230 



PRESSURE OF THE ATMOSPHERE. 



remain in the vessel, maintained by atmospheric 

 pressure, as long as the mouth is kept horizontal, that 

 is, as long as at all points the pressure of the air and 

 that of the water are in equilibrium, as shown in fig. 

 158 A] but if the jar be held somewhat inclined, as in 

 1581>, then the depth of the liquid particles varies 

 from point to point ; at a the downward pressure of water 



FIG. 157 (real size). 



FIG. 158 (| real size). 



is less than the upward pressure of the atmosphere, 

 at b it is greater, hence the air forces its way into the 

 vessel at a, while the water drops out of it at b. 



If a bottle which is quite full be closed with 

 the finger and inverted into a vessel contain- 

 ing water, as in fig, 159, no water will leave 

 the bottle when the finger is withdrawn ; for 

 the pressure of the air upon the surface of 

 the water in the tumbler is transmitted 

 through the liquid in all directions and main- 

 ^ ams the water in the bottle. If the latter 

 has a narrow neck (5 mm or less, as in a small 



Fir 159 



medicine bottle), the water and air cannot conveniently 

 pass by one another, and the water will remain in the 

 bottle even if the mouth is not immersed. 



The pressure of the air is so considerable that, 



