W. Hopkins on Changes of Climate. 251 
itis important for us to notice, in order that we may be able the 
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, 
us suppose a case in which the temperature should be entire- 
ly 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 de- 
pend ceteris paribus on the quantity of snow produced in the 
atmospheric region directly over that upper portion of the moun- 
tain which should be bounded by the snow-line. If the quanti- 
ty of snow thus formed and falling on the mountain should be 
very small, the destructive causes would not allow it to remain 
Permanently at so low a level as that of the line of freezing tem- 
siow-line. On the contrary, if a comparatively large quantity 
of snow should fall on the mountain, the snow-line might de- 
Scend to a considerable distance below that of freezing temper- 
But let us now suppose the annual temperature to vary from 
summer heat to winter cold, the mean annual temperature re- 
maining the same. It is manifest that the variable snow-line du- 
ting the year would in winter be below, and in summer above 
the permanent snow-line of the previous case, the extent of this 
8scillation being proportionate to that of the temperature during 
the year, But the highest or summer position of this variable 
how-line is what is properly called the snow-line. Thus, while, 
wha temperature in which the variation from summer to win- 
ter 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 
to the line of 32°, the conditions which produce its low- 
“st positions are those which secure a moist atmosphere with an 
-boximately equable annual temperature. Comparing places 
in the same latitude, an insular position will have a lower posi- 
i € snow-line than a continental one. In comparing a 
i. Hear the equator with one in the higher latitudes, there will 
4; uch greater humidity in the atmosphere and much less vari- 
tion in the annual temperature in the former case than in the 
later, and both these causes tend roduce a much lower posi- 
