1851.] HOPKINS ON CHANGES OF CLIMATE. 79 



same authority as the above, the height of the snow-line 18,500 feet, 

 or upwards of 2600 feet above the former line. 



These calculated results as to the difference of heights of the 

 snow-line and that of the temperature of 32° may not of themselves 

 be entitled to much confidence, compared with those deduced from 

 observation ; but they show that the data on which the calculations 

 are founded are in accordance with the results of observation in 

 other cases. 



22. It appears from the preceding facts, that the height of the 

 snow-line, with reference to the hne of 32° F., increases, under 

 similar conditions, as we proceed northward from the equator. For 

 this phsenomenon we may assign two principal causes, which it is 

 important for us to notice, in order that we may be able the better 

 to understand the real analogies between actual cases of observation 

 and the hypothetical cases of past geological epochs. 



At places near the equator, and especially at great elevations, there 

 is little variation of temperature from one season to another. Let us 

 suppose a case in which the temperature should be entirely equable. 

 The snow-line would be absolutely stationary. Above this line the 

 snow would tend to accumulate by constant deposition, until this 

 tendency should be exactly counteracted by destructive causes, such 

 as the direct action of the sun's rays, evaporation, drifting by the 

 wind, avalanches, &c. The snow-line would be that beneath which 

 these antagonistic causes would cease to be in equilibrium, and its 

 position would manifestly depend cceteris paribus on the quantity of 

 snow produced in the atmospheric region directly over that upper 

 portion of the mountain which should be bounded by the snow-line. 

 If the quantity of snow thus formed and falling on the mountain 

 should be very small, the destructive causes would not allow it to re- 

 main permanently at so low a level as that of the line of freezing 

 temperature, which might in this case be considerably below the 

 snow-line. On the contrary, if a comparatively large quantity of 

 snow should fall on the mountain, the snow-line might descend to a 

 considerable distance below that of freezing temperature. 



But let us now suppose the annual temperature to vary from sum- 

 mer heat to winter cold, the mean annual temperature remaining the 

 same. It is manifest that the variable snow-line during the year 

 would in winter be below, and in summer above the permanent snow- 

 Hne of the previous case, the extent of this oscillation being propor- 

 tionate to that of the temperature during the year. But the highest 

 or summer position of this variable snow-line is what is properly called 

 the snow-line. Thus, while, with a temperature in which the varia- 

 tion from summer to winter should be comparatively small, the snow- 

 line should be below the line of freezing temperature, it might be far 

 above that line if the oscillation of temperature were great, although 

 the mean annual temperature should be the same in both cases. 



Considering then the position of the snow-line with reference 

 always to the line of 32°, the conditions which produce its lowest 

 positions are those which secure a moist atmosphere with an approxi- 

 mately equable annual temperature. Comparing places in the same 



