"Twice during the night," he says, "I was aroused by the unearthly complaints of _The_ Geyser; but beyond the vast clouds of vapor which invariably follow each detonation, and a gentle overflowing of the basin, they were false alarms. As morning was breaking it sounded an unmistakable 'reveille,' which would have roused the dead: and I had barely time to take up my position at the brink of the old 'Strokr' before full power was turned on. Jet succeeded jet with fearful rapidity, earth trembled and the very cone itself seemed to stagger under the ordeal. Portions of its sides, rent with the uncontrollable fury it had suddenly generated, were ripped off and flew up in volleys, soaring high above water and steam, whilst the latter rolled away in fleecy clouds before the light north wind, and catching the rays of the morning sun just glistening over the Jökul tops in the East, was lustrous white as the purest snow. Discharge succeeded discharge in rapid succession for upwards of four minutes, when, apparently exhausted and its basin empty, I scrambled up to the margin, intending to have a good look down the tube, which I imagined must also be empty; but the water was still within a few feet of the brink, and boiling furiously. Hastening back to my former position, the basin filled rapidly, and I was just in time to witness the most magnificent explosion of all. Everything seemed to depend on this superhuman effort, and a solid, unbroken column of water twenty-five feet in circumference, was hurled upwards, attaining an altitude very near 100 feet. Here the column paused for a moment before reversing its motion, then fell listless and exhausted through the volumes which followed it into its throbbing cup, again to undergo its fiery ordeal at the threshold of the infernal regions." Grand as this display must have been, it was but a momentary spasm, a feeble effort compared with the terrific force which sustains the Giant's river-volume, with a steady uprush two hundred feet high, for the space of three hours and a half. There are many, perhaps scores, of geysers in the Firehole Basin, which--even in midsummer, when their action is weakest--far surpass the glory of Iceland. But what is the origin of the power that sustains these wonderful eruptions? And what is the cause of its intermittent action? Fortunately these questions are not only answerable, but the answers are susceptible of demonstration, as Professor Tyndall has shown in his admirable lectures on heat considered as a mode of motion, wherein he gives the following lucid description of the mechanism and development of the Great Geyser of Iceland: in principle the description applies equally to the geysers of Firehole Basin, and all other springs of the kind.