258 Vineyard Culture. 



Be this as it may, the crop thus obtained is far from 

 equaling that which has been destroyed. Moreover, it 

 often happens, if these late frosts are severe enough to 

 destroy all the young shoots of a plant completely, that 

 the vine grows no more, but withers. In this manner, 

 one-tenth of all the plants in the vineyards of the Jura 

 are destroyed annually. 



These late frosts almost invariably proceed from ra- 

 diation during the bright nights at the end of April and 

 beginning of May. They are then called " white 

 frosts." We know that all bodies on the surface of the 

 earth possess a certain degree of heat peculiar to them. 

 When the sky is clear at night, these bodies lose a part 

 of their heat, in the shape of caloric rays, which rise 

 into space. This radiation continuing, these bodies 

 cool gradually, and the fall of temperature they un- 

 dergo, is such, that the atmospheric vapor condenses 

 on their surface. It is this first phenomenon which 

 produces dew. If the cooling of these bodies goes on 

 increasing, the dew covering them congeals, and gives 

 rise to white frosts. 



If the sky be cloudy, there is no dew, and, of course, 

 no white frost, because the caloric rays, radiating from 

 earthly bodies toward the sky, meet the clouds, and are 

 sent back to the ground ; there is an exchange of caloric 

 rays between bodies on the earth and these clouds, but 

 there is no decided cooling. The slightest obstacle in- 

 terposed between woody or grassy plants, and the sky, 

 would therefore suffice to preserve them from the ac- 

 tion of white frosts. 



As may be supposed, these white frosts are the more 

 to be feared when the air is charged with moisture. 



