MR JOHN AITKEN ON DEW. 13 



case on those nights when there is wind, and the air is not saturated, a con- 

 dition which seems to be very frequent in our climate at ordinary elevations. 

 We must remember that the air may not be saturated when dew is forming; and 

 the dew-collecting surface requires to be cooled below the temperature of the 

 air before it collects moisture. 



In experimenting with these trays different kinds of ground were selected, 

 and the trays placed over them after sunset, that is, after the earth had ceased 

 to receive heat, and the heat-tide had begun to ebb. They were generally 

 examined between 10 and 11 p.m., and again in the morning. 



Dew on Grass. 



Confining our attention to the trays placed over grass, the result of the 

 experiments was that, on all occasions yet observed, there was — 1. Always 

 more moisture on the grass inside the trays than outside. 2. There was always 

 a deposit of dew inside the trays. 3. There was often a deposit outside the 

 trays, but the deposit on the outside was always less than on the inside, and 

 sometimes there was no deposit outside when there was one inside. 



Now I think these facts prove that far more vapour rises out of the ground 

 during the night than condenses as dew on the grass. This excess is evidenced 

 by the greater amount of moisture on the grass inside the trays than outside, 

 and by the amount of dew condensed inside the box. Under the ordinary con- 

 ditions found in nature, this excess is carried away by the wind and mixed up 

 with the air, while some of it is deposited on bodies further away from the ground. 

 It should be noticed that the inside of the tray was more heavily dewed than 

 the outside. This shows there was a higher vapour tension inside than outside 

 the enclosure, which proved that the vapour rising from the ground outside 

 the tray had got mixed up with drier air, as it did not form so heavy a coating 

 of dew as the inside air, even though it had the advantage of a slightly lower 

 temperature than the inside, on account of it being the side of the metal 

 from which the heat was radiating. 



It may be as well to notice here some objections that may be made to this 

 way of testing the point. It may be said, that though so much vapour does 

 rise under these trays, yet if they were removed and the grass freely exposed, 

 the vapour would not rise, and that the vapour rises because the tray keeps 

 the ground under it warm. Observation certainly shows that the ground under 

 the trays is kept slightly warmer than outside them. At night a ther- 

 mometer is higher on the grass under the tray than on that outside, and next 

 morning the ground at 3 inches below the surface is from 1 to 2 degrees warmer 

 under the tray than outside its influence. This objection to the protecting 

 influence of the trays has an appearance of reason about it; but if we examine 

 the facts, I think it will be admitted that instead of being an argument 



