INVARIABLE STRATUM. 



much more long-continued and extensive than have hitherto been 

 made, to render manifest. 



27. The thennometric observations on the periodical changes 

 which take place above the stratum of invariable temperature are 

 not so numerous as could be desired ; nevertheless, the following 

 general conditions have been ascertained, especially in the middle 

 latitudes of the northern hemisphere : 



1. The diurnal variations of temperature are not sensible to a 

 greater depth than 3| feet. 



2. The difference between the extreme temperatures of the 

 strata decreases in geometrical progression for depths measured in 

 arithmetical progression, -or nearly so. 



Thus, at the depth of twenty-five feet the difference between the 

 extreme temperatures T and t, is reduced to two degrees ; at fifty 

 feet it is diminished to the fifth of a degree, and at sixty or eighty 

 feet to the fiftieth of a degree. 



3. Since the effects of the superficial variation must require a 

 certain time to penetrate the strata, it is evident that the epoch at 

 which each stratum attains its maximum and minimum tempera- 

 tures will be different from those at which the other strata and the 

 surface attain them. The lower the strata the greater will be 

 the difference between the times of attaining those limits as com- 

 pared with the surface. 



28. The same uniformity of temperature which prevails in the 

 invariable stratum is also observed at all greater depths ; but the 

 temperature increases with the depth. Thus, each successive 

 stratum, in descending, has a characteristic temperature, which 

 never changes. The rate at which this temperature augments 

 with the depth below the invariable stratum is extremely different 

 in different localities. In some there is an increase of one degree 

 for every thirty feet, while in others the same increase corresponds 

 to a depth of 100 feet. It may be assumed, in general, that an 

 increase of one degree of temperature will take place for every fifty 

 or sixty feet of depth. 



29. The permanency of the temperatures of the inferior strata is 

 rendered manifest by the uniformity of the temperature of springs, 

 of which the water rises from any considerable depths. At all 

 seasons of the year the water of such springs maintains the same 

 uniform temperature. 



It may be assumed that the temperature of the water proceeding 

 from such springs is that of the strata from which they rise. In 

 these latitudes it is found in general to be a little above the 

 mean temperature of the air for ordinary springs, that is from 

 those which probably rise from strata not below the invariable 

 stratum. In higher latitudes the excess of temperature is 



