1847.] Value of Kon-condudors of Heat. Ill 



ter; where except in the temperate zones is this process going on? 

 In reasoning then, from the present to the past, we find nothing 

 which goes to sustain the assumption that the climate of the coal 

 era was a tropical one. 



VALUE OF NON-CONDUCTORS OF HEAT. 



Non-conductors of heat seem to perform two functions. 1st, 

 to prevent the escape of heat from a body, and 2d, to prevent 

 the access of heat to a body ; for example, wrap a cake of ice in 

 tow or cotton, and its temperature is not diminished nor increased. 

 Its own heat is preserved by the covering, and the outward heat 

 of the atmosphere cannot gain access to the body. The princi- 

 ple is an important one, and its application and use is often wit- 

 nessed in the common operations of nature. In fact this princi- 

 ple is among the most common in the world. The warmth of 

 the earth is maintained by the non-conduction of air and snow, 

 the warmth of lakes and rivers by the non-conduction of ice. 



Men avail themselves of the principle in preserving ice for 

 summer, in preventing the freezing of bodies by surrounding thera 

 with porous bodies which are always non-conductors, and we have 

 no doubt but that the principle may be more frequently applied 

 than it is. In certain situations for example, where vegetation 

 comes forward so early as to endanger it by subsequent frosts, this 

 principle may be applied successfully. The object will be to re- 

 tain the temperature of winter about the roots and trunks of trees, 

 till the danger of frosts has passed. This may be effected by keep- 

 ing the earth frozen about their roots by surrounding the trunk 

 and covering around it by hay, straw, or horse manure in- 

 termixed with its litter. When a covering of this kind is 

 applied during the continuance of snow, it will not melt as the 

 outward heat cannot penetrate through the non-conductors with 

 which they are surrounded, and it will not be melted and remov- 

 ed until the heat of the earth has considerably accumulated, 

 when it can gain access to the snow and ice laterally by conduc- 

 tion. But the earth conducts heat slowly, and hence the dissipa- 

 tion of the snow will not take place for a long time. 



Practically, then, gardeners as well as farmers, may secure 

 important ends by availing themselves of the non-conductibilty 

 of matter, taking any or all light and porous materials to pre- 

 serve one uniform temperature. The use of these materials about 

 fruit trees to keep vegetation back is only one among many. Of 

 these materials, horse manure will be the best, as it is not only 

 effectual in guarding against the access of heat, but is useful in 

 promoting vegetation when the proper time has arrived. 



