448 
Proceedings of the Royal Society 
All these different methods of testing point to the conclusion that 
during dewy nights, in this climate, vapour is constantly being 
given off from grassland, and almost always from bare soil; that the 
tide of vapour almost always sets outwards from the earth, and but 
rarely ebbs, save after being condensed to cloud and rain, or on 
those rarer occasions on which, after the earth has got greatly cooled, 
a warm moist air blows over it. 
It seems probable that when the radiation is strong, that soil, 
especially if it is loose and not in good heat-communication with 
the ground, will get cooled below the dew-point, and have vapour 
condensed upon it. On some occasions the soil certainly got wetter on 
the surface during the night, but the question still remains, Whence 
the vapour 1 Came it from the air, or from the soil underneath ? The 
latter seems the more probable source : the vapour rising from the hot 
soil underneath will be trapped by the cold surface-soil, in the same 
way as it is trapped by grass over grass land. During frost, oppor- 
tunities are afforded of studying this point in a satisfactory manner, 
as the trapped vapour keeps its place where it is condensed. On these 
occasions the under sides of the clods, at the surface of the soil, are 
found to be thickly covered with hoar-frost, while there is little on 
their upper or exposed surfaces , showing that the vapour condensed 
on the surface-soil has come from below. 
The next division of the subject is on dew on roads. It is gene- 
rally said that dew forms copiously on grass, while none is deposited 
on roads, because grass is a good radiator and cools quicker, and 
cools more, than the surface of a road. It is shown that the above 
statement is wrong, and that dew really does form abundantly on 
roads, and that the reason it has not been observed is that it has 
not been sought for at the correct place. If a road is examined on 
a dewy night, and the gravel turned up, the under sides of the stones 
are found to be dripping wet. 
For studying the formation of dew on roads, slates were found to 
be useful. One slate was placed over a gravelly part of the road, 
and another over a hard dry part. Examined on dewy nights the 
under sides of these slates were always found to be dripping wet, 
while their upper surfaces, and the ground all round, were quite dry. 
The importance of the heat communicated from the ground may 
be illustrated by a simple experiment with two slates or two iron 
