3 1 6 VARIA TJONS OF TEMPERA TURE 



under the influence of local as well as general causes. Had 

 this fact been known to the philosophers of antiquity, they 

 would have taken advantage of it to liken the earth to an 

 animal whose skin is more or less sensible of heat, not only 

 according to the difference of the seasons, but according to 

 the different hours of the day. 



The diurnal thermometrical variations are those which 

 penetrate the least profoundly into the interior of the soil. 

 At a depth of about five feet they cease to be perceptible. 

 The maxima and minima of the year, however, can be 

 detected at a sufficiently considerable depth. The limit 

 descends as low as 80 to 100 feet. Below 100 feet, the 

 terrestrial stratum is found invariable, that is, inaccessible 

 to the thermometrical changes of the atmosphere. It is 

 remarkable that the temperature of this stratum differs but 

 little from the mean annual temperature of the air, which, in 

 the latitude of London, is 49. 



The maxima and minima of the yearly heat are propagated 

 very slowly in the earth, and their difference gradually be- 

 comes less and less. Thermometers buried 26 feet deep 

 in the ground, mark, in our latitudes, the maximum of tem- 

 perature only on the loth of December, and the minimum 

 on the 1 5th of June. But there are certain elements which 

 we must take into account. Thus, the depth of the invari- 

 able thermometrical curve depends both on the latitude of 

 the place, on the conductibility of the strata, and the differ- 

 ence between the highest and lowest temperature of the year, 



