IMPRISONED HEAT. 97 



into it to the height of the place of outcrop. If the place of 

 boring is lower than the outcrop, the water will rise above the 

 surface. The water comes up with nearly the temperature 

 acquired at the bottom of the well. 



The sun's warmth penetrates daily but a foot or two in 

 summer; and at night, much of this is lost by radiation. 

 Not all, however, for the deeper warmth continues to descend; 

 and next day's excess of warmth follows this. Thus the sum- 

 mer heat accumulates, and continues to descend. It grows 

 less ' and less, however, and at fifty feet, can no longer be 

 discerned. The winter's cold also penetrates slowly, and 

 diminishing in intensity at every foot, ceases to influence the 

 temperature at the depth of about fifty feet. At this depth 

 then, the temperature is constant the year round. The depth 

 of constant temperature varies, however, with the nature of 

 the climate. If the surface fluctuations are excessively great, 

 you can understand that the contrasts must be felt at a greater 

 depth. In Minnesota, therefore, the depth of uniform tem- 

 perature would be greater than fifty feet. In Florida, how- 

 ever, where the climatic extremes are much less, the depth of 

 uniform temperature would be less than fifty feet. The uni- 

 form temperature under any region must be about the same 

 as the mean annual temperature at the surface. 



The heat of midsummer and the cold of midwinter pene- 

 trate the earth at the rate of about one foot per week. Hence 

 the cold of January 1st is felt at a depth of twenty-five feet 

 about July 1st ; and so of the cold or heat of any other date. 

 At twenty-five feet the temperature of water is higher in 

 winter and lower in summer. So the popular opinion about 

 certain wells is not entirely unfounded. 



If, however, we employ means to ascertain the tempera- 

 ture at depths below the plane of constant temperature, we 

 find it regularly increasing as we descend. We do not find 

 the rate of increase exactly the same at different localities, 

 but the average is about one degree (Fahrenheit) for every 

 fifty or sixty feet of descent. The Artesian well at Charles- 

 ton, South Carolina, is 1,250 feet deep, and the bottom 



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