Inclement Weather, Diseases, Etc. 259 



For thrs reason, frosts are more prevalent in the valleys 

 than on the hills. On a vine-stock, the young shoots 

 nearest the ground, and hence more exposed to cold, 

 freeze more frequently than those at the top. A vine- 

 yard in which the soil has been quite recently stirred, 

 freezes more easily than one in which the surface is 

 hardened ; for, in the former case, the evaporation of 

 moisture is more abundant. Dew and white-frosts are 

 not produced when the air is sufficiently in motion to 

 carry away, by evaporation from the surface of bodies, 

 the cold which is there produced by radiation. 



[It is singular that after M. Du Breuil had given so distinct 

 an explanation of the formation of dew and frost, which he 

 very properly attributes to the eiFects of radiation, he should 

 still be disposed to refer the evils of frost to moisture. And 

 in the case of the freshly-plowed ground, to which ref- 

 erence is made on a previous page, our author attributes its 

 freezing to the presence of increased moisture ; whereas, it 

 is the better surface of radiation, and its increased amount in 

 the plowed ground, that causes the difficulty. The radiation 

 being perfect, the cooling process will go on, whether the 

 soil and air be wet or dry, but if the former be full of moist- 

 ure, and its dew-point be low, the precipitation will be greater, 

 and the dew or frost will be more apparent. 



And so of the effects of a spring frosf upon the lower part 

 of a vine-stock being more disastrous than upon the higher 

 shoots, the result is easily explained upon the common prin- 

 ciples of physics. The radiation takes place from the surface, 

 chiefly at the level of the ground, and the air is chilled only 

 as it comes in contact with bodies thus cooled ; the lower 

 portion of the atmosphere must first have its temperature re- 

 duced, and the cooling of the next upper layer must take 

 place very slowly, since the air is a non-conductor, and 



