174 



WELLS'S NATURAL PHILOSOPHY. 



the liquid layer adhering to an orange after it has been dipped in water, ia 

 to the entire mass of the orange. Of the whole bulk of the atmosphere, the 

 zone, or layer which surrounds the earth to the height of nearly 2 3-4 miles 

 from its surface, is supposed to contain one half. The remaining half being 

 relieved of all superincumbent pressure, expands into another zone, or belt, 

 of unknown thickness. Fig. 163 will convey an idea of the proportion which 

 the highest mountains bear to the curvature of the eartli, and the thickness 

 of the atmosphere. The concentric lines divide the atmosphere into six layers, 

 containing equal quantities of air, showing the great compression of the lower 

 layers by the weight of those above them. 



Fig. 163, 



¥hW/jiii//'''M'''i'''/^'0>m'mmM'ii^^^^^^^^^^^ 



HIMiXATAB. 



Water is about 840 times the weight of air, taken bulk for 



bulk, and the weight of the whole atmosphere enveloping our 



globe has been estimated to be equal to the weight of a globe 



of lead sixty miles in diameter. 



If the whole air were condensed, so as to occupy no more space than the 



same weight of water, it would extend above the earth to an elevation of 



thirty-four feet. 



385. All aeriform, or gaseous substances, 

 like liquids, transmit pressure in every direc- 

 tion equally ; therefore, the atmosphere presses 

 upward, downward, laterally, and obliquely, 



with the same force. 



386. The amount of pressure which the at- 

 mosphere exerts at the level of the ocean is 

 equal to a force of 15 pounds for every square 

 inch of surface. 



The surface of a human body, of average size, measures 

 about 2,000 square inches. Such a body, therefore, sustains 

 a pressure from the atmosphere amounting to 30,000 pounds, 

 or about 15 tons. 



The reason we are not crushed beneath so enormous a load, 

 is because the atmosphere presses equally in all directions, 

 and our bodies are filled with liquids capable of sustaining 

 pressure, or with air of the same density as the external air ; 



What is the 

 comparative 

 weight of the 

 atmosphere ? 



Hoir is the 

 pressure of 

 aeriform sub- 

 stances exert- 

 ed? 



What is the 

 amount of 

 pressure ex- 

 erted by the 

 atmosphere ? 



What pressure 

 is sustained 

 by the human 

 body J 



Why are we 

 not crushed by 

 the pressure of 

 the atmosphere? 



