34 DOMESTIC WATER SUPPLIES FOR THE FARM 



season to season or from year to year. This is known as the 

 normal temperature of the water for a given region, and it agrees 

 very closely with the mean annual temperature of the same 

 locality. The depth of uniform temperature varies somewhat in 

 different localities, but is commonly from 50 to 60 feet below the 

 surface. The temperature varies from about 40 degrees or 45 

 degrees in New England to about 65 degrees to 70 degrees in the 

 Gulf States. 



Waters occurring nearer to the surface than the zone of uni- 

 form temperature vary in temperature according to season, being 

 warmer than the normal in summer months and colder in the 

 winter months. The temperature of waters warmer than the nor- 

 mal may also be due to the great depth from which the waters 

 have come. 



The main cause of rise in temperature below the line of invari- 

 able temperature is the internal heat of the earth. This internal 

 heat increases rapidly downward, the rate of increase varying from 

 i degree in 30 feet to I degree in 100 feet, the average increase being 

 about I degree to 50 feet. The temperature of the water is very 

 little affected in passing through the upper 50 feet of its course, 

 hence its temperature is a fair indication of the depth from which 

 it is derived. 



Besides the internal heat of the earth the heat of igneous 

 masses below the surface of the earth has been thought to give 

 rise to the hot springs of many localities, and in some instances 

 the heat evolved by the chemical decay of rocks has been cited 

 to explain the temperature of hot springs. 



