Transactions. 7 



vegetation as compared with the material of which our roads are 

 composed. But the true explanation is rather that the stones or 

 compact earth on the surface of the roads are in contact with the 

 ground, and in such good heat communication with it that the 

 temperate of their upper surface is kept from falling below the 

 dew-i)oint. But if the under surfaces of the stones are examined 

 on a dewy evening they will generally be found to be copiously 

 bedewed, while the upper surfaces are dry — a good deal of the 

 vapour rising from the ground being trapped by the under sides of 

 the stones, while that which escapes is not enough to saturate the 

 air at the temperatures of the exposed surfaces. All these facts 

 seem plainly to prove that under most conditions of our "climate 

 vapour rises from the ground at night as well as during the day, 

 and even from parts of it which at first sight would seem least 

 likely to yield it — that is, from hard, dry-looking roads. They 

 shew that under ordinary conditions there is a sufficient amount of 

 heat in the soil (the heat stored up during the day) to cause evapo- 

 ration to a certain extent at night of the moisture it contains ; and 

 so long as the soil is warmer than the air above, this vapour will 

 tend to diffuse itself upwards, and part of it will be condensed into 

 dew on the blades of grass cooled at the time below the dew 

 point. And Mr Aitken's conclusion from the experiments re- 

 ferred to is that dew on bodies near the surface of the earth is 

 almost entirely formed from the vapour thus rising at the time out 

 of the ground. Mr Andson concluded his paper with some general 

 remarks on the conditions, favourable or the reverse, to tlie pro- 

 duction of dew. In the course of his j^aper he remarked that the 

 removal of stones from dry and stony fields, which had sometimes 

 been tried with a view to the improvement of the land, had been 

 found to be prejudicial to the crops in dry seasons. The explana- 

 tion of this is that the stones lying on the surface of the gniund 

 prevent evaporation to a considerable extent during the day, and 

 that their under surfaces during the night are bedewed by the 

 vapour rising from the ground, and in this way the moisture is 

 conserved and returned to the soil, whereas otherwise it would 

 have escaped into the atmosphere. 



