THE ATMOSPHERE. 55 



which is, their feet are pressed down upon that surface 

 by the external atmosphere. On this principle, other 

 insects possess the power of locomotion on similar situ- 

 ations. The same law applies to the sea-horse, who is 

 thus enabled to climb perpendicular hills of ice ; and to 

 some kinds of lizard, who ascend vertical walls at plea- 

 sure. Many sea-shells depend on this law for their 

 tenacity to rocks. The animal has the power of expel- 

 ling the air between himself and the rock, whereby he 

 is pressed there with a force proportioned to his size. 

 Cupping, whether affected by a syringe, or by the re- 

 moval of the air by burning anything in the cup, be- 

 longs to the same law. And until within a very few 

 years, the steam-engine, the mightiest of all the inven- 

 tions of man, depended for its power on the pressure of 

 the air. 



Another evidence of the effect of this pressure is 

 shown in the process of boiling. We know by the 

 thermometer, that water boils at the surface of the earth 

 at 212, and that other liquids require a certain quan- 

 tity of heat to be absorbed, before they exhibit the 

 phenomenon of boiling. But we find that in proportion 

 as we rise from the level of the sea, these substances 

 do not require the absorption of so much heat, to exhibit 

 the same peculiarities. Water, for instance, boils on 

 Mont Blanc at 180 Fahrenheit; and ether, which boils 

 at the level of the sea at 98, cannot be retained there in 

 any form, if there be the slightest communication with 

 the atmosphere. This is produced by the diminished 

 pressure of the air ; and in fact, the boiling of water at 

 different heights, if properly attended to, would be one 

 means of ascertaining the height of mountains. 



A beautiful illustration of the removal of pressure 

 from the surface of water, may be exhibited by boiling 

 it in a common oil flask, and corking it tightly during 

 the ebullition. The glass being removed from the 

 fire, shows the continuance of the commotion, which 

 may be immediately checked by holding it near the fire 

 or dipping it into hot water. This very curious experi- 

 ment may be easily explained. The addition of heat 

 checks the commotion by expanding the vapor on the 



