524 SCIENTIFIC RECORD FOR 1883. 



[As this is a matter that in 1871 and 1872 became to the writer very- 

 important in his daily weather predictions, it is allowable to remark 

 that the amount of heat given up by the upper layer of the soil or rock 

 to the air or snow seemed to him very small and hardly worth men- 

 tioning in the above explanation; it rarely amounts to enough to 

 cause the melting of an inch of snowfall into a tenth inch of water. 

 The special low night temperature over snow and ice should be 

 wholly attributed to the clear, dry air, free from slightest haze or 

 dust, which allows freer radiation, and to the exceptionally large ra- 

 diating power of snow and ice for the red and ultra red or heat rays. 

 The low temperature during sunshine is due to the fact that solar heat 

 is consumed in melting ice (latent heat of liquefaction 79.) instead of 

 warming the air (specific heat 0.267). Similarly we should abstain from 

 assuming, as is too frequently done, that warm air flowing up cold 

 mountain sides is cooled by contact with the earth ; i. e., by conduction of 

 its heat into the earth. This is wholly insignificant in comparison with 

 the cooling due to expansion and to the evaporation of moisture. The 

 heat given to the air by the earth surface at midday is not conducted 

 upwards from any depth, but is a purely surface action, by which solar 

 radi tiou is converted into heat, or the short waves of the upper end of 

 the spectrum and beyond are degenerated to the red end and returned 

 to the air mostly by contact, conduction, and convection — slightly by 

 radiation and absorption.] 



Maquenue has investigated the absorption and dissipation of heat by 

 foliage. The results were — 



(1.) All leaves dissipate a part of the heat vertically incident upon 

 them; this dissipation amounts to 0.25 of the total heat when the radia- 

 tion comes from a Bourbouze lamp,%ut only a few hundredths when it 

 comes from a Leslie cube. 



(2.) The leaves dissipate different amounts of heat from their two sur- 

 faces ; ordinarily the lower side dissipates more than the upper, but oc- 

 casionally we observe the contrary. 



(3.) The leaves absorb a sensible proportion of the heat radiated from 

 a Bourbouze lamp ; this absorption depends upon the presence of ab- 

 sorbents in the texture, especially of chlorophyl and water, and upon 

 the dissipation that takes place in the interior on the surface of each 

 cell ; it is ordinarily greater on the upper than on the lower side. 



(4.) The thick leaves absorb more than the thin ones. 



(5.) The absorbing power for the heat of (bodies of the temperature 

 of) boiling water is very nearly equal to that of lampblack. 



(6.) Leaves transmit heat better in proportion as they are thinner or 

 younger. 



(7.) The radiating power of leaves is for great differences of tempera- 

 tures nearly like that of lampblack ; it diminishes a little as the incli- 

 nation increases. 



(8.) The absorbing power of chlorophyl is, on the average, like that 

 of water for the radiation from the Bourbouze lamp, and increases in 



