Most deep wells have a lower origin than the zone of climate temperature, which in Britain is between 200 and 300 feet. It is well known that a regular increase in the tem , perature is observed after this zone is passed, equal to 1° of Palm for every 55 feet. As wells have a temperature corresponding to that of the strata from which they spring, it follows that the deeper the spring the higher will be its temperature Local conditions may affect the thermal state of springs, as in the ease of the Geysers in the active vol. clinic district in Iceland, and the warm springs near Naples; but where no such local influ• noes exist., the depth of the b«1 ft ore which thevater comes may be to some extent esti• mated by its temperature. Thermal springs occur iu Britain at Matlock (66° Fain%) and .Thixton (82') in, Derbyshire, at Bath (117°) in Somerset, and at Clifton (76°) in Gloucester. shire. Artificial communications have been opened with deep-lying strata, by which the water they contain has been brought to the surface, and in these the temperature is found to increase in proportion to the depth of fhe bore. See ARTESIAN WELLS. The most
remarkable thermal springs are the Geysers of Iceland, which are fully described under GEYSER.
• Intermittent springs are sometimes produced by the ebb and flow of the tide, as at Riclunond, where the rise at high water is seen in the wells which flow from the arena• eeous strata on the banks of the Thames; and sometimes they depend on the supply of rain-water. But there is a kind of spring the intermittcnces of which are believed to be owing to the structure of the internal cavities from which the supply is obtained. A large reservoir is fed by the rain percolating through the rock. It cr,mmunicates with the sur face by a siphon-shaped tithe. As long as the water in the reservoir is at a lower I.;vel than the arch of the siphon, no water can escape; but as soon as it reaches its level, the whole of the water in the eavity will be drawn off, the spring will then cease, and will only make its appearance when sufficient water has accumulated to permit the siphon again to act.