12 MR JOHN AITKEN ON DEW. 



Then again, all such hygrometers, as well as wet and dry bulb thermometers, 

 will have their indications affected by radiation ; they will surround themselves 

 with an envelope of cooled air, as there is but little wind during the time the 

 observations require to be made. Their indications would therefore be of little 

 value, and investigation by means of them had to be abandoned. 



What first caused me to doubt the present theory, and led me to suppose 

 that dew is formed from vapour rising from the ground, was the result of some 

 observations made in summer on the temperature of the soil at a small depth 

 under the surface, and of the air over it, after sunset and at night. On all 

 occasions in which these temperatures were taken, the ground a little below 

 the surface was found to be warmer than the air over it. It is evident that, so 

 long as these conditions exist, and provided the supply of heat is sufficient to 

 keep the surface of the ground above the dew-point, there will be a tendency 

 for vapour to rise and pass from the ground into the air, the moist air so 

 formed will mingle with the air above it, and its moisture will be condensed, 

 forming dew wherever it comes in contact with a surface cooled below its 

 dew-point. 



These considerations suggested another method of experimenting than by 

 the use of hygrometers. If vapour is really rising from the ground during 

 night, it seemed possible that it might be trapped on its passage to the air, 

 and that this might be accomplished by placing over the soil something 

 that would check the passage of the vapour, while it allowed the heat to 

 escape. To carry out this idea, I placed over the soil shallow boxes or trays, 

 made of tinplate and painted. These trays were 3 inches (76 mm.) deep, 

 and more than a foot (305 mm.) square in area; they were placed in an 

 inverted position over the soil to be tested. 



The action of these trays will be somewhat as follows : — Supposing the 

 roof of the small enclosure formed by the covering tray is not by the passing 

 air or by radiation cooled below the temperature of the ground. Then 

 evaporation will cease when the air between the tray and the ground is 

 saturated, and no dew will collect on the inside of the enclosure. But if the 

 tray is cooled below the temperature of the ground, vapour will condense on the 

 inside, and more vapour will rise from the ground to supply its place, and this 

 will go on so long as the ground is the warmer of the two. The effect of these 

 trays will be very much the same as if there was no enclosure, and the air 

 over the grass was nearly saturated, motionless, and of a lower temperature 

 than the soil. But it is evident the trays will check the evaporation on most 

 nights, on account of the slow circulation inside, and also on account of the 

 air inside being always nearly saturated, which is not the case outside the 

 enclosure, so that under most conditions.it seems likely there will be less 

 evaporation under the trays than outside them. This will be particularly the 



