Hot and Thermal Springs, 371 



strata, the one of air and the other of earth, both of which 

 would be warmer than itself. A permanently low temperature 

 of the external crust can be conceived in no other way, even 

 considering it to be a much worse conductor of heat than it 

 really is, than by supposing the existence of a never-failing cool- 

 ing principle ; but, then, what could that principle be ? Such an 

 hypothesis is shewn to be still more contradictory by the above- 

 mentioned observations of Boussingault ; namely, that, between 

 the Tropics, the temperature of the earth is constant at the depth 

 of one foot below the surface. For how can it be supposed that 

 the crust of the earth should, at the depth of one foot from its 

 surface, have a constant temperature lower than that of the air, 

 with which it is continually in contact ? But that supposition is 

 most completely refuted by Boussingault''s comparative observa- 

 tions, as will be seen from the following table : 



Between 5° south and 11° north latitude, the mean tempera- 

 ture of the air exactly accords with the temperature of the earth, 

 taken at one foot below the surface of the earth, in a place pro- 

 tected from the sun by a roofing. 



In order to obtain a similar comparison in the temperate 

 zones, it would be very desirable that similar observations on the 

 temperature of the soil should be made in our latitudes which, 

 however, it would be necessary to continue for at least a year. 

 Till now, little has been done towards this object. According 

 to the observations of Rudberg, the mean temperature of the 

 earth at Stockholm is 1°.62 higher than that of the air. Three 

 years'* observations, conducted by Herrenschneider at Strasburg, 

 gave the mean temperature of the air 0°.54 higher than that of 

 the soil. The mean of each year, however, differed as much as 

 1.CC5 one from the other. It is, therefore, necessary for the 

 comparison that the observations on the air and on the soil 



