138 DOMESTIC WATER SUPPLIES FOR THE FARM 



stances, are that there be a double-tubed well, in which the air is 

 able to pass between the pump-pipe and outer casing, and a 

 porous, water-free stratum by which air may be absorbed or ex- 

 pelled according to the external barometric pressure. When, on 

 the approach of a storm, the pressure at the surface is reduced, 

 the air confined in the earth rushes out until equilibrium is re- 

 established; but when, upon the return of fair weather, the 

 pressure again increases, air is forced back through the well into 

 the earth. In the few wells from which water is spouted during 

 the period of "blowing," the casing probably extends virtually 

 to the water, but not far below it. 



The freezing of wells seems likewise to be due to the indraft of 

 cold air at periods of high barometer, the air passing down the 

 well and freezing the water. When, on a change of weather, the 

 direction of the air current is reversed the well thaws and the 

 snow about the well mouth melts. 



The fluctuation of head and flow are also due to variation of 

 atmospheric pressure. The water in many deep wells is under 

 more or less hydrostatic pressure, which is opposed by the pressure 

 of the air, the level at which the water stands representing the 

 result or the balance of the two forces. If the pressure of the air 

 is lessened and the hydrostatic pressure remains the same the 

 water level in the well will rise, and if the atmospheric pressure 

 increases the water level will fall. In some non-flowing wells 

 the increased head will cause the water to flow; in flowing wells 

 it increases the volume of discharge. 



The roiliness of the water is apparently dependent on the 

 same general causes, i.e., the fluctuation of barometer pressure. 

 As low air pressure causes increased discharge in certain flowing 

 wells, and as increased discharge produces increased velocity of 

 the water both in the well and in the material from which the 

 water is derived, it often happens, when this material includes 

 more or less silt which is too coarse to be affected by ordinary 

 currents, that quantities of silt are loosened under the increased 

 velocity and taken up by the water, producing milkiness. Iron 



