175 



Now this is only true when the temperature is constant. 

 When it is not, we have seen that the true law is that 



p = x p (1 + a t). 



If, however, we attempt to use this equation, we require 

 to know the law according to which t, the temperature, 

 varies as we ascend into the air and descend into the 

 earth. We can have but little assistance in determining 

 this from observation. As Humboldt* has remarked, our 

 experimental knowledge of the interior of the earth is 

 limited in the extreme. The greatest depth below the 

 surface of the sea that has yet been obtained, is probably 

 that of the salt-works of Xew-Salzwerk, near Minden, 

 in Prussia; yet this was only 1993 feet, or less than 

 Y^-Joo^ part of earth s radius. The observations even on 

 these small depths are liable to serious errors, as the 

 different periodic variations of temperature caused by the 

 diurnal or annual heating of the surface, the greater 

 exposure to the surface air, &c. The temperature of 

 water at the bottom of the salt mine was 90 8 Fahren 

 heit, giving a mean decrease of 1 Fahrenheit for every 

 53 -8 feet. If we tried to make our observations on the 

 law of density instead of that of temperature, for the 

 knowledge of either would enable us to integrate the 

 equations, we can succeed no better. The dippings of 

 strata beneath the surface, which rise again at known 

 distances, only reach some twelve thousand feet below the 

 surface of the sea ; and if to this we added the height of 

 the highest mountain, we have only a knowledge of 

 ji T th part of earth s radius. 



We have also observations made on the temperature of 

 the air at the summits of mountains, and in balloon 



* Kosmos, i. 150. 



