened space, to 

 illuminate the atmosphere. Upon the summit of Mont Blanc, the report of a 

 pistol at a short distance can scarcely be heard. When Gay Lussac reached the 

 height of 23,000 feet, he breathed with great pain and difficulty, and felt dis- 

 tressing sensations in his ears, as though they were about to burst. Upon the 

 high table-lands of Peru, the lips of Dr. Ischudi cracked and burst ; and blood 

 Uowed from his eyelids. 



641. What is the amount of atmospheric pressure at the 

 earth's surface ? 



The pressure of the atmosphere at the earth's surface is fifteen 

 'pounds to every square inch of surface. That is to say, that the 

 column of air, extending fifty miles over a square inch of the earth, 

 presses upon that square inch with a weight equal to fifteen 



642. Is that the weight of dry or moist air? 



That is the weight of air at what is called the point of saturation, 

 when it is fully charged with watery vapour. 



643. WJiat is the proportion of watery vapour in the 

 atmosphere ? 



The proportion constantly varies. Evaporation is not a result of 

 accident ; it seems an established law that the air shall constantly 

 absorb vapour until it has reached the maximum that it can hold. 

 Experiments have been tried, in which dry air has been pressed 

 upon the surface of water with great force, but no degree of 

 pressure could prevent the formation of vapour. (See 431.) 



644. What is the total amount of atmospheric pressure on 

 the earth's surface ? 



The total amount of atmospheric pressure on the earth's surface, at 

 151bs. to the square inch, amounts to 12,042,604,800 ,000,000,0001bs. 

 This pressure is equal to that of a globe of lead of sixty miles in 

 diameter. 



645. What is the pressure of the atmosphere upon the 

 human body? 



Estimating the surface of man's body to be equal to fifteen 

 square feet, he sustains an atmospheric pressure of 32,4001bs., or 

 nearly fourteen tons and at-half. The mare variation of weight, 



