The time required for the construction of the Great Geyser tube has been estimated by Commander Forbes as ten or eleven centuries, on the following grounds: a bunch of grass, placed under a little fall made by the ejected water, receives, in twenty-four hours, a coating of silica the thickness of a thin sheet of paper, or about one five-hundredth part of an inch. At this rate it would take 1036 years to build up the 762 inches, which, according to his measurement, is the depth of the tube. In evidence of the probable truth of this estimate he makes note of the following facts: first, there is no notice of this fountain in the early history of the colonization of the island 986 years ago, at which time the tube would have been only three feet deep, and its eruptions too slight to attract attention; second, 436 years afterwards, when the tube would have been twenty-six feet deep, and the eruptions proportionately important, the Geyser is mentioned; third, accurate records of all occurrences were kept by the early inhabitants, and if so remarkable a phenomenon had existed at the time, it could not have been left unnoticed. The phenomena attending a geyser-eruption--the filling of the basin with water, the agitation of the water, with deafening detonations, the escape of steam, and so on--have been sufficiently described in the preceding chapters. Their causes have been ingeniously explained by Professor Bunsen, who succeeded in determining the temperature of the geyser-tube, throughout its entire length, a few minutes before an eruption. The annexed diagram shows on the left the observed temperatures of the water at different depths, and on the right the temperatures at which water would boil, taking into account the pressure of the atmosphere increased by the presence of the superincumbent column of water. The degrees have been changed from Centigrade to our familiar Fahrenheit standard, disregarding fractions. [Illustration] It will be observed that in no part of the tube does the water reach the boiling point. The nearest approach to it is at A, thirty feet from the bottom; out even here the water is some four degrees below the temperature at which it could boil. How then is an eruption possible?