OF TEMPERATURE. 



91 



What we want to know, as respects vegetation, is, not 

 the mean temperature of the earth at some distance 

 from its surface, but the temperature immediately be- 

 low the surface; i. e. of that part of the soil into which 

 the roots of plants penetrate, and whence they derive 

 their food. It is also requisite that this should be as- 

 certained monthly, so as to furnish the means of com- 

 paring the terrestrial temperature with the periodical 

 state of vegetation.* Such being the case the tempe- 



* The following proportions between the mean temperature of 

 the earth, as indicated by springs, and that of the atmosphere, 

 have been collected from various sources : — 



Berlin .... 

 Carlstrom . . 

 TJpsal .... 

 Paris .... 



Charleston . . 

 Philadelphia 

 Virginia . . . 

 Massachusetts . 

 Vermont . . . 

 Raith .... 

 Gosport . . . 

 Kendal . . . 

 Keswick . . . 

 Leith .... 

 South of England 

 Torrid Zone . . 



It must be obvious, from these returns, imperfect as they are, 

 that the results are of little value with respect to vegetation, and 

 that indications from springs, from their very nature, can be but 

 little employed in inquiries where it is necessary to determine 

 the fluctuating terrestrial temperature of the surface of the earth 

 in different months. For example, Mr. Ferguson found the 



