THE POBMATION OF DEW. 49 



to become heavy enough to sink to the surface of the earth, 

 where it constitutes the cold air that is often felt in the 

 evenings succeeding warm days in the summer and autumn ; 

 — in some places known as the land breeze. When the day 

 cloud is very large, the atmospheric mass is sometimes suffi- 

 ciently cooled to cause it to descend to the surface of the 

 earth, before the globules of the water constituting the cloud 

 are all evaporated. These globules are then found floating 

 iu the lower air, and any object passing through them is soon 

 wetted by them as if by rain, though they do not, like drops 

 of rain, fall freely to the ground. In Lancashire, in the 

 month of September, the clouds raised daily from the Irish 

 Sea frequently descend in the evening to the earth, and they 

 are abundant over the lower levels, particularly on the river 

 Irwell, where they are well known to boatmen under the 

 common appellation of " falling dew," and are remarkable 

 for their intense cold, the effect of previous evaporation of a 

 portion of the cloud in the higher part of the atmosphere. 

 As these masses of floating particles of water are formed in 

 the higher part of the atmosphere, they are in their origin 

 clouds, though called falling dew when they reach a low 

 level. It is obvious that either the floating dew formed 

 near the surface, or this *' falling dew," if carried along by 

 a light breeze, will impinge upon, and attach to, any pro- 

 jecting object. In like manner, persons passing through a 

 mass of this nature come in contact with the globules and 

 are wetted by them, as is well known to coachmen, boat- 

 men, and other persons similarly exposed. 



But dew also forms on objects by a process differing from 

 those just named, and this dew is found attached to different 

 substances in very unequal quantities. The daily clouds 

 are often evaporated early in the evening, and the sky left 

 clear, yet highly charged with transparent aqueous vapour. 

 At such times, radiation of heat cools the surface of the earth 

 until its temperature sinks below the dew-point of the 



