MR JOHN AITKEN ON DEW. 23 



neatli. On lifting the small clods on the surface, it was observed that their 

 under surfaces and sides, when close to each other, were all thickly covered 

 with hoar-frost so thickly as to be nearly white, while the upper surfaces 

 exposed to the passing air had but little deposited on them, — the interpreta- 

 tion of which seems to be, that the vapour rising from the hot soil underneath 

 had got trapped in its passage through the cold clods. Its presence under- 

 neath and on the sides of the clods was an evidence that the moisture was 

 on its passage from the ground, when it met with the cold surface which im- 

 prisoned it. 



This hoar-frost on the sides and under the clods could not be due to vapour 

 condensed from the passing air, because the upper surfaces of the clods had 

 scarcely any deposited on them, and that in spite of the fact that the upper 

 surfaces would be the colder, as they were those from which the radiation 

 was taking place. It seems probable that even the vapour condensed on the 

 upper surfaces of the clods was part of the vapour escaping from the soil, and 

 was not taken from the passing air. 



The occasions when the earth is most likely to receive vapour condensed 

 upon it from the passing air, are not on clear nights when the radiation is 

 strong, but rather when after strong radiation and cooling of the surface 

 the weather changes, becomes cloudy, and a warm moist wind blows over the 

 land. Occasions of this kind are seen most frequently after frosts, and 

 undoubtedly much moisture is then condensed on the soil, but the moisture 

 so condensed is not what we call dew. 



Dew on Koads. 



There is considerable difference among works on dew as to the absence 

 of dew on roads, but almost all agree in stating that it is never formed on 

 roads ; and the presence of dew on grass, while none is visible on roads, is 

 generally attributed to the greater radiating power of vegetation over that of 

 the material of which our roads are composed. Now I find that this state- 

 ment as to facts is wrong, and the explanation is also inaccurate. Dew really 

 does form on roads in great abundance on dewy nights, and the material of 

 the road is practically as good a radiator as the grass. 



The reason why it is generally saicl that dew is not seen on roads is 

 owing, not to the less radiating power of the stones, but to the fact that dew 

 has not been looked for at the proper place. The blades of grass are practi- 

 cally non-conductors of heat, while stones conduct fairly well. The result of 

 this is that we are not entitled to look for dew on the upper surfaces of stones, 

 as on grass, but it must be sought for on their under sides, because the stones 

 are good conductors, and the vapour tension under them is much higher than 

 at their upper surfaces, owing to the higher temperature of the air laden with 



