XIELLONI ON THE NOCTURNAL COOLING OP BODIES, 539 



Every one has doubtless remarked that the dews are less 

 copious in the earlier part of spring than in the equally long 

 nights of the latter part of autumn. To see clearly the cause of 

 this difference, it will be sufficient for us to observe, that the 

 leaves, whence arises the greatest portion of the cold manifested 

 at night in the lower strata of the atmosphere, are few and small 

 at the beginning of the former season, large and numerous at 

 the end of the latter ; so that the cold, and consequently the in- 

 creased degree of humidity, being greater in the latter case, the 

 precipitation of dew is also more abundant. Add to this, that 

 the quantity of elastic vapour existing in a given space increases 

 more rapidly than the temperature ; and since the diurnal heat 

 is generally greater in autumn than in spring, we see that under 

 the influence of the same radiation, there ought to be a greater 

 quantity of vapour precipitated in the former season. The thick- 

 ness of the stratum of air cooled at night by the contact of plants, 

 will evidently depend on the nature and on the luxuriance of the 

 vegetation ; it will be large in meadows abundantly clothed with 

 long grass of thick and vigorous growth, less in those where the 

 herbage is low and poor, and still less on naked soils. The same 

 theory will hold with regard to the position of the minimum 

 of temperature, which will be found quite close to the soil in 

 naked places, and can scarcely exist, as we have just seen, either 

 at the base or at the summit of the grass, and will maintain 

 itself near the numerous and compact leaves which are subject 

 to the action alone of the zenithal part of the heavens. 



These direct consequences of the theory have been perfectly 

 confirmed by those persons even who deny the origin of dew 

 founded on nocturnal radiation, and who think to explain the 

 phaenomenon by the exhalation from the soil. In fact, these 

 gentlemen have found the maximum of cold at the height of 

 7 inches in a meadow covered with a luxurious vegetation, at 

 the height of 2 inches in a meadow recently mown, and at a 

 fraction of a line above the soil beaten down and entirely de- 

 prived of grass. Their thermometers, badly prepared for these 

 sort of observations, being placed in contact with the leaves of 

 different species of plants, gave indications sometimes equal, 

 sometimes lower, and scarcely ever higher, to those of thermo- 

 meters freely suspended at the same elevation above the soil. 

 And in spite of results so little conformable to their views, they 

 have persisted in maintaining that the depression of temperature 



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