In nature it often happens that a layer of water collects between two strata, such as clay, through which water cannot penetrate. If the ground from which the water has been gathered is high, the pressure at the bottom of the layer will sometimes be very great; and on boring through the retaining bed, the pressure of the water will be sufficient to force it up the shaft to the surface of the ground, and in some cases to cause it to spring into the air from the mouth of the well. This is in accordance with the hydrostatic law that water rises to its own level. In Europe, this method of boring was first practiced in the ancient French province of Artois (hence the name Artesian); but it is now extensively applied in all parts of Europe, in America, and in other parts of the world. The artesian well at Grenelle, near Paris, is 1,798 feet deep; another at Passy, near Paris, is 1,923 feet deep. In America the borings reach a depth of from 2,000 to 3,000 feet. The water from these deep wells being always warmer than surface water, maintains a constant temperature in hospitals and manufactories, warms greenhouses, and reduces variations of cold in fish-ponds.