4£ HEAT OF THE EAETH. 



material that conducts heat very slowly, and to 

 leave it there until yon are quite sure that it must 

 have taken the temperature of the place. Of the 

 time necessary for this you must have satisfied 

 yourself by previous experiments. The move- 

 ments of the instrument are so much retarded by 

 the covering, that there can be no change of its 

 indication while you are withdrawing it, and 

 reading off the degree recorded. In fact this 

 method, with careful management, leaves nothing 

 to be desired. 



But there are also instruments specially con- 

 trived for the purpose of measuring temperature 

 at times and places at which the observer cannot 

 be present. The best known is the Registering 

 Thermometer, which shows both the highest and 

 lowest temperature which has occurred during any 

 length of time that may be desired. You will find 

 it described in works on physical science. 



The regular increase of the temperature of the 

 ground in proportion to the depth, agrees altogether 

 with the law, which physicists have ascertained to 

 govern the propagation of heat through walls of uni- 

 form material, and exposed on their different sides to 

 unvarying but different degrees of heat. We find one 

 of these constant temperatures in the solid earth- 

 crust, at the imaginary surface which limits the 

 influx of heat from without : of the place of 

 the other we know nothing, except that, consi- 



