124 



Revieivs mid Notices of Books. 



" It may be doubted, however, whether even the mean tem- 

 perature of the summer season affords a sufficient clue to all the 

 variations in the character of the vegetation which are attributable 

 to heat. A plant is not like a spring which is pushed forwards a 

 certain number of degrees by the application of a definite force, 

 and when that pressure is removed, returns again to its original 

 position ; for when the stimulus of heat is applied to it, its organs 

 undergo a degree of development, which they retain even although 

 the temperature should afterwards be reduced. Hence it is ne- 

 cessary to note the extremes of temperature to which a country is 

 liable, as well as the mean of its summer and winter climate. 



" Should it happen, for instance, that the cold in a low or sub- 

 tropical latitude ever approaches even for a single night to the 

 zero of Fahrenheit, certain trees, such as the Orange, would in- 

 fallibly perish ; and hence they can never be indigenous in countries 

 subject to such contingencies. 



" The general climate of the British Isles is so exceptionally 

 mild, that we have introduced the plants of warmer regions gene- 

 rally into cultivation, and begun to consider them as in a manner 

 naturalized ; but that they are not so, and could never have estab- 

 lished themselves in the soil without the aid of man, became evi- 

 dent from the effects of the rigorous winter of 1860-61, — one of 

 those seasons of unusual severity which^ however, are sure to occur 

 within a certain cycle of years, and to entail the destruction of all 

 these denizens of a more temperate climate, which, rashly pre- 

 suming upon the mildness of many preceding winters, had begun 

 to regard this as their home." 



Among the causes which affect climate are the relative distribu- 

 tion of land and w^ater ; the proximity to a large continent or to a 

 wide expanse of sea ; the chilling influence of mountains which 

 retain the winter snow during the greater part of the year, or push 

 down glaciers into the valleys ; and the shelter afforded by forests 

 or hills of moderate height. Of these, the relative power of land 

 and water, with respect to the reception, radiation, and transmis- 

 sion of heat is the most important influence. The following ex- 

 tracts show the changes produced by different arrangements of sea 

 and land in different countries : — 



"Thus it is only on extensive plains, at a distance from snow- 

 capped mountains, that a high range of temperature can maintain 

 itself even during the day in a northern latitude. On the other 

 hand, over such level tracts as those of Kussia and Siberia, great 

 heats prevail in summer even in comparatively northern regions. 



" ^Moscow, for instance, in north latitude 55° 48', where the 

 mean temperature of the coldest month is only 13° of Fahr., 

 enjoys in July a heat of 66° 4' ; and at Yakoutzk, a Siberian 

 town, in latitude 62°, at which the mean temperature of January 



