«» MB. T. H0PKIN3 ON 



dew;^ as, in the absence of the sun, the cooling inflaence 

 of terrestrial radiation increases, more vapour is condensed, 

 and the condensation takes place at a greater distance from 

 the earth's surface. This extension of condensation may 

 seem to an observer to be a rising of the dew, because it 

 will appear successively at greater distances from the sur- 

 face : yet the floating globules may not really have risen, 

 but, from the operation of the causes just pointed out, may 

 have been all formed in the part in which they are seen, the 

 apparent rising of the dew being an optical deception. At 

 other times, however, floating dew may rise. Wherever 

 aqueous vapour is condensed into water, heat is liberated, 

 and this liberated heat may possibly expand the air in the 

 locality sufficiently to cause it to ascend and carry the floating 

 dew with it to a greater height. In this way the dew may really 

 rise, if a sufficient amount of heat be liberated. When the 

 dew extends to a moderate height it is generally called mist, 

 and may frequently be seen filling our valleys, and at a 

 distance looking like water. 



"What is called " falling dew" may be often felt in the 

 evenings in this part of the world. But it is very common 

 in the latter end of summer and in autumn in calm weather, 

 and appears to be produced in the following way. During 

 the hotter part of the day, and until about four o'clock in 

 the afternoon, the heat of the sun raises vapour to the higher 

 part of the atmosphere, and forms cumuli or day-clouds. 

 Soon rfter this time these day-clouds begin to evaporate, and 

 consequently to cool the atmosphere in the part. The portion 

 of the atmosphere thus cooled is thereby made heavier 

 than adjoining portions at the same height not similarly 

 affected — the previous equilibrium of atmospheric pressure 

 is then destroyed, and the cooled part sinks to a lower level. 

 As evaporation of the cloud proceeds greater cold is pro- 

 duced, and by the time that the whole of the cloud is 

 evaporated, the mass of air is so much cooled as frequently 



