492 Mr. Charles Tomlinson's Remarks 



its lower side before a single drop of liquid appears on the 

 upper surface. The body submitted to the frigorific action 

 of the sky is in contact with two masses of air — the one at 

 rest and humid, because it is sheltered and situatecl close to 

 the earth's surface ; the other less humid and exposed to the 

 changes of the atmosphere. The former then will be more 

 disposed than the latter for the precipitation of vapour, and 

 the dew ought to show itself first on the side turned towards 

 the soil; it may even exist only on this surface, if the air 

 has but little moisture, or is agitated by wind. Hence the 

 experiment of a plate covered with waxed cloth, which being 

 placed on the grass was found sometimes to be moistened 

 only on its lower surface, by no means proves that the dew is 

 exhaled from the ground like those clouds of vapour which 

 are seen to arise from a vessel full of hot w^ater." 



Melloni, in two letters to Arago, describes some experiments 

 with thin tin plates in which the surface looking towards the 

 ground is covered with dew while the upper portions remain dry. 



While Melloni insists on the influence of radiation in the 

 formation of dew, whereby bodies become colder than the air 

 and so condense its vapour, he claims to have first pointed 

 out the great influence exerted by the reaction of the air on 

 the radiating body; and he defines dew, not as an immediate 

 effect of the cooling produced by nocturnal radiation, but as a 

 consequence of a series of actions and reactions between the 

 cold due to the radiation of plants &c. and the cold trans- 

 mitted to the surrounding air. The grass becomes slightly 

 cooled below the temperature of the air, but it soon imparts 

 to it a portion of the cold thus acquired ; and as the difference 

 in temperature between the radiating body and the air is 

 independent of the absolute temperature at the time, grass 

 surrounded by the colder air becomes lower in temperature 

 and imparts a fresh amount of cold to the air, which in its turn 

 reacts on the grass and gives to it a still lower temperature, 

 and so on. In the mean time the air acquires a kind of vertical 

 circulatory movement, the upper portions condensed by cold 

 descending and those at the surface ascending. As this 

 gradual cooling goes on, the surface air gradually becomes 

 saturated with moisture and the slight amount of cold pro- 

 duced by the direct action of radiation is sufficient to 

 condense the vapour contained in the air. 



If in the above statement we read a thin metal tray instead 

 of grass, the process and its result remain the same. 



If we now turn from the grass meadow to grander natural 

 objects, abundant proof will be found that dew is condensed 

 from the air and does not rise out of the ground. The 



