38 OPEN AIE GRAPE CULTUKE. 



warmed it no longer ascends, and the colder and 

 heavier air again accumulates in the valleys. Thus 

 it will be seen, that during the night the air in the 

 valleys is colder than that in other places, while the 

 reverse is the case during the day. The stillness of 

 the air in valleys and sheltered situations also con- 

 tributes to this result in a remarkable degree. 



'Now it is obvious, that if for any fruit tree, the air 

 in the valleys should be sufficiently cold to kill the 

 buds, no orchard could succeed. And if, on the 

 other hand, sufficient light and heat to ripen the fuit 

 could not be found on the hill-tops, such situations 

 also would be unavailable. 



ISTor is the mere existence of such extremes of 

 temperature the worst evil. The destructive influ- 

 ence of a hot sun upon frozen vegetation is well known, 

 and in low valleys, the circumstances are such as to 

 give the greatest effect to this adverse influence. For 

 not only are the plants chilled by the extra cold night- 

 air, they are also completely protected from the rays 

 of the sun, until it has attained a greater power than 

 it usually exerts at its first appearance upon plants in 

 more exposed situations. And then, owing to the 

 dense atmosphere through which they pass, tlie rays 

 strike suddenly with concentrated energy so as to 

 thaw the buds with a rapidity completely destruc- 

 tive to their vitality. In such situations also, the soil 



