i oo The Realm of Nature CHAP. 



whole body of a man of ordinary size. A common limpet 

 / weighing perhaps half an ounce sticks to a smooth level 

 rock as if its weight were from I o to 15 pounds, because the 

 soft tough foot is planted so closely on the stone as to 

 exclude all air from below and the pressure comes from the 

 outside only. The limpet sticks as firmly to a vertical 

 or an inverted surface as to a level one. The vacuum 

 brake is a powerful illustration of the pressure of air, for by 

 it the pressure of the atmosphere applied to a very small 

 part of the surface of a rapidly moving train brings it to a 

 stand in a very few minutes. 



148. Density of Air. The mass of the air has been 

 measured with great accuracy, but the height to which it 

 extends, the depth of our aerial ocean, is difficult to estimate. 

 If the density of the air ocean were uniformly the same as 

 it is at the Earth's surface (about -\- of the density of 

 water), its height would be five miles. That this is not the 

 case was proved by Mr. Glaisher, who once ascended more 

 than seven miles in a balloon and still found air around him, 

 though of much less density than at the Earth's surface. 

 But the fact was known by theory two centuries earlier. 

 Boyle, in 1662, announced the discovery of the law known 

 by his name : 



The density of any gas is proportional to the pressure it 

 supports. 



The pressure of the atmosphere produced by its own 

 weight is greatest on the Earth's surface or in a mine, where 

 the density is accordingly greatest also. As one ascends 

 in the atmosphere the pressure falls steadily, because less 

 air remains above, and the density of the remaining air is 

 consequently less. Thus the barometer can be used to 

 measure heights : near sea-level a fall of one inch in the 

 barometer corresponds to a rise of I ooo feet. One half of 

 the atmosphere lies beneath the height of 3^ miles, or 1 8,500 

 feet, from the Earth's surface, and the half which is above this 

 height can exert a pressure only equal to about 1 5 inches of 

 mercury at that level. Another rise of 3 J miles (to 7 miles) 

 leaves half of the half atmosphere below, and only one 

 quarter above, the pressure being equal to 7^ inches. At 



