By Professor Lindley. 



299 



for the first, there is no doubt some foundation. In all cases, the 

 roots of trees suffer from frost less than the stems, partly perhaps 

 because the vitality of a root is greater than that of a stem, as 

 Mr. Knight long since shewed ; but more especially because they 

 are so much less exposed to cold. That the earth, being a bad con- 

 ductor of heat, should remain in winter at a higher temperature 

 than the superincumbent atmosphere will excite no surprise ; but 

 probably few persons are aware, to how small a degree the tempe- 

 rature of the earth is lowered in this country, during even long con- 

 tinued and severe frost. In order to measure the exact difference be- 

 tween the temperature of the earth, and the air, experiments have for 

 some time been in progress in the Society's garden. Two ther- 

 mometers have been buried in the earth, one at the deptb of I foot, 

 the other at the depth of 2 feet, and their indications have been 

 noted daily. It will be seen, from the following table, that the 

 ground was never frozen to the depth of a foot, in the Society's 

 garden, even while the temperature of the surface was as low as 

 4|° below zero, and that it did not fall to within 5° of freezing at the 

 depth of 2 feet during the same period. I cannot pretend to ex- 

 plain the discrepancy between this statement, and the observations 

 of those who have found the earth frozen to the depth of more 

 than 2 feet during the past winter, especially, as the soil in which 

 my observations were made is far from dry ; but the fact as now 

 stated is certain. In order to check the geothermometrical obser- 

 vations, I caused the earth of the garden to be broken up during 

 the frost, for the purpose of ascertaining how deep the soil was 

 hardened, and the result was as follows : — In the Kitchen garden 

 quarter, 9 inches; in a hard loam footpath, 10 inches; in the 

 Arboretum, adjoining the geothermometers, 8 inches ; in the Ar- 

 boretum, where the turf is chiefly composed of moss, 5 inches. 



