252 WRITINGS OF JOSEPH HENRY. [1855- 



ature of the surrounding air, and that the amount of radia- 

 tion was nearly the same at all temperatures. The explana- 

 tion therefore of the great cold of the air between the blades 

 of grass is as follows: By the radiation of the heat the grass 

 is at first cooled two degrees lower than the air at the sur- 

 face of the earth, and next the thin stratum of air which 

 immediately surrounds the grass is cooled by contact to the 

 same degree. It then sinks down and another portion of 

 air comes in contact with the blade of grass, and is in its 

 turn cooled to the same extent, and so on until all the air 

 between the blades is two degrees lower than that of the air 

 farther up. The radiation however continues, and a stratum 

 of air from the mass already cooled is cooled two degrees 

 more, which sinks down as before, and so on until the air 

 between the blades is cooled to 4° below its normal condi- 

 tion ; and in this way the process may be continued until 

 the temperature descends to 8° or 10° below that of the strat- 

 um of air a few feet above. In this way we can readily ex- 

 plain the small amount of dew deposited on the tops of trees, 

 since the air as soon as it is cooled sinks down toward the 

 ground, and its place is continuously supplied by new por- 

 tions of the atmosphere. To the same cause we may at- 

 tribute copious deposition of dew on wool and other fibrous 

 materials which, though they do not radiate heat more freely 

 into space, yet entangle and retain the air between their 

 fibres, and thus allow the cooling process we have described 

 to go on. It would appear that spider-webs radiate heat 

 freely into space, since they are generally covered with a 

 large amount of dew ; their insulated position prevents them 

 from renewing their heat, but according to the above prin- 

 ciple a much larger amount of deposition ought to be pro- 

 duced by the same material were it loosely gathered up into 

 a fibrous mass. The fact of the screening influence of the 

 clouds teaches us that a thin cloth or even a slight gauze 

 supported horizontally over tender plants is sufficient to 

 neutralize the radiation and to prevent injury from frost 

 during the clear nights of spring or autumn. The same 

 effect is produced by artificial clouds of smoke. 



