24 MR JOHN AITKEN ON DEW. 



moisture rising from the ground. If we examine a gravel walk on a dewy 

 evening, we shall find the under sides of the stones, especially those near the 

 solid ground, to be dripping wet; and we may occasionally see isolated patches 

 of stones wet on the upper surface, probably due to an openness in the ground 

 at the place permitting a free escape of vapour. 



Another reason why the upper surface of the gravel does not get wet, is 

 that it is in good heat communication with the ground; the stones are thus 

 kept warm; and as a good deal of the vapour rising from the ground is trapped 

 by the under surfaces of the stones, the vapour which escapes these surfaces is 

 not enough to saturate the air at the temperature of the exposed surfaces of 

 the gravel. The following temperature, taken at 10 p.m. on the 25th September, 

 will give an idea of the difference in temperature on the surface of grass and 

 on gravel, and show why no dew is formed on the top of the stones while it 

 collects on the grass. A thermometer placed on the surface of the gravel was 

 34°, while one placed near it, but on grass, was 30°, or 4° lower. At the 

 surface of the soil under the grass the temperature was 40°, and it was almost 

 exactly the same temperature at the bottom of the gravel which was 1\ inches 

 deep. 



We see from the above that hot vapour, rising from the ground under 

 grass, ascends till it comes into contact with the cold blades, and is condensed 

 on their exposed surfaces; whereas on the gravel road the under sides of the 

 stones are nearly as cold as their exposed surfaces, and much of the warm 

 vapour gets condensed under them, while the vapour which escapes to the 

 surfaces has its dew-point lowered by mixing with the surrounding air, and 

 the upper surfaces of the stones being in good heat communication with the 

 ground, are not cool enough to condense this vapour and form dew. 



A simple manner of studying the formation of dew on roads is to take, say, 

 two slates, and place one of them on the gravel and one on a hard part of the 

 road. If these slates are examined on a dewy night, their under sides will be 

 found to be dripping wet, though their upper surfaces and the road all round 

 them are quite dry. This experiment also shows us that under most conditions 

 of our climate vapour does rise from hard dry-looking roads on dewy nights. 



In studying questions of this kind, and for showing the importance of the 

 heat communicated by the earth to the radiating body, the following experiment 

 may be useful. Place on the grass, soil, or road, a slate and a piece of iron, 

 say an ordinary 7 lb. weight. Alongside of these place another slate and 

 weight; but instead of the latter resting on the ground, elevate them a few 

 inches on small wooden pegs driven into the earth. If we examine the 

 surfaces of these bodies on dewy nights, the following will be the general 

 result. While the grass all round is wet with dew, we shall find that the 

 upper surfaces of the slate and the weight resting on the ground keep dry, and 



