OF TEMPERATURE. 93 



a similar kind, have been made in the garden of the 

 Horticultural Society, from which we learn that, in 

 the valley of the Thames, the maximum mean of ter- 

 restrial temperature, at one foot below the surface, 

 has been found to be 64-81° in July, which is the 

 hottest month of the year ; but that the greatest differ- 

 ence between the mean temperature of the earth and 

 atmosphere is in the month of October, when it 

 amounted, in the two years during which the ob- 

 servations were made, to between 3 and 4 degrees ; 

 and that, in general, the mean temperature of the 

 earth, a foot below the surface, is at least one degree, 

 and more commonly a degree and a half, above the 

 mean of the atmosphere. In these cases, if the ter : 

 restrial temperatures be compared with those of the 

 atmosphere, it will be found that in the spring, when 

 vegetation is first generally set in motion, the tempe- 

 rature of the earth not only rises monthly, but re- 

 tains a mean temperature higher than that of the 

 atmosphere by from one to two degrees ; and that, in 

 the autumn, when woody and perennial plants re- 

 quire that their tissue should be solidified, and their 

 secretions condensed, in order to meet the approach 

 of inclement weather, the terrestrial temperature re- 

 mains higher in proportion than that of the atmo- 

 sphere, the earth parting with its heat very slowly.* 

 117. There appears to be no series of direct ob- 

 servations, upon the superficial temperature of the 

 earth, at the different periods of vegetation, in other 



* Quarterly Averages of Temperature obtained from Thermometers 

 buried in the Earth in the Garden of the Horticultural Society ; re 



