HYDROSTATICS. 



135 



Fig. 120. 



What is an Ar- 

 tesian Well ? 



304. 



ficient strength to bear with security a bursting pressure of nearly 5 tons for 



each foot of its length. 



Upon the principle that water tends to rise to the level of its source, orna- 



Biental fountains may be constructed. Let water spout upward through a pipe 

 communicating with the bottom of a deep vessel, and 

 it will rise nearly to the heiglit of the upper sur- 

 face of the water in the vessel. The resistance of 

 the air, and the falling drops, prevent it from rising 

 to the exact level. Let A, Pig. 120, represent a 

 cistern filled %vith water to a constant height, B. 

 If four bent pipes be inserted in the side of the 

 cistern at different distances below the surface, the 

 water will jet upward from all the orifices to nearly 

 the same level 



The phenomena of Artesian "^ells, and the plan 

 of boring for water, depend on the same principle. 



An Artesian Well is a cylindrical 

 excavation formed by boring into the earth 

 with a species of auger, until a sheet or vein of water is 

 found, when the water rises through the excavation. Such 

 excavations are called Artesian, because this method was 

 employed for obtaining water at Artois in France. 



,„^ , ,^ The reason that the water rises in Artesian, and sometimes 



Why does the 



water rise in m ordinaiy wells, to tne surface, is as follows : The surface 



w ,j,'^''®^'*'^ of the globe is formed of different layers, or strata, of diflerent 

 materials, such as sand, gravel, clay, stone, etc., placed one 

 upon the other. In particular situations, these strata do not rest horizontally 

 upon one another, but are incUned, the different strata being like cups, or 

 basins placed one within the other, as in Fig. 121. Some of these strata are 

 composed of materials, as sand or gravel, through which water will soak most 



readily; while other strata, 

 like clay and rock, will not 

 allow the water to pass 

 through them. IfJ now, 

 we suppose a stratum like 

 sand, pervious to water, to 

 be included as at a a, Fig 

 121, between two other 

 strata of clay or rock, the 

 water falling upon the un- 

 covered margin of the sandy stratum a a, will be absorbed, and penetrate through 

 its whole depth. It will be prevented from rising to the surface by the im- 

 pervious stratum above it, and from sinking lower, by the equally impervious 

 Btratum below it. It will, therefore, accumulate as in a reservoir. If, now, we 



Fig. 121. 



