CH. VII.] THE FRUIT AND SEED. 221 



is a known worse conductor of heat than hemp, and 

 not absorbing moisture so easily, is not so liable to 

 the cold always produced by its drying. 



Snow is a protection to plants from the three fore- 

 going reasons — it prevents heat radiating from them — 

 protects them from the chilling blasts — and is one of 

 the worst conductors of heat. I have never known the 

 surface of the earth below a covering of snow colder 

 than 3-2°, even when the temperature of the air above 

 has been 28°. A similar protection, though less effec- 

 tual, is afforded by straw. 



Strange as it may appear, yet it is nevertheless 

 true, that a shelter is more beneficial in preseiwing 

 the temperature of trees, when from three to six 

 inches from them, than when in immediate contact 

 with their surfaces. When a woollen net was 

 suspended four inches from the wall, on which a 

 peach-tree was trained, the thermometer fell very 

 slowly, and the lowest degree it reached was SS'^; 

 when the same screen was twelve inches off, it 

 fell to 34° ; and when draAvn tightly over the tree, 

 it barely kept above 32", the temperatiu'e of the 

 exterior air. When at twelve inches from the wall, 

 it permitted the too free circulation of the air; 

 and when in immediate contact with the polished 

 bark of the peach, perhaps another law of cooling 

 came into operation. That law is, that polished 

 surfaces radiate heat slowest. Thus if two glass 



