IVeal/ier, U\Ui'r uiuf Disease. 15 



chanj^es, the barometric pressure is suddenly lowered to what 

 it would be at a much higher level, and that it, therefore, 

 comes in as one element in tlie uncomfortable effects notice- 

 able in a good many people during storms. The storm effect 

 is the same in kind, if not in degree, as the altitude rarefac- 

 tion and diminished ox\gen brings about. There is an 

 approximate standard of air pressure in every locality. If 

 there be sudden and considerable change from it by the 

 approach and movements of storm, it contributes its share to 

 the healthy or unhealthy effect. As a prognostic, a sudden 

 change means a sudden reaction. The capacity of living 

 under extreme \-ariations from sea level to 15,000 feet or more 

 is demonstrated by well known facts, but Jourdanet, the 

 French writer on climatology, after his own experience and 

 observation at high levels in Mexico, where he lived, says 

 that people living above 8,00c feet are not health}'. 



HUMIDITY. 



Where there are land and water, and an atmosphere coexist- 

 ing, with the solar rays pouring their heat down upon them, 

 there must be humidity or the vapor of water in that atmos- 

 phere. The direct genesis is from the water and heat. The 

 land is a receptacle and reservoir for the precipitation of the 

 vapor in the form of rain, and returns the vapor to the atmos- 

 phere in varying degrees and rates. The condition of a rapid 

 return is that the radiant heat shall not be obstructed by 

 bodies of visible vapor, by fogs, or clouds. The dryer the 

 superimposed air the quicker the radiant heat passes into the 

 space above, and the lower the temperature of the ground and 

 layers of air immediately above become. The barometric 

 pressure is greater and oxygen more abundant. If fog, mist 

 or cloud overlie the ground, they absorb the radiant heat, and 

 the earth and atmosphere are warmer. Rapid evaporation 

 means cooler temperature and dryer air — hence, the great 

 difference between sun and shade and liability to nocturnal 

 chill. A cloud passing between the sun and a thermometer 

 not suspended directly in the sun will sometimes produce an 

 increase of heat in a few minutes, because of a diminution or 

 suspension of radiant heat.-'' Tyndall's graphic statement as 



•TyndaU, p. 428. 



