o36 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



irrigation is practised, The air, in this case, becomes completely 

 saturated every time it comes in contact with the soil ; the quan- 

 tity of vapour which it deposits by superficial contact on sub- 

 stances cooled by radiation, is much greater than in the case of 

 a dry or nearly dry soil ; and since these effects always ensue in 

 virtue of the circulation established in the lower stratum of the 

 atmosphere, we see that the air of this stratum forms a sort of 

 vehicle, by means of which the liquid spread over the surface of 

 the earth is successively carried to the surface of plants and 

 other bodies cooled by nocturnal radiation. Now, it will be under- 

 stood, that in order for this transportation to go on regularly, the 

 atmosphere must be calm ; the slightest breeze disturbs it, and 

 it is entirely destroyed by strong winds, which moreover (as 

 Wells had already observed) introduce another cause of disturb- 

 ance into the process, by communicating their own temperature 

 to plants, and thus causing that slight difference between the 

 temperature of solid bodies and that of the surrounding me- 

 dium to disappear, on which, in fine, the plijenomenon of dew 

 depends. 



Some have pretended to discover proofs of the existence of a 

 current of warm vapour exhaled by the earth, and an objection 

 against the principle of nocturnal radiation, in the different pro- 

 portions of water deposited, during calm and clear nights, on the 

 two surfaces of a bell glass inverted on the ground; for it often 

 hajDpens that the dew is more copiously formed on the inside 

 than on the outside of the vessel. But this fact by no means 

 justifies the conclusion ; for the phccnomena of circulation and 

 aqueous precipitation just described with reference to the air and 

 grass of a meadow, are also produced in the interior of the vessel, 

 the sides of which are cooled by radiation : these actions be- 

 come even more intense in this case, because the imprisoned air 

 is sheltered from the least atmospheric disturbance ; and we have 

 just seen that the quantity of water condensed on the outside 

 depends, on the contrary, on the degree of calm in the atmo- 

 sphere. Hence, the slightest degree of wind will suffice to ren- 

 der more abundant the precipitation on the interior of the bell- 

 glass, without leading to the conclusion of an increase favouring 

 the pretence of an exhalation of vapour from the earth, and con- 

 trary to the theory of dew founded on the cold produced by noc- 

 turnal radiation. 



Nothing then is simpler now than to comprehend why a 



