on a new Theory of Dew. 489 



dew, or its diffusion in quantities sensibly equal on all low- 

 growing plants, whatever their position with respect to the 

 sky. He also insists that dew is never formed except when 

 the air is nearly saturated : also that radiation under a clear 

 sky is always a fixed quantity, whatever the temperature may 

 be ; so that in nights equally serene the same substance 

 always cools to the same extent, whatever the temperature of 

 the air at the time. 



The nocturnal differences in heat, moisture, and aqueous 

 condensation do not proceed, as hitherto supposed, from the 

 direct action of the cold due to the radiation of plants and of 

 exposed portions of the soil, but most of the phenomena which 

 precede and accompany the formation of dew result from the 

 more or less prolonged sojourn of the air around the radiating 

 surfaces. In a meadow while dew is forming under a clear 

 sky and a calm air, let us divide the low^er region of the 

 atmosphere into two strata — -the lower, which scarcely rises 

 above the grass ; the higher, which extends upwards from it 

 30 or 40 metres ; and suppose the cold due to the nocturnal 

 radiation of the herbage to be only 1° ; this degree of cold 

 will always remain the same whatever the temperature of the 

 atmosphere. If the air be at 20°, the higher portions of the 

 grass will descend to 19° a few minutes after sunset ; the air 

 in contact with them will be cooled, will descend into the 

 interior of the meadow, and reach the ground. This move- 

 ment of descent along the leaves and stems will necessarily 

 restore to the air a portion of the lost heat, and will force it 

 to reascend towards the higher part of the meadow, where it 

 will undergo a fresh cooling which will cause a second descent, 

 and so on ; so that the air of the meadow or of the lower stratum, 

 impelled by two opposite influences, will soon take a circulating 

 or convective motion. The cold produced at the surface of 

 the meadow wall be gradually transmitted by this aerial circu- 

 lation to the lower parts, which will also be cooled ; and, on 

 the other hand, both by radiation and by their contact with 

 the superior portion of the stems, the temperature of the whole 

 mass of air which is put in motion will fall. If it be at 19° # 5, 

 the grass will sink to 1 8°*5. By repeating the process the 

 air will fall to 19°, and the grass to 18° ; the air to 18°'5, the 

 grass to 17 0, 5, and so on ; so that by the action of the grass on 

 the air and the reaction of the air on the grass, the temperature 

 of the lower stratum will be gradually diminished by several 

 degrees, and the space occupied by the herbage retaining its 

 vapour will approach the state of saturation. A thermometer 

 introduced into this space will mark a temperature much 

 lower than that of the higher stratum ; the hygrometer will 



