Another remarkable fact is, that, even in countries exposed to violent and frequent earthquakes, so many subterranean watercourses have remained unaltered for 2,000 years at least. The sources of Greece still flow apparently as in the times of Hellenic antiquity. The spring of Erasinos, two leagues south of Argos, on the declivity of the Chaonian mountains, is mentioned by Herodotus. At Delphi the Cassotis (now Wells of Saint Nicholas) still flow under the ruins of the temple of Apollo, and the hot baths of Aidepsos still exist in which Sylla bathed during the Mithridatic war. Many springs exhibit the singular phenomenon of an intermittence which is independent of the quantity of rain falling in the district, or of the flux and reflux of the tide in a neighbouring river. In many cases the simple and well-known hydrostatical law exemplified in the common siphon[3] affords a very ready and sufficient explanation of the phenomenon. Footnote 3: A siphon, as is well known, is a bent tube, having one leg longer than the other. When this tube is filled with any liquid, and the shorter end is immersed in a vessel containing liquid of the same kind, the weight of the column in the longer leg will cause the liquid to begin to run out, and it will continue running till the vessel is emptied. This arises from the pressure of air on the exposed surface of fluid, forcing it up through the tube to prevent vacuum, which would otherwise be formed at the highest point; and the extreme limit of length at which the siphon will act is therefore determined by the height of a column of the fluid equal to the pressure of the atmosphere (fifteen pounds on the square inch). The limit in the case of water is something more than thirty feet. [Illustration: siphon] In the annexed diagram the vessel _a_ communicates, by a tube _c_, with the siphon tube _b_, and it is manifest that when the water in a rises above the level of the top of _b_, it will begin to flow over and escape, as at _d_. But as soon as this is the case the tube _b_ begins to act as a siphon, and draws off all the water in _a_, so that if a constant supply is poured into _a_, but at a rate slower than the rate of the discharge at _d_, there will be an intermittent discharge, the interval depending on the relation of the rate of filling to that of emptying. [Illustration: SECTION OF AN INTERMITTENT SPRING.] The case of a subterranean cavity in a limestone rock, slowly fed by drainage from the cracks and fissures of the rock above, and communicating at a distant point with the surface by a bent or siphon tube, is evidently strictly analogous. [Illustration: GEYSIRS OF ICELAND.]