They may be summed up, in Lyell's words, as different combinations of sea and land, for on these depend the secondary conditions which affect temperature. Thus elevation of land is as certain a cause of cold as high latitude, and even Kilimanjaro, under the equator, retains patches of unmelted snow throughout the year. It is estimated that a rise of 1000 feet in height is about equivalent to a fall of 3° F. in mean annual temperature, and that the line of perpetual snow is, on the average, a little higher than the line where this mean annual temperature is at 32° F., or freezing-point. If there is any mass of land so high as to be below this temperature, snow accumulates and forms glaciers, which descend some 4000 feet below the snow-line before the excess of ice pushing down is melted off by the summer heat unless it has been previously floated off in icebergs at a higher level. Now the mean temperature of the north of Scotland at sea-level is about 46° F., so that an elevation of 8000 or 10,000 feet would bring a great part of it well above the snow-line, and vast glaciers would inevitably accumulate, which would push down through the principal valleys almost to the sea-level; a state of things which actually exists in New Zealand, where glaciers from the Southern Alps at about this elevation descend, in some instances to within 700 feet of the sea-level, in the latitude of Devonshire. But a still more important factor of temperature is found in aërial and oceanic currents, which again, to a great extent, are a product of the configuration of sea and land. The most familiar instance is that of the Gulf Stream, which raises the temperature of Western Europe some 10°, and in Norway as much as 15° F., above that due to latitude, and which prevails on the other side of the Atlantic. The northern extremity of the British Islands in Shetland is on the same parallel of latitude as the southern extremity of Greenland, Cape Farewell. One is buried under perpetual ice, in the other there is so little frost in winter that skating is an unknown art. What is the reason of this? We must go to the tropics to find it. A vast mass of vapour is raised by the sun's heat from the oceans near the equator, which being lighter rises and overflows, the trade winds rushing in from the north to supply its place, and being deflected to the west by the earth's rotation. This prevalence of easterly surface winds sweeps the waters of the Atlantic to the west, where they are intercepted by South America, turned northwards into the Gulf of Mexico, where they circle round under a tropical sun and become greatly heated, and finally run out through the Straits of Florida with a rapid current, and spread a surface return current eastwards over the Northern Atlantic. The shores of North-west Europe are thus in the position of a house warmed by hot-water pipes, while their neighbours over the way in North-eastern America have no such apparatus.