A very broad belt of the earth’s surface is occupied by countries that may be said to possess hot climates; as they include not only those within the Tropics, but also the sub-tropical zones. Practically speaking, the whole of Africa, much of South America, the Southern States of North America with the West Indian Islands, Asia Minor, Arabia, Persia, India and the Malay Peninsula, the greater part of China, Australia and the islands lying between it and the continents of the northern hemisphere, may be said to be included within this general term. In so wide an area, it is needless to remark that the widest range of climatic conditions may be found, the only condition common to all being that of subjection to a fiercer heat than that to which we Europeans are accustomed, for some or all the months of the year. It must be remembered, too, that climate is determined not only by the relation of a place to the parallels of latitude, or to speak more exactly, to isothermal lines, but is also affected by the elevation of a site above the sea. Even under a vertical sun, an ascent of 18,000 feet will land the climber in an absolutely arctic climate, as far as the temperature of the air is concerned; but it is only in this one item that we have any similarity to arctic conditions, as the sun’s rays blaze down even more fiercely than they can at the sea-level, where they have been tempered by passage through miles of denser air and watery vapour. On the crest of the Wakujrui Pass, at 16,500 feet, while the air temperature at noon stood at 20° below freezing point, the sun thermometer registered 165°, in May, 1886, when the writer was crossing the great divide between India and Central Asia; and yet it was sufficient to remove one’s hat for a minute to realise that to do so might result in sunstroke. Moreover, apart from the effects of the rarification of the air on respiration, radiation was so rapid as to be painfully apparent; one side of the hand turned to the sun would be scorched, while the other chilled so rapidly that the sensation conveyed was that of being in contact with a cold liquid; and one was constrained to wrap up even the face as closely as possible, though the air was fortunately well-nigh still, whereas cold of similar severity at the sea-level is quite tolerable as long as there is no wind. Thus, in the consideration of a given climate, not only geographical position, but also elevation above the sea must always be taken into account.