on a new Theory of Dew. 491 



certain amount of dryness will be produced, evaporation will 

 take place, and the vapour thus formed will be again precipi- 

 tated on the upper parts of the blade. But in order to effect 

 this, the ground must be comparatively dry. This appearance 

 of dew on the upper parts of plants led to the idea that dew 

 rose out of the earfh. 



The process above described will of course be disturbed or 

 destroyed by wind. On the other hand, what is called "a 

 perfectly calm night " does not exist in nature. Air is so 

 very mobile a fluid, that the least difference in temperature sets 

 it in motion in the form of imperceptible currents ; and there 

 are always numerous inequalities of temperature depending 

 on the nature of the soil, the greater or Jess amount of vege- 

 tation, the degree of cultivation, the presence of houses, all 

 of which prevent equilibrium from being attained in the air. 

 But supposing the air to be tranquil so far as our perceptions 

 are concerned, then the processes of radiation, convection, 

 and condensation sufficiently account for the presence of dew, 

 whether on the herbage or on the inner surface of Mr. 

 Aitken's thin metal trays. 



But let us hear Melloni. He says : — " Some have pre- 

 tended 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 proportions of 

 water deposited during calm and clear nights on the two 

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

 happens 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 phenomena 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 become even more intense in this case, because the 

 imprisoned air is sheltered from the least atmospheric dis- 

 turbance ; 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 atmosphere. Hence the slightest 

 degree of wind will suffice to render 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 contrary to the 

 theory of dew founded on the cold produced by nocturnal 

 radiation. 



" Nothing, then, is simpler now than to comprehend why a 

 radiating body such as a piece of wood or stone, placed on a 

 moist soil towards sunset, is abundantly covered with dew on 



