54 
rOPULAK SCIENCE REVIEW. 
A very curious condition of the conversion of falling snow into 
ice sometimes occurs in those severe hail-storms, in which really 
oval-shaped masses of ice fall to the earth, doing great damage. 
If those are examined, they will be found to have a nucleus of 
snow, and over this a layer, sometimes two or three layers, of 
ice (see fig. 33). These have evidently been formed rapidly, 
and there is no doubt but that they are the result of a refrigera- 
tion of layers of air b}^ electrical disturbances. Snow has been 
first produced ; then, the falling flakes have passed through air 
saturated with cold moisture, and lastly through air below the 
freezing temperature. It has been already shown, that the snow 
which clothes the mountain-tops with a permanent robe of 
whiteness, has been lifted to those heights from the surface of 
the ocean by the action of the sun’s rays. Water has been 
vapourised by the solar heat, and borne to the upper regions of 
the air, where, meeting with a temperature below the freezing 
point of water, it is condensed as snow. The limits of perpetual 
snow are fixed by the temperature of those elevated regions, 
and of course the snow-line varies greatly as we pass from the 
equator towards the pole. 
Humboldt fixed the altitude of perpetual snows under the 
equator at 15,748 feet. On the northern sides of the Himalaya 
mountains it is about 17,000 feet, and on Chimborazo 15,802 
feet. On the Alps and the Pyrenees it is about 8,850 feet; at 
the North Cape, in latitude 71°, it is estimated at little more 
than 2,000 feet. But, beside the constant data of latitude and 
elevation, the position of the snow-line depends on variable 
causes ; such as the form of summits, the comparative altitude, 
and other physical features, of the surrounding country; the 
particular exposure of the mountains, and even the character of 
the neighbouring vegetation ; therefore no general rule can be 
given for fixing the limits of perpetual snow in any given latitude. 
Our own temperate island, upon which we have no mountains 
high enough to be constantly snow-clad, is an example in illustra- 
tion of this. The evidence exists, which tells us with unmistakable 
force, that there was a time, however, when the mountains of 
Scotland and Wales were within the limits of the line of per- 
petual snow — when, indeed, glaciers moved down the vale of 
Llanberris, and, according to some geologists, scooped out the 
lakes of North Wales and Scotland. We are not in a position 
to say at what period this state of things existed, but we do 
know under what conditions it might be renewed. 
The present temperature of the British Isles is mainly due to 
the action of the Gulf stream. This great river of water, flowing 
through the sea, comes warmed by a tropical sun from the 
shores of Central America, and washes our land. It is found in 
the winter, that, off the Scilly Islands, the western coast of 
