158 



ATMOSPHERIC PRESSURE. 



computed in the same way as for liquids ( 226, 228 and 

 231). vOwing to the great compressibility of aeriform 

 bodies, the lower layers of the atmosphere are much more 

 dense than the upper ones, but density and pressure alike 

 are constant in value throughout any horizontal layer. 

 The weight of a column of air one inch square extending 

 from the sea-level to the upper limit of the atmosphere is 

 about fifteen pounds; a similar column, a cm. square, 

 weighs about 1 Kg. "We express this by saying that the 

 atmospheric pressure at the sea-level is fifteen 

 pounds to the square inch) or 1 Kg. to the sq. cm. 

 Several illustrations of atmospheric pressure will be given 

 after we have considered the air-pump. 



274. Torricelli's Experiment. The intensity of 

 this pressure may be measured as fol- 

 lows: Take a glass tube a yard long, 

 about a quarter of an inch in internal 

 diameter. Close one end and fill the 

 tube with mercury. Cover the other 

 end with the thumb or finger and in- 

 vert the tube, placing the open end 

 in a bath of mercury. Upon removing 

 the thumb, the mercury will sink, 

 oscillate, and finally come to rest at 

 a height of about 30 inches, or 760 

 mm., above the level of the mercury 

 in the bath. This historical experi- 

 ment was first performed in 1643, 

 by Torricelli, a pupil of Galileo. 

 The apparatus used, when properly 

 graduated, becomes a barometer. FIG. 94. 



