The desert has its own meteorology; it is the theatre of peculiar phenomena, which one observes in no other part of the globe. Its climate, at least in the sandy region, is remarkably uniform; it varies only, according to latitude, in a greater or less elevated thermometrical mean. Hippocrates, the ancient philosopher, rightly called "the Father of Medicine," states the three elements of climate to be, the atmosphere, the soil, and the waters. Throughout the desert these are identically similar, and consequently originate identically similar phenomena. The atmosphere, in fact, is everywhere of an almost unchanging purity. It is only in the neighbourhood of mountains that clouds accumulate, to spend themselves at periodical seasons in more or less abundant rains. In the plains it never rains, and during the day no veil is interposed between the earth and the sun's burning glare, nor during the night do any refreshing dews weaken the force of the terrestrial radiation. There result constant alternations of devouring heat while the sun is above the horizon, and of rapid and frequently intense cooling when he has disappeared. The soil is everywhere as smooth as "the liquid main." This uniformity contributes, in addition to its silicious, argillaceous, or calcareous character, to render more abrupt the changes of temperature which occur from morning to evening and from evening to morning. In truth, the earth reflects the sun's heat in proportion as it receives it; it absorbs but insignificant quantities, which it loses in a few minutes when the calorific source begins to fail. On the other hand, in these immense plains where no inequality of surface can oppose the atmospheric movements, the wind acquires an increasing force and swiftness, _vires acquirit eundo_, and soon assumes all the characteristics of a tempest. Hence arise those terrible typhoons, those appalling hurricanes, of whose destructive effects history records so many instances, and of which I shall presently be called upon to speak. As for water, we have seen that its entire absence is a characteristic feature of the Sandy Desert. To sum up, an overpowering degree of heat during the day,--a freshness, often even an excessive cold, during the night (in the Sahara the thermometer frequently rises above 120° F. at noon, and not infrequently sinks below 32° about two or three o'clock A.M.); an ever transparent and azure sky, "Darkly, deeply, beautifully blue;" the absence of rains and dews, of gales and thunder; but a frequent recurrence of terrible hurricanes: such is the meteorological constitution of the arid zone, which embraces all the northern districts of Africa, except the Mediterranean region--that is, from the snowy heights of Atlas to the fertile pastures of Soudan--and which extends in Asia from the west to the north-east, for all but one narrow belt, as far as the 119th meridian of longitude. * * * * *