ter from the upper stratum, which does not attain so high a level, passes up through the outer pipe to aa; by this means, should the water from the lower stratum be pure and that from the upper impure, the former may be brought up and discharged separately without being mingled with or contaminated by the former.
Both these streams are used for supplying the canal basin at St. Ouen, which is above the level of the Seine.
The well at Calais is 1,138 feet, and that at Douchery, in the Ardennes, France, 1,215 feet, in depth.
The English wells are of less depth, varying from 70 or 80 to 620 feet. The fountains in Trafalgar Square, London are supplied by wells of this kind, 393 feet deep.
Those of London are all in the chalk, and it is believed that by deeper boring, so as to reach either the upper or lower green-sand formations, a more ample supply of water could be obtained.
The essential apparatus for boring as generally practiced consists of an auger or borer attached to rods