METEOROLOGY. 191 
increase on an average amounting to about one degree for every forty or 
fifty feet, although the exact law of this increase has not yet been determined. 
Its cause is to be found in the original heat of the earth, which, radiated from 
the surface, is still retained in the centre. Should the heat increase in the 
same ratio with increasing distance from the surface, water would boil at a 
depth of 10,000 feet, and granite would melt at a depth of five miles; 
consequently, the heat at the centre of the earth must be so great as to melt 
even the most refractory bodies. ‘The reason that this heat is not sensible 
at the earth’s surface is to be found in the badly-conducting character of her 
crust. 
The opposite phenomenon, a decrease of heat, is observed as we ascend 
in the air, the higher Jayers being colder than the lower; nevertheless, the 
diminution of temperature is not exactly proportional to the elevation. As 
the lower strata of air become heated by contact with the earth, they 
ascend, and expanding as they ascend, a great amount of heat is rendered 
latent; the temperature is thus necessarily reduced. On high mountains, 
particularly those of Central and South America, this decrease in 
temperature is well shown by the change in the vegetation as we ascend. 
Thus on the slope of a single mountain we may pass in comparatively few 
hours from the climate and vegetation of the tropics to the stunted flora 
and icy temperature of polar regions. The precise diminution of 
temperature depends upon the character of the particular mountain on 
which it is observed. The variation of temperature experienced in 
ascending in a balloon differs from that on high mountains, the latter 
absorbing heat during the day, which is again radiated or transmitted to 
the incumbent layers, thus elevating the temperature. Elevation and other 
circumstances being equal, it is warmer on elevated planes and connected 
mountain ranges than on isolated mountains ; on the former the periodical 
_ variations of temperature are also greater. Gay Lussac, in his celebrated 
balloon ascension, found that the temperature diminished on an average 
about one degree Fahrenheit for every 615 English feet of ascent. In the 
Cordilleras the elevation necessary for a decrease of one degree F. in 
temperature was about 300 feet (French) from 0 to 3000 feet of elevation ; 
444 feet from 3000 to 9000 ; 228 feet from 9000 to 12,000; and about 306 
feet from 12,000 to 15,000: the average of all was thus about 318 feet to 
the degree. From a comparison of the temperature of Geneva and the St. 
Bernard, an elevation of 335 feet was found to correspond on the average 
to a decrease of one degree of temperature ; nevertheless, this ratio varies in 
different months very considerably. In other regions of the Alps the ratio 
is less than that just mentioned. 
At a certain height above the level of the sea, varying with the latitude, 
a temperature prevails so low in degree, that ice and snow once formed do 
not melt, but remain throughout the year. This snow line, or the greatest 
height up to which snow can melt, is higher as we approach nearer the 
equator. The following table of snow lines is taken from Humboldt ; the 
elevations are in English feet. 
