50^ MR. T. HOPKINS ON 



atmosphere, when that part of the vapour which is in contact 

 with the earth is condensed into water, and becomes liquid 

 dew. As the earth is successively cooled by radiation below 

 the dew-point of the air, more dew is deposited, until in this 

 way a considerable part of the vapour of the lower portion 

 of the atmosphere may be abstracted from it, and collected 

 on the surface in the form of water. 



A French chemist, C. A. Prieur, has maintained, " that 

 the moisture deposited on bodies soon after sunset, is not 

 the same with that we find on them again at sunrise. There 

 TS consequently (he says) an interruption in the pheno- 

 menon—an evaporation of the serein or evening dew, and a 

 new production in the morning," rosee. 



It has been shown that what is called evening dew is often 

 descended cloud, and it is commonly found only for a short 

 time after the sun has set, as it soon evaporates or is de- 

 posited. The cold, too, produced by the evaporation of the 

 cloud, ceases in a short time to be experienced, and a period 

 then occurs, comparatively warm, when evaporation from 

 the surface may be renewed. But, in the absence of cloud, 

 radiation cools the surface, and acts with increasing effect 

 upon it until some time before sunrise, when the surface 

 is cooled to the greatest extent by radiation, and much dew 

 is deposited. But morning, thus separated from evening dew, 

 by the time of its formation, is produced by the cooling 

 influence of radiation from the surface of the earth ; whilst 

 the falling evening dew is descended cloud brought down by 

 the cold of cloud evaporation. 



The circumstances favourable to the formation of dew, 

 are, an abundance of aqueous vapour in the air, and a clear 

 sky. The dew-point over the Mediterranean is sometimes 

 as high as 75°, and the transparent vapour of the part so 

 circumstanced will, by diffusion, and the action of gentle 

 breezes, be conveyed to the adjoining countries, where clear 

 sties and great radiation condense large portions of the 



