&80 THERMAL SPRINGS. [SECT. xx^i. 



perature was found to increase regularly and uniformly with 

 the depth at the rate of about 1 of Fahrenheit for every 52 

 feet. Professor Reich of Freyberg has found that the mean 

 of a great number of observations both in mines and wells is 

 1 of Fahrenheit for every 55 feet of depth, and, from M. 

 Arago's observations in an Artesian well now boring in Paris, 

 the increase is 1 of Fahrenheit for every 45 feet. Though 

 there can be no doubt as to the increase of temperature in 

 penetrating the crust of the earth, there is still much un- 

 certainty as to the law of increase, which varies with the 

 nature of the soil and other local circumstances ; but, on an 

 average, it has been estimated at the rate of 1 for every 50 

 or 60 feet, which corresponds with the observations of MM. 

 Marcet and De la Rive. In consequence of the rapid in- 

 crease of internal heat, thermal springs, or such as are inde- 

 pendent of volcanic action, rising from a great depth, must 

 necessarily be very rare and of a high temperature, and it is 

 actually found that none are so low as 68 of Fahrenheit : 

 that of Chaudes Aigues in Auvergne is about 136. In many 

 places warm water from Artesian wells will probably come 

 into use for domestic purposes, and it is even now employed in 

 manufactories at Wurtemberg, in Alsace, and near Stutgardt. 

 It is hardly to be expected that at present any information 

 with regard to the actual internal temperature of the earth 

 should be obtained from that of the ocean, on account of the 

 mobility of fluids, by which the colder masses sink down- 

 wards, while those that are warmer rise to the surface. 

 Nevertheless, it may be stated, that the temperature of the 

 sea decreases with the depth between the tropics ; while, on 

 the contrary, all our northern navigators found that the 

 temperature increases with the depth in the polar seas. The 

 change takes place about the 70th parallel of latitude. 

 Some ages hence, however, it may be known whether the 

 earth has arrived at a permanent state as to heat, by com- 

 paring secular observations of the temperature of the ocean 

 if made at a great distance from the land. 



