122 Mr. Darnell on Climate. [Feb. 



carried to that extent which its simplicity and efficacy would 

 suggest. Neither does the manner of fixing the screen exhibit 

 at proper acquaintance with the principle upon which it is 

 resorted to : it is generally bound tight round the tree which it 

 is required to protect, or nailed in close contact with its fohage. 

 Now it should be borne in mind that the radiation is only 

 tranferred fiom the tree to the mat, and the cold of the latter 

 will be conducted to the former in every point where it touches. 

 Contact should therefore be prevented by hoops or other means 

 properly applied, and the stratum of air which is enclosed will, 

 by its low conducting power, effectually secure the plant. 

 With their foliage thus protected, and their roots well covered 

 with litter, many evergreens might doubtless be brought to sur- 

 vive the rigour of our winters, which are now confined to the 

 stunted growth of the green house and conservatory. 



The secondary effect which radiation has upon the climate of 

 particular situations, is a point which is less frequently consi- 

 dered than the primary one which we have been investigating, 

 but which requires perhaps still more attention. The utmost 

 concentration of cold can only take place in a perfectly still 

 atmosphere : a very slight motion of the air is sufficient to 

 disperse it. A low mist is often formed in meadows in particu- 

 lar situations, which is the consequence of the slow extension 

 of this cold in the air, as before described ; the agitation of 

 merely walking through this condensation is frequently sufficient 

 to disperse and melt it. A valley surrounded by low hills is 

 more liable to the effects of radiation than the tops and sides of 

 the hills themselves ; and it is a well known fact that dew and 

 hoar frost are always more abundant in the former than in the 

 latter situations. It is not meant to include in this observation 

 places surrounded by lofty and precipitous hills which obstruct 

 the aspect of the sky, for in such the contrary effect would be 

 produced. Gentle slopes which break the undulations of the 

 air without naturally circumscribing the heavens, are most 

 efficient in promoting this action, and it is worthy of remark 

 and consideration, that by walls and other fences, we may arti- 

 ficially combine circumstances which may produce the same 

 injurious effect. 



But the influence of hills upon the nightly temperature of the 

 valleys which they surround is not confined to this insulation ; 

 radiation goes on upon their declivities, and the air which is 

 condensed by the cold rolls down and lodges at their feet. 



Their sides are thus protected from the chill, and a double 

 portion falls upon, what many are apt to consider, the more 

 sheltered situation. Experience amply confirms these theore- 

 tical considerations. It is a very old remark, that the injurious 

 effects of cold occur chiefly in hollow places, and that frosts 

 are less severe upon hills than in neighbouring plains. It is 



