The cloud, as a sort of "pillar by day," has conducted him to the spot, whilst it has, at the same time, warned him of his danger. But the cloud--how came it there, why does it remain so faithfully at its post, and what are its functions? One of the most beautiful of the phenomena of tropical countries is the alternation, with the regularity of clock-work, of the land and sea breezes; by day, the sea breeze blowing toward the land, and by night the land breeze blowing toward the sea. The reason of this is as follows. The land absorbs heat, and radiates it, more rapidly than the sea. The consequence is, that when the sun has risen, an hour or two, the land becomes warmer than the surrounding sea, and there is an in-draught toward it; in other words, the sea breeze begins to blow. When, on the contrary, the sun has set, and withdrawn his rays from both land and sea, and radiation begins, the land, parting with its absorbed heat, more rapidly than the sea, soon becomes cooler than the sea. As a consequence, there is an out-draught from the land; in other words, the land breeze has commenced to blow. The reader now sees how it is, that the "pillar by day" hangs over the little coral island; the bank of coral absorbing heat by day more rapidly than the surrounding sea, there is an in-draught setting toward it, and as the lazy trade-winds approach it, they themselves become heated, and ascend into the upper air. There is thus a constantly ascending column of heated atmosphere over these banks. This ascending column of atmosphere, when it reaches a certain point, is condensed into cumuli of beautiful, fleecy clouds, often piled up in the most fantastic and gorgeous shapes. It is thus that the cloud becomes stationary. It is ever forming, and ever passing off; retaining, it may be, its original form, but its nebulæ constantly changing.
When a cooler blast of trade-wind than usual comes along, the condensation is more rapid, and perfect, and showers of rain fall. The sea-birds are already hovering, in clouds, over the inchoate little island, fishing, and wading in its shallow waters, and roosting on it, when they can get a sufficient foothold. Vegetation soon ensues, and, in the course of a few more ages, nature completes her work.