42 National. Geographic Magazine. 



great elliptical area to the southwestward. The circulation of 

 the wind about these areas of low barometer, and the correspond- 

 ing changes of temperature, are indicated graphically on the 

 map: the isobars, or lines of equal barometric pressure, are, as a* 

 rule, somewhat circular in form, and the winds blow about and 

 away from an area of " high " in a direction with the hands of a 

 xoatch (in nautical parlance, " with the sun "), toward and about 

 " low " with an opposite rotary motion, or against the hands of a 

 watch; in front of a " low " there will therefore be, in extra tropi- 

 cal latitudes, warm southeasterly winds, and behind it cold north- 

 westerly winds, the resulting changes of temperature being shown 

 by the isotherms, or lines of equal temperature. Moreover, in a 

 cyclonic system of this kind the westerly winds are generally far 

 stronger than the easterly winds, the motion of the whole system 

 from west to east increasing the apparent force of the former 

 and deci-easing that of the latter. Upon reaching the coast, such 

 areas of low barometei*, or storm systems, almost invariably 

 develop a great increase of energy, largely due to the moisture 

 in the atmosphere overhanging the ocean, which, when the air 

 is chilled by contact with the cold dry 'air rushing in from the 

 "high," is precipitated and becomes visible in the form of clouds, 

 with rain or snow. The latent heat liberated by the condensa- 

 tion of this aqueous vapor plays a most important part in the 

 continuance of the storm's energy and, indeed, in its increase of 

 energy: the v/arm light air flowing in towards the central area 

 of the storm rises rapidl}'^ into regions where the pressure is less, 

 that is, where the thickness and consequently the weight of the 

 superincumbent atmosphere is less; it therefore rapidly expands, 

 and such expansion would result in a much more rapid cooling, 

 and a corresponding decrease in its tendency to rise still higher, 

 were it not for the latent heat liberated by the condensation of 

 the moisture which it contains. Thus the forces that are con- 

 spiring to increase the energy of the storm are powerfully 

 assisted by the presence and condensation of aqueous vapor, and 

 the increasing updraught and rarefaction are at once marked by 

 the decreasing barometric pressure at the center. For example^ 

 a storm was central over the Great Lakes on Jan. 25th, with 

 lowest barometer 29.7; the following day it was central off Kan- 

 tucket, barometer 29.2; and on the SHh and 28th, over the Gulf 

 of St. Lawrence, with barometer below 28.6. But such instances 

 are so common as to make it the rule, and not the exception. 



