538 MELLONI ON THE NOCTURNAL. COOLING OF BODIES. 



but also by the evaporation, more or less abundant, of the sea, 

 lakes and rivers ; the winds afterwards transport it, and spread 

 it even to those countries where water is scarcest. If the air 

 and soil are impregnated with moisture, which is the case in 

 regard to calm and clear nights that succeed a season of rain, 

 the dew shows itself everywhere in the greatest profusion. But 

 when the weather is extremely dry and the air calm, the local 

 action predominates, especially during the night, when the equi- 

 librium of the atmosphere is not disturbed by the presence of 

 the sun ; and in this case the atmospheric humidity is in pro- 

 portion to the proximity of the sources. Now in order to cause 

 the air to deposit its vapour, it is necessary that there be a fall 

 of temperature, more or less considerable according to the degree 

 of humidity prevailing : the precipitation of the atmospheric 

 vapour, therefore, will be more slow and scanty in proportion as 

 we remove further off from the reservoirs of water, and will cease 

 entirely at a certain distance, if the air be sufficiently dry, what- 

 ever may be the degree of cold which bodies acquire under the 

 influence of a pure and calm sky. This is the reason why, in 

 seasons of great dryness, dew no longer shows itself except on 

 plants situated in marshy or watered places, along the borders 

 of lakes, ponds and rivers. 



The nocturnal frigorific action exerted by vegetables on the 

 surrounding air, and the reaction of this fluid on the vegetables, 

 can never cease until the heat communicated by the earth to the 

 plants is equal to the heat lost by the radiation and the contact 

 of the air. And this state of equiUbrium in a system of bodies 

 so heterogeneous appears to require a considei-able time ; for if 

 the sky be clear and the air calm during the whole of the night, 

 the temperature goes on decreasing at the earth^s surface, even 

 till sunrise. Hence in calm and clear weather, the lower strata 

 of the air ought to be the more humid in proportion as the night 

 is the more advanced. It is for this reason that, cateris paribus, 

 the dew is precipitated in greater abundance, and penetrates 

 more deeply into the interior of the tufts of plants, hedges, and 

 groves, towards morning than in the earlier hours of the night ; 

 and that the phaenomenon shows itself more copiously in autumn 

 than in summer, when, in consequence of the short absence of 

 the sun, the radiation of plants and the circulating movement of 

 the surrounding medium do not last long enough to pi'oduce any 

 great humidity in the lower region of the atmosphere. 



