PROTECTION OF VEGETA TION FROM COLD. 5 



and the air engaged among it, even on the stillest 

 night, to sink as low as the dew-point. Thus- 

 either clouds, by their counter radiation, or wind, 

 by mixing a comparatively thick stratum of air 

 with that next the earth, keep the grass and deli- 

 cate parts of other plants from sinking to the dew- 

 point. When there is not enough of clouds and 

 wind to afford this degree of protection, clew 

 begins to form, and by preventing the temperature 

 of any leaf or flower from sinking below the 

 dew-point, saves them all from destruction, unless, 

 as when hoar-frost appears, the dew-point itself is 

 below the freezing-point. 



[Added December 15, 1892.] Thus when neither 

 clouds, nor wind blowing among the plants, 

 suffice to protect them from sinking to the dew- 

 point, the temperature to which leaves, flowers 

 and grass sink is lower the dryer is the air, not 

 because dry clear air is more diathermanous 

 than moist clear air, 1 but because the dew-point 

 is lower the dryer is the air. 



1 Tyndall, "On Terrestrial Radiation," Proc. Roy. Soc., 

 February, 1883. 



