OF PLANTS. 249 



example, the average number of degrees of heat are taken 

 from a number of daily observations, and these average 

 numbers collected for every day in the year, and again 

 another average drawn, this last average varies but few 

 degrees from that of the preceding or following year. 

 Taking a greater number of years, for instance, twenty, a 

 value is obtained, which differs scarcely the tenth part of a 

 degree from the foregoing or succeeding twenty years. 

 Humboldt then first arrived at the ingenious idea of con- 

 necting, by a line upon a map, all the places upon the 

 globe which have the same mean temperature, obtained by 

 the mode of estimation just described (Isothermal Lines, or 

 lines of equal heat) ; and now it was found, that although 

 the curves of these Isothermal Lines deviate so much from 

 the parallels of latitude, yet the boundaries of vegetation 

 cling much closer to them than to the latter. But many 

 riddles yet remain unsolved. Drontheim, for instance, has 

 the same mean temperature as the most southern point of 

 Iceland ; the Hebrides, Orcades and Shetland Islands, have 

 something like 5 higher mean temperature. Nevertheless, 

 in Drontheim fruits and Wheat are still cultivated, while the 

 growing of Wheat first begins at Inverness, in Scotland, and 

 of fruit still farther south. Thus inquirers were at last led 

 to draw within the circle of their investigations, the distri- 

 bution of heat within the Seasons, since it is evident that 

 the vegetation is often more essentially determined by this, 

 than by the mean temperature, or sum of the heat which 

 it receives. The mean summer and winter temperatures 

 are now estimated in the manner indicated, and the places 

 which agree in these relations are connected in the same 

 way by lines Isotheral (lines of equal summer heat) and 

 Isochimenal (lines of equal winter cold) . Now Drontheim, 

 for example, has a mean winter cold of 24 34', while the 



