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Show 218 THE THUNDER·STORl\r. but if the surface of the earth is flat, the lower sur· face of the cloud also continues flat till it is near the earth; and then its approach is not without danger, as the longer that the cloud holds together, the stroke is the more violent; but then, although more powerful, the flashes of lightning are fewer than if partial discharges took place. 'Vhen the discharges from an equal accumulation of cl<?ud are partial, there is little action between the d~fferent strata of the cloud, until there has been actwn between the lower stratum and the earth; and in those cases each stratum of cloud descends and thunders to the earth. At such times the curlings of the different parts of the cloud are very striking, for thev are so dense that they all seem solid, and as there is air between them, the openings appear to penetrate many miles into the sky, and yet it may happen that the most distant cloud is the blackest; as the lower ones, that have discharged their thunder, are melting in rain, and as they then allow a passage to the red light, the lower sky is exceedingly murky. The fall of the rain is often as fantastical. After each peal, which reverberates as if a stone arch were rattling down in pieces, the rain falls with the headlong rush of water when it bursts its barrier; but the rain is often over before the last echo of the thunder-clap has ceased. Yet the silence and cessation of rain are of very short duration; for it is barely fair, when another black mass descends, discharges its thunder, and lets fall its rain; and that is succeeded by another, and another, till the whole cloud is exhausted. Sometimes those splendid clouds sail majestically over without disturbing the atmosphere througb which they pass ; but when they do break, there are no atmospheric phenomena so sublime, or that imbody so much of varied information in so short a time. The impressiveness of thunder-storms renders them among the best studies for beginners in the observation of nature. There is often a sul- THE GEYSER. 219 phureous smell accompanying thunder, just as therf:l is accompanying earthquakes, which shows that there are other atmospheric ingredients acted on by the commotion of the water and the heat. When water remains on the ground in the liquid state, its operations are more open to unguided observation than when it is on its aerial passage from the sea to the land; and its natural uses there are much more profitably viewed in connexion with those substanc€s and productions to which it is useful. . But on the earth's surface, and even in cavities within the earth, heat has the same kind of effect npon it as when it is in the atmosphere. Boiling springs are among the most curious of these phenomena; and one of the most remarkable is the great Geyser in Iceland, which is a sort of natural steam engine, wbich, like some of the high-pressure engines, blows its steam into the air. THE GEYSER. Great part of Iceland is volcanic, and there is no doubt that there are many curious caverns that have be.en formed below ground, as there are in all volcanic PDtmtries. The Gevser appears to have a boiler, |