580 LAWS OF TEMPERATURE. 



of deserts, which we might call the Deserts of Cold, since the coldness 

 of the climate is the dominant cause which in both renders the soil 

 more and more unproductive and uninhabitable. In effect, it is not 

 only in departing from the Tropic Zone that we see the mean tem- 

 perature gradually sinking even to the point whereat all liquids con- 

 geal and all terrestrial life becomes impossible. The same pheno- 

 menon occurs in proportion as we ascend in the atmosphere. It is 

 a consequence of the properties of the gaseous medium which envelops 

 our globe, and takes place in obedience to certain laws which science has 

 been able bo ascertain and define. We know now that the decline of 

 the temperature is always in proportion to the elevation of places or of 

 the atmospheric strata ; but the value of the relation which exists be- 

 tween the two terms may be modified by various circumstances such 

 as the direction of the prevailing wind, the hygrometrical state of the 

 atmosphere, the hour of the day, and particularly the climate, or, to 

 speak more exactly, the thermic latitude. The warmer the climate, 

 the more sensible the difference between the temperature of the air 

 at the level of the sea and that which we observe at a certain height ; 

 greater, nevertheless, is the height to which we must rise to find the 

 region where the thermometer never descends below 0, and where, con- 

 sequently, the snows and ices of the mountains do not melt in any season. 

 As a mean, we estimate every 580 feet of elevation in the Torrid 

 Zone as equal to one therm ometrical degree, and in the Temperate 

 Zone at one degree for every 450 feet, the cooling of the air. That 

 is, for every 580 feet in the one instance, and every 450 feet in the 

 other, as we ascend above the sea's level, the temperature decreases 

 one degree. In the Polar regions the decrease of temperature is 

 insensible up to a certain height, which has not yet been ascertained. 

 At Ingloolich, in 69 21' north latitude, Captain Parry flew a kite to 

 a height of 400 feet, with an a minima thermometer attached. At 

 this elevation the temperature of the air was 31 below zero, or the 

 same as on the ice-fields of the sea. Humboldt counted one degree 

 of declination for every 550 feet on Chimborazo. De Saussure 

 obtained one degree for every 440 feet on Mont Blanc. 



