246 APPLICATIONS OF ATMOSPHERIC PRESSURE. 



a longer column of air to have the same weight and 

 to exert the same pressure. The diminution of the 

 pressure as we ascend in the atmosphere is therefore 

 not uniform, but proceeds at a rate which gets continually 

 slower and slower, and can only be exactly deter- 

 mined by taking all circumstances of the case into 

 consideration. The calculation required for this pur- 

 pose would exceed the scope of this work; but not 

 only can the diminution of the atmospheric pressure 

 from one point to a higher one be precisely calculated, 

 but, conversely, from the observed difference in the 

 atmospheric pressure at two places the difference in their 

 heights may be determined. On this fact rests the appli- 

 cation of the barometer to the measurement of heights. 



o 



27. Applications of Atmospheric Pressure and of 

 Mariotte's Law. The pressure and elasticity of the air, 

 varying in accordance with Mariotte's law, are applied in 

 a variety of mechanical contrivances, depending on the 

 mutual action of gases and liquids, or of gases, 

 liquids, and solid bodies. 



Fig. 169 is a flask or balloon of glass, 

 containing air and water, the latter filling 

 about one third, or one half of the flask. 

 The neck is closed by a cork through 

 which passes a tube, which reaches 

 nearly to the bottom of the flask ; the 

 projecting end of the tube is drawn out to 

 a fine aperture. If the air in the flask is 

 compressed by blowing strongly into the 

 tube, and the latter is then withdrawn 

 [ real size). f rom the mouth, a jet of water issues 

 from the tube which has at first a height of perhaps 

 l m or more, but gradually diminishes in height until 



