the animals become torpid with cold, so here, under the influence of the parching drought, the crocodile and the boa become motionless and fall asleep, deeply buried in the dry mud. . . . The distant palm-bush, apparently raised by the influence of the contact of unequally heated and therefore unequally dense strata of air, hovers above the ground, from which it is separated by a narrow intervening margin. Half-concealed by the dense clouds of dust, restless with the pain of thirst and hunger, the horses and cattle roam around, the cattle lowing dismally, and the liorses stretching out their long necks and snuffing the wind, if haply a moist current may betray the neighbourhood of a not wholly dried-up pool. ... At length, after the long drought, the welcome season of the rain arrives; and then how suddenly is the scene changed! . . . Hardly has the surface of the earth received the refreshing moisture, when the previously barren steppe begins to exhale sweet odours, and to clothe itself with killingias, and a variety of grasses. The herbaceous mimosas, with renewed sensibility to the influence of light, unfold their drooping, slumbering leaves to greet the rising sun; and the early song of birds and the opening blossoms of the water-plants join to salute the morning."
327. Are the great deserts centres of circulation?—The arid plains and deserts, as well as high mountain ranges, have, it may well be supposed, an influence upon the movements of the great aerial ocean, as shoals and other obstructions have upon the channels of circulation in the sea. The deserts of Asia, for instance, produce (§ 299) a disturbance upon the grand system of atmospherical circulation, which, in summer and autumn, is felt in Europe, in Liberia, and away out upon the Indian Ocean, as far as the parallel of the 10th degree of south latitude. There is an indraught from all these regions towards these deserts. These indraughts are known as monsoons at sea; on the land, as the prevailing winds of the season. Imagine the area within which this indraught is felt, and let us ask a question or two, hoping for answers. The air which the indraught brings into the desert places, and which, being heated, rises up there, whither does it go? It rises up in a column a few miles high and many in circumference, we know, and we can imagine that it is like a shaft many times thicker than it is tall; but how is it crowned? Is it crowned like the stem of a mushroom, with an efflorescence or ebullition of heated air flaring over and
