210 WRITINGS OF JOSEPH HENRY. [1855- 



Effect of cold on the ground. — The depth to which ground 

 is frozen in some places from year to year, is also an indica- 

 tion of the severity of the seasons ; the effect of cold will 

 penetrate very differently however in dry and moist soil; 

 in the first it will depend entirely on the conducting power 

 of the material, and in the second, it will also depend upon 

 the amount of water to be congealed. The conducting capac- 

 ity being the same, the depth to which the given degree of 

 cold will penetrate will be much greater in dry than in wet 

 soil, on account of the great amount of latent heat given off' 

 by the water before it is solidified. In dry conducting soil 

 the propagation of cold downwards may continue some time 

 after the surface of the ground has become considerably 

 heated. 



In a conducting body all parts tend to an equilibrium of 

 temperature. If the upper end of a vertical iron bar be 

 heated and then removed from the source of heat, it gradu- 

 ally becomes cooled, while the other parts increase in tem- 

 perature, until gradually an equilibrium is established ; con- 

 versely, if we cool the upper end of the bar, it will take heat 

 from the next lower part ; and this from the next, and so on, 

 until the cooling reaches the extreme end, which will be 

 cooled last. If, before the cooling has reached the lower end, 

 we heat the upper part, the next below will be heated, and 

 so on, proceeding downwards ; thus waves, as it were, of 

 heat and cold may be sent through the length of the bar, 

 becoming less and less in intensity as they descend. In this 

 way explanations have been given of the phenomenon of 

 caverns colder in summer and warmer in winter, — the cold 

 wave due to a lower temperature requiring six months to 

 reach the point of observation. 



The freezing of the ground in certain soils is hurtful to 

 vegetation ; the frozen stratum expanding irregularly from 

 below heaves up the surface, and frequently loosens or breaks 

 the roots of the plant. A covering of snow is a protection, 

 since this sul)stance from its flocculent nature and the air 

 entangled in it is a bad conductor of heat. As a general 

 rule during cold weather a thermometer in air on the snow 



