136 DOMESTIC WATER SUPPLIES FOR THE FARM 



In open wells, including in this class the dug wells not provided 

 with covers, the cold air often many degrees below zero is 

 free to enter and displace the air of the wells, which, owing to its 

 contact with the water and the unfrozen earth, is generally con- 

 siderably warmer. Under such conditions, although the tem- 

 perature of the entering air is somewhat modified by mixture with 

 that already in the well and by contact with the walls, freezing 

 often occurs at considerable depths, and the well is rendered use- 

 less during the continuance of cold weather. 



In dug wells protected by covers there is generally little 

 trouble from the freezing of the water unless it happens to stand 

 very near the surface. Although few well coverings are tight 

 enough to exclude the cold air, it penetrates so slowly that the 

 temperature in the well, owing to the warmth given off by the 

 earth and the ground water, seldom reaches the freezing point. 

 In some wells, however, where open, water- free gravels occur above 

 water level, much trouble is experienced. 



In the simpler type of driven wells, consisting of a single con- 

 tinuous casing or of double tubes, both of which are carried below 

 the ground- water level, little or no trouble is caused by the freez- 

 ing of the water in the well, except, perhaps, when its level is very 

 near the surface. The amount of cold air entering through the 

 pump is insignificant, and there is no material circulation of air 

 in the surrounding materials, and, therefore, no adequate cause 

 for freezing. 



Most of the wells subject to freezing are drilled or double- 

 tube driven wells in which the inner or pump tube is carried be- 

 low the outer casing, stopping in some porous stratum (Fig. 62), 

 or wells drilled in limestone or other rocks that contain open solu- 

 tion passages (Fig. 64). The cause of freezing in these wells is 

 discussed on pp. 138-139. 



Pumps are certain to freeze if the cylinders are near the sur- 

 face, as the water left in the valves and box after pumping freezes 

 before it can drain back into the well. Where the cylinder is 

 placed at a considerable depth, however, this difficulty is avoided, 



