CHAP, vii The Atmosphere 99 



the atmosphere could not press upon it. He knew that in 

 a liquid at rest every point in the same horizontal plane 

 must be at the same pressure, so he argued that every point 

 in the line a b (Fig. 20) must be at the same pressure. 

 The points between c and d were pressed 

 upon by the weight of 30 inches of mercury, 

 but were free from the weight of the air, 

 while the points from a to c and d to b 

 were free from the weight of mercury, but 

 subject to the pressure of the weight of 

 the air. Thus the pressure of the atmo- 

 sphere on a given area is equal to the 

 weight of 30 inches of mercury, or 14^ 

 pounds on a square inch. This reasoning 

 proved that the atmosphere presses as 

 heavily on the Earth's surface as if it were 

 an ocean of mercury 30 inches deep, or, 

 since mercury is about 13^ times denser FIG. 20. - Mercurial 

 than water, an ocean of water 34 feet deep. Barometer and 



. 11 yard measure. 



Exact observation shows that the column 

 of mercury balanced by the atmosphere at sea-level over 

 the whole Earth averages 29.9 inches, and it is calculated 

 from this that the whole mass of the atmosphere is 5500 

 million millions of tons. Since the mercury tube enables 

 one to measure the weight of the atmosphere it has been 

 called the Barometer (see also 439). 



147. Pressure of the Atmosphere. Torricelli's experi- 

 ment made it clear that the piston of a common suction- 

 pump lifts the atmosphere from above the piece of water in 

 which the pipe dips, and that the pressure, of the atmosphere 

 on the rest of the surface forces up the water over that 

 space until the weight of the column is equal to the pressure 

 on an equal area of the free surface : this height never exceeds 

 about 34 feet, which is the limit of lifting power in a pump. 

 Air, and fluids generally, exert pressure equally in all 

 directions ; and on account of this uniform pressure of the 

 air all round us and through the tissues of our bodies, we 

 do not feel the pressure to which we are always subjected 

 of 1 4|- pounds on every square inch, or 1 4 tons for the 



