-1*0 INTERNAL HEAT OF THE EARTH, 



between the winter and summer temperatures, ( 40 ) the winter 

 temperature being the higher. If we take the mean of many 

 observations of temperature at and below the surface of the 

 ground between the parallels of Zurich (lat. 47 22') and 

 Upsala (lat. 59 51'), we find a mean increase of 1 Cent, for 

 the depth of 67i French, or 72 English, feet. The diffe- 

 rences due to latitude are not more than from about % 13 to 16 

 English feet, and do not show any regular systematic varia- 

 tion from south to north ; probably because the undoubtedly 

 existing effect of latitude is at present inextricably blended 

 with the influence of the varying heat-conducting powers of 

 the different rocks, and with errors of observation. 



As, according to the theory of the distribution of heat, the 

 underground stratum in which we begin to find no difference 

 of temperature in the course of the annual cycle, is so much 

 the less deep beneath the surface as the maxima and minima 

 of temperature in the year are less distant from each other, 

 my friend Boussingault was led by this consideration to the 

 ingenious and convenient method of determining the mean 

 temperature of places within the tropics (more particularly 

 within 10 on either side of the equator) by observing a 

 thermometer buried about a foot deep within a well-covered 

 space. At the most different hours, and even in different 

 months, (as is shown by the experiments of Colonel Hall 

 near the sea-shore of Choco, in Turnaco ; those of Salaza at 

 Quito ; and those of Boussingault in the Yega de Zupia, at 

 Marmato, and Anserma Nuevo in the Cauca Valley), the 

 temperature did not vary two-tenths of a centesimal degree 

 (0'36 Fahr.), and was identical within almost the same 

 limits with the mean atmospheric temperature at places 

 where the latter had been derived from hourly observations. 



