246 ISOTHERMAL LINES. SECT. XXV. 



climate in the polar Ocean, probably not under 15 Fahrenheit.* 

 It is believed that two corresponding poles of maximum cold 

 exist in the southern hemisphere, though observations are want- 

 ing to trace the course of the southern isothermal lines with the 

 same accuracy as the northern. 



The isothermal lines, or such as pass through places where 

 the mean annual temperature of the air is the same, do not 

 always coincide with the isogeothermal lines, which are those 

 passing through places where the mean temperature of the 

 ground is the same. Sir David Brewster, in discussing this sub- 

 ject, finds that the isogeothermal lines are always parallel to the 

 isothermal lines ; consequently the same general formula will 

 serve to determine both, since the difference is a constant quan- 

 tity obtained by observation, and depending upon the distance 

 of the place from the neutral isothermal line. These results are 

 confirmed by the observations of M. Kupffer of Kasan during 

 his excursions to the north, which show that the European and 

 the American portions of the isogeothermal line of 32 of Fahren- 

 heit actually separate, and go round the two poles of maximum 

 cold. This traveller remarked, also, that the temperature both 

 of the air and of the soil decreases most rapidly towards the 45th 

 degree of latitude. 



It is evident that places may have the same mean annual 

 temperature, and yet differ materially in climate. In one, the 

 winters may be mild and the summers cool; whereas another 

 may experience the extremes of heat and cold. Lines passing 

 through places having the same mean summer or winter tempera- 

 ture are neither parallel to the isothermal, the geothermal lines, 

 nor to one another, and they differ still more from the parallels 

 of latitude. In Europe, the latitude of two places which have 

 the same annual heat never differs more than 8 or 9 ; whereas 

 the difference in the latitude of those having the same mean 

 winter temperature is sometimes as much as 18 or 19. At 

 Kasan, in the interior of Kussia, in latitude 55'48, nearly the 

 same with that of Edinburgh, the mean annual temperature is 

 about 37'6 ; at Edinburgh it is 47-84. At Kasan the mean 

 summer temperature is 64'84, and that of winter 2'12 ; whereas 



* ' Meteorology,' by Sir J. Herschel. 



