CHAPTER VII 



THE ATMOSPHERE 



145. The Ocean of Air. We live and move at the bottom 

 of a shoreless ocean of invisible fluid to the surface of which 

 we are powerless to rise. The existence of this ocean is 

 revealed to us by its power of exercising pressure, but the 

 substance composing it was long supposed to have no 

 weight, and the phrases "light as air," u an airy nothing" 

 are remnants of that idea. The simple experiment of 

 inverting a tumbler over a cork floating in a basin of water 

 shows that air can exert pressure and that it occupies space. 

 .By means of the air-pump a glass vessel can be nearly 

 emptied of air, and on weighing it before and after empty- 

 ing, it is ascertained that a pint of air has the mass of about 

 10 grains, or a cubic foot that of i^- ounce. 



146. The Barometer. Torricelli, an Italian mathema- 

 tician of the seventeenth century, when investigating the 

 action of the common sucking-pump, made a discovery which 

 laid the foundations of scientific knowledge of the atmo- 

 sphere. He took a tube closed at one end and about 33 

 inches long, filled it with mercury, and placing his thumb on 

 the open end inverted it (Fig. 20) in a basin of mercury. 

 The column of mercury in the tube sank gradually and 

 stood just 30 inches above the level of the mercury in the 

 basin. Mercury placed in a tube open above and below 

 and set in the same manner would immediately run out by 

 its own weight. Torricelli argued that the only difference 

 in the mercury in the closed tube was that the weight of 



