16 Discrepancy qf Terrestrial and Atmospheric Temperature, 



a greater number of cases in which the difference is considerably 

 increased, from various causes powerfully tending to lower the 

 temperature of the border. 



In order to illustrate some of the causes which have this ten- 

 dency, let us make an experiment wherein the agencies bear 

 close analogy to what actually occurs. Let a box, water-tight, 

 be nearly filled with loose mould or peat, or sand, or even the 

 composition of a vine border; let this be tolerably dry, in order 

 to increase its capacity for water cooled to the temperature of 

 50° ; then saturated with snow water a little above the freezing 

 point. This process, with the aid of a frosty night, will bring 

 the contents of the box below 40°, and is supposed to take 

 place in the winter or early part of spring. If the box be 3 ft. 

 deep, the sun's rays will not affect it far below the surface, since 

 water is not easily heated from above ; and, besides, the heat 

 through the day will, at that season, be greatly counteracted by 

 the cold at night. As the season advances, rain may be expected 

 of a temperature above 60°; and this, under some circumstances,' 

 to be hereafter explained, would produce a very considerable 

 effect. Unfortunately, however, the box was made water-tight; 

 and its contents, being completely saturated with nearly gelid 

 water, cold and heavy, and mechanically immovable by the 

 descending warmer and lighter fluid, the latter must recede by 

 the surface in quest of an unoccupied lower level, after producing 

 only a slight effect on the surface of the contents of the box. 



The analogy between this and a badly drained outside vine 

 border is so obvious that it need not be traced : similarity might, 

 indeed, be substituted for analogy. Nor is it necessary to point 

 out the injurious consequences that must accrue to the forcing 

 vine. He that is not sufficiently impressed with this, has only 

 to imagine his feet to be plunged in the one temperature, and 

 his body in the other : the bare idea of such will make him 

 shudder. 



Let us now, with the same box, and similar materials, en- 

 deavour to produce a different result ; commencing by piercing 

 the bottom, and making a complete drainage ; and, further, 

 keeping the contents as dry as possible, and neither exposing 

 them to the sleety shower, nor placing them exterior to the front 

 of the vinery, where it would receive an overshot of snow from 

 the slippery roof; but letting the surface be snugly covered, or 

 thatched from all such drenching, till genial showers of the tem- 

 perature of 60° or more begin to fall. The latter may find it 

 below 50°, but it is not likely to be much below. If it should 

 be thus low, the water of the above warmth would percolate 

 speedily from top to bottom, and communicate its heat during 

 its progressive descent. The soil being dry, a portion of the 

 water would be held by capillary attraction among the particles 



