The Weather 11 



the Eustachian tubes are opened, admitting air and equalizing 

 the pressure upon both sides. 



At very high altitudes, as on mountains, the pressure is not 

 quickly and easily equalized, and much discomfort and even 

 illness may result. This is due to the expansion of the air in 

 the blood in response to the decreased external pressure upon 

 the body. But the important fact is that air in usual situations 

 exerts pressure equally and in all directions, and because it is 

 equal and in all directions, the pressure is not felt. The pres- 

 sure is felt only when it is suddenly made unequal. 



9. Pressure exerted by air. How great is the pressure of 

 the air upon things ? You may get an idea by covering a glass 

 full of water with a sheet of paper and then inverting the glass. 

 The air pressure upon the paper is enough to support the water 

 in the glass. This shows that pressure is upward as well as 

 downward. A simple experiment will illustrate the important 

 fact. 



Exercise : Showing air pressure. Connect a flat-sided tin can 

 by a piece of rubber tubing with an air pump and exhaust the air 

 from the can. The effect of the pressure of the air upon the outside 

 surface of the can is evident. Or this experiment may be performed 

 more easily, by putting a very small quantity of water in a can that 

 may be tightly stoppered with a rubber stopper. Bring the water 

 to boiling and when the steam is issuing freely from the can, cork it 

 securely. Cool and observe the result. The air is expelled by the 

 steam which, condensing to water, leaves a partial vacuum in the 

 can. Another illustration of the fact of air pressure may be had in 

 this way. An ordinary electric incandescent bulb incloses a fairly 

 complete vacuum. If the glass near the brass part is softened by a 

 hot flame, it cannot withstand the outer pressure of the air and is 

 forced inwards. These simple experiments show clearly that the air 

 exerts pressure. 



That the air has weight has been shown by many scientific 

 experiments, and the weight of a given volume of air is known 

 definitely. A strong vessel with a capacity of one cubic foot 

 is weighed with its air content ; the air is removed by an air 



