54 MR. T. HOPKHfS ON 



season, when the dew-point is very low. In the latter part 

 of the winter, as there is not sufficient vapour to permit much 

 of it to be liquefied by the cold of radiation, that cold may, 

 and frequently does, go on increasing without counteraction 

 during the absence of the sun. 



Thus we find that vapour, when condensed into liquid by 

 cold, always gives out heat; whether it is in the formation 

 of the cumulus cloud in the higher regions of the atmo- 

 sphere, in producing mist near the surface of the earth, or in 

 the production of dewdrops on the surface, the same effect 

 is experienced ; and, wherever heat is liberated, it must have 

 its degree of influence in counteracting the cold of radiation. 



It has been stated, that " metals give to glass, near which 

 they are placed, the property of more speedily attracting calo- 

 ric from hot air; and, on the contrary, that of yielding it more 

 speedily to cold air," because a mercurial thermometeraccom- 

 modates itself to a higher temperature sooner than an air ther- 

 mometer. But this may be because the heat which passes into 

 the glass tube of the thermometer is rapidly absorbed by the 

 mercury. In like manner, when placed in a colder medium, 

 the heat of the mercury is conducted to the inner surface 

 of the glass tube more rapidly than is the heat of the in- 

 closed air ; the mercury therefore cools quicker than the air. 

 But these results are consequences of mercury being a better 

 conductor of heat than air is. And when a piece of foil is 

 placed on the inside of a pane of glass, the outside of the 

 glass opposite the foil is not so soon cooled by radiation, 

 because the metal furnishes heat to supply the place of that 

 lost by radiation from the glass. 



In conclusion, we may then say that ^* falling dew"^ is pro- 

 duced by the descent of the cumulus or day-cloud, which, 

 cooled by evaporation in a higher part of the atmosphere, 

 sinks in the evening to the surface of the earth. Floating 

 dew is found in parts which have much vapour in the air 

 in proportion to the temperature, along with a clear atmo- 



