Earth Te^nperatures. 



65 



thermometers at Addiscombe, at depths up to 60 ft., and that he 

 finds that the seasonal changes of temperature do not extend so 

 deep as 40 ft., but that even at 50 ft. there are slow changes of 

 small amount extending over a series of years. 



Neither the Greenwich observations nor my own show the 

 diurnal range of temperature, but some information on this 

 point is given by Mr. G. T. Symons in an Abstract of Meteoro- 

 logical Observations made in the Eoyal Botanical Society's 

 Gardens, Regent's Park, during the years 1871-76. Theje 

 observations were made three times daily, viz., at 9 a.m., 3 p.m., 

 and 9 p.m., and at depths of 3, 6, 12, 24, and 48 in. The means 

 for the period were as follows : — 



These observations show that the course of the daily heat- 

 wave through the badly conducting earth resembles that of the 

 annual heat-wave, as shown by the Greenwich observations, only 

 that it does not extend so deep, not reaching to a depth of 2 ft. 

 It will be seen that of the three times of observation the air 

 temperature is highest at 3 p.m., and is lower at 9 p.m., when 

 the sun is down, than at 9 a.m., when the sun is above the 

 horizon. At 3 in. depth the mean temperature is also highest 

 at 3 p.m., but is higher at 9 p.m. than at 9 a.m., indicating that 

 the highest point duiing the day must have been attained after 

 3 p.m.°; and this is still more the case at 6 in. depth, while at 

 1 it. depth the highest temperature of the 24 hours appears to be 

 attained at 9 p.m. 



There is a difference between the conducting power of different 

 soils, light dry soils being worse conductors than dense clay 

 soils. Hence the former are subject to greater extremes of 

 temperature at the surface than the latter, but the heat and cold 

 do not penetrate so far down. This helps to explain how it is 

 that some plants, such as wallflowers, will stand the winter on 

 dry soils, but are killed by severe frost in clay soils, the frost 

 in the latter case penetrating sufficiently deep to kill the spongy 



