_Theory of Geysers._--No satisfactory explanation of the phenomena of geysers was advanced till near the middle of the 19th century, when Bunsen elucidated their nature. Sir George Mackenzie, in his _Travels in Iceland_ (2nd ed., 1812), submitted a theory which partially explained the phenomena met with. "Let us suppose a cavity C (fig. 1), communicating with the pipe PQ, filled with boiling water to the height AB, and that the steam above this line is confined so that it sustains the water to the height P. If we suppose a sudden addition of heat to be applied under the cavity C, a quantity of steam will be produced which, owing to the great pressure, will be evolved in starts, causing the noises like discharges of artillery and the shaking of the ground." He admitted that this could be only a partial explanation of the facts of the case, and that he was unable to account for the frequent and periodical production of the necessary heat; but he has the credit of hitting on what is certainly the proximate cause--the sudden evolution of steam. By Bunsen's theory the whole difficulty is solved, as is beautifully demonstrated by the artificial geyser designed by J. H. J. Müller of Freiburg (fig. 2). If the tube ab be filled with water and heated at two points, first at a and then at b, the following succession of changes is produced. The water at a beginning to boil, the superincumbent column is consequently raised, and the stratum of water which was on the point of boiling at b being raised to d is there subjected to a diminished pressure; a sudden evolution of steam accordingly takes place at d, and the superincumbent water is violently ejected. Received in the basin c, the air-cooled water sinks back into the tube, and the temperature of the whole column is consequently lowered; but the under strata of water are naturally those which are least affected by the cooling process; the boiling begins again at a, and the same succession of events is the result (see R. Bunsen, "Physikalische Beobachtungen über die hauptsächlichsten Geisire Islands," _Pogg. Ann._, 1847, vol. 72; and Müller, "Über Bunsen's Geysertheorie," ibid., 1850, vol. 79). [Illustration]