138 ON THE ACTION OF WATER ON PLANTS. 



Caroline. I imagined that the dew fell from a consid- 

 erable height ; for trees afford a shelter from it : you sel- 

 dom find any dew beneath a tree ? 



Mrs. B. The radiation of the earth is stopped by the 

 canopy of the tree and reflected back to the ground, thus 

 preventing it from so rapidly cooling as to occasion a 

 deposition of dew. For the same reason, when the sky 

 is covered with clouds, the heat is reflected back to the 

 earth by them, and little or no precipitation of dew takes 

 place ; while, on a clear night, the radiation goes on un- 

 interruptedly, the earth cools rapidly, and an abundant 

 dew is deposited. 



Emily, How admirably this provision is proportioned 

 to the wants of the vegetable creation! A clear sky, 

 which leaves it exposed throughout the day to the ardor 

 of the sun's rays, insures it an abundant supply of re- 

 freshing dew in the evening. 



Caroline. I have seldom perceived this radiation of 

 heat in England ; but in Switzerland it is very sensibly 

 felt on a summer's evening, from trees, walls, and other 

 buildings which have been heated by the sun during the day. 



Mr*. B. It is for this reason, that, in hot climates, 

 the public walks are less planted with trees, than those 

 of more temperate regions ; in the former you can walk 

 out only after sunset, when the neighborhood of trees is 

 attended with every disadvantage. They prevent the 

 free circulation of the cool evening air. They reflect 

 back the heated radiation of the earth, and are, them- 

 selves, a source of heat by their own radiatron. 



Emily. In our more temperate climate, when we fre- 

 quently walk out during the day, trees afford us a grate- 

 ful shelter from the sun, and in the evening they have 

 the advantage of retaining the heat and preventing the 

 deposition of dew. 



Mrs. B. In regard to the third mode which nature 

 employs to water plants, the melting of snow, as it relates 

 only to plants growing on the Alps, or other mountains 

 whose summits are constantly covered with snow, it is 

 unnecessary to make any observations upon the subject. 



767. Why is there little or no dew under trees'? 768. And why 

 less in a cloudy than a clear night'? 769. What does Caroline say 

 she has seen of radiation in England and Switzerland 1 ? 770. What 

 does Mrs. B. say of trees upon the temperature in the eveningl 771. 

 What is the third mode of nature in watering plantsl 



