10 Introduction to the Study of Science 



the instrument by which the pressure of the atmosphere is 

 measured. 



Isobars are drawn without regard to altitudes, as on moun- 

 tains or high plateaus. The legend on the weather map is : 

 " Barometric readings are reduced to sea level and standard 

 gravity." Air pressure diminishes with ascent above sea 

 level, and it increases with descent below sea level, as in a 

 mine or shaft. This general fact is recognized by the observer 

 at any weather station when he adds to the actual pressure 

 of the elevated locality enough to equal the pressure which 

 the locality would have if it were at sea level. You may 

 correct weather map isobars for high altitudes, as explained on 

 page 15. 



Isotherms are similarly constructed without reference to altitude. 

 The snow line of high mountains must be at or below the freez- 

 ing point, 32 F. It would be impossible to give on weather 

 maps the temperatures of high elevations without distorting 

 the isothermal lines and giving facts which would be generally 

 misinterpreted. When you desire to obtain from a weather 

 map approximately the temperature of a high altitude, you 

 may subtract one degree from that recorded, for each 270 feet 

 of elevation (page 392). 



III. AIR PRESSURE 



One may find in personal experience evidence that the air 

 exerts pressure. In ascending a high mountain or descending 

 into a deep shaft, or again in riding up or down in a swift ele- 

 vator in a tall building, one may feel the effects of the change 

 of air pressure upon the ear drums. In the swiftly moving 

 elevator the discomfort experienced may be relieved by swallow- 

 ing. But what does the fact observed signify? Ordinarily the 

 pressure of the air is exerted equally upon both sides of the 

 ear drums and is therefore not felt. A sudden change in pres- 

 sure upon one side, as in an elevator ascending twenty to 

 thirty-five stories, causes discomfort. By the act of swallowing, 



