Upwards of fifty geysers have been counted in the space of a few acres at the southern end of the great valley. Some are constant, some periodical, some stagnant, some only slightly agitated. The grandest and most celebrated are the Great Geyser and Strokr, thirty-five miles north-west from Hekla. These, at regular intervals, hurl into the air immense columns of boiling water, to the height of one hundred feet, accompanied by clouds of steam and deafening noises. In the case of the Great Geyser, the jet issues from a shaft about seventy-five feet deep, and ten in diameter, which opens into the centre of a shallow basin, about one hundred and fifty feet in circumference. The basin is alternately emptied and filled: when filled, loud explosions are heard, the ground quivers, and the boiling water is forced upwards in gigantic columns. Thus the basin is emptied, and the explosions cease until it is refilled. Messrs. Descloiseaux and Bunsen, who, according to Mrs. Somerville, visited Iceland in 1846, found the temperature of the Great Geyser, at the depth of seventy-two feet, to equal 260° 30’ F. prior to a great eruption, reduced, after the eruption, to 251° 30’ F.; an interval of twenty-eight hours passing in silence. [Illustration: THE GREAT GEYSER.] About one hundred and forty yards distant is the Strokr (from _stroka_, to agitate), a circular well, forty-four feet deep, with a tube eight feet wide at its mouth, diminishing to little more than ten inches at a depth of twenty-seven feet. The surface of the water is in constant ebullition, while at the bottom the temperature exceeds that of boiling water by about twenty-four degrees. It appears, from experiments made by Donny, that water, long boiled, becomes more and more free from air, and that thus the cohesion of the particles is so much increased, that when the heat is sufficiently increased to overcome that cohesion, the production of steam is so considerable and so instantaneous as to induce an explosion. In this circumstance M. Donny finds an explanation of the phenomena of the Geysers, which are in constant ebullition for many hours, until, being almost purified from air, the intense internal or subterranean heat overcomes the cohesion of the particles, and thus an explosion takes place.