ABSOEBITTG Am) EMITTING SUEFACE. 167 



IS very difficult, nor is it in any case practicable to distinguish 

 how far a reduction of temperature produced by vegetation ia 

 due to. radiation, and how far to exhalation of the gaseous and 

 watery fluids of the plant ; for both processes usually go on to- 

 gether. But the frigorific effect of leafy structure is well ob- 

 served in the deposit of dew and the occurrence of hoarfrost on 

 the fohage of gi'asses and other small vegetables, and on other ob- 

 jects of similar form and consistence, when the temperature of 

 the air a few feet above has not been brought down to the dew- 

 point, still less to 32°, the degree of cold required to congeal dew 

 to frost.* 



We are also to take into account the action of the forest as a 

 conductor of heat between the atmosphere and the earth. In 

 the most important countries of America and Europe, and espe- 

 cially in those which have suffered most from the destruction of 

 the woods, the superficial strata of the earth are colder in winter, 

 and warmer in summer, than those a few inches lower, and their 

 sliifting temperature approximates to the atmospheric mean of 

 the respective seasons. The roots of large trees penetrate be- 

 neath the superficial strata, and reach earth of a nearly constant 

 temperature, corresponding to the mean for the entire year. As 

 conductors, they convey the heat of the atmosphere to the earth 



* The leaves and twigs of plants may be reduced by radiation to a tempera- 

 ture lower than that of the ambient atmosphere, and even be frozen when the 

 air in contact with them is above 32°. Their temperature may be communi- 

 cated to the dew deposited on them, and thus this dew be converted into frost 

 when globules of watery fluid floating in the atmosphere near them, in the 

 condition of fog or vapor, do not become congealed. 



It has long been known that vegetables can be protected against frost by 

 diffusing smoke through the atmosphere above them. This method has been 

 lately practiced in France on a large scale : vineyards of forty or fifty acres 

 have been protected by placing one or two rows of pots of burning coal-tar, or 

 of naphtha, along the north side of the vineyard, and thus keeping up a cloud 

 of smoke for two or three hours before and after sunrise. The expense is said 

 to be small, and probably it might be reduced by mixing some less combustible 

 substance, as earth, with the fluid, and thus checking its too rapid burning. 



The radiating and refrigerating power of objects by no means depends on 

 their form alone. Melloni cut sheets of metal into the shape of leaves and 

 grasses, and found that they produced little cooling effect, and were not moist- 

 ened under atmospheric conditions, which determined a plentifiU deposit of 

 dew on the leaves of vegetables. 



