256 ESSAY 



Walls, I believe, as far as warmth is con- 

 cerned, are regarded as useful, during a cold 

 night, to the plants which touch them, or are 

 near to them, only in two ways ; first, by the 

 mechanical shelter which they afford against 

 cold winds, and secondly, by giving out the 

 heat which they had acquired during the day. 

 It appearing to me, however, that, on clear and 

 calm nights, those on which plants frequently 

 receive much injury from cold, walls must be 

 beneficial in a third way, namely, by prevent- 

 ing, in part, the loss of heat, which they would 

 sustain from radiation, if they were fully ex- 

 posed to the sky, the following experiment was 

 made for the purpose of determining the just- 

 ness of this opinion. 



A cambric handkerchief having been placed, 

 by means of two upright sticks, perpendicularly 

 to a grassplat, and at right angles to the course 

 of the air, a thermometer was laid upon the 

 grass close to the lower edge of the handker- 

 chief, on its windward side. The thermometer 

 thus situated was several nights compared with 

 another lying on the same grassplat, but on a 

 part of it fully exposed to the sky. On two of 

 these nights, the air being clear and calm, the 

 grass close to the handkerchief was found to be 

 4 warmer, than the fully exposed grass. On a 



