INTRODUCTION. XXIX 



air that surrounds it, and excess of cold, accompanied 

 by perfect stillness, is incomparably less injurious 

 than when coupled with rapid motion. Thus our 

 travellers in Polar regions speak of intense cold, as 

 indicated by the thermometer, having been scarcely 

 inconvenient to them if the atmosphere were per- 

 fectly still ; but if the wind rose, although the quick- 

 silver simultaneously fell, as was almost invariably the 

 case, the cold was most distressing. In England, 

 if Fahrenheit's thermometer be at 30% we walk about 

 or stand exposed to it without any sensation of pain, 

 but if we face it in travelling by railway at the rate of 

 thirty miles an hour, the cold becomes perfectly 

 intolerable. In fact, it has been abundantly proved 

 by experiment, that a much greater extreme of heat 

 or cold may be borne by plants, by animals, and even 

 by the human frame, if both the atmosphere and the 

 objects of experiments be in a state of perfect quies- 

 cence. In closed cases we thus not only avoid rapid 

 changes of temperature, but the active motion in 

 extremes of temperature, which is the most injurious 

 property of such extremes. The deleterious effect of 

 boisterous winds on the fragile fronds of Ferns needs 

 no exemplification ; it is so great, that if a specimen 

 of Cystopteris be moved from its protected habitat, 

 and placed where it may receive the full force of the 

 wind, that alone will, in a few weeks, work its utter 

 destruction : to such a plant how grateful must be 

 the motionless atmosphere thus provided ! 



The solution of the problem appears to me to be 



