MR JOHN" AITKEN ON DEW. 19 



between the minimum on and under the grass, often amounting to a considerable 

 number of degrees, this difference being greatest on nights when radiation is 

 strongest, and least when windy and cloudy. It is only as the day advances 

 that the temperature on the grass approaches that under it; this is caused by 

 the upper thermometer being heated by solar radiation sooner than the lower ; 

 but as the air is by this time drier, there is no tendency for it to lose moisture 

 by contact with the colder soil, though some of the dew condensed on the grass 

 will, after it evaporates, diffuse downwards, and condense on the soil. It may 

 therefore be safely concluded that, on almost all nights in this climate, vapour 

 does rise from grass-covered land, and it is this vapour that we see as dew 

 on the exposed surfaces of the grass. 



Dew on Soil. 



While the experiments previously described were being made on ground 

 covered with grass, parallel ones were made on bare soil. The inverted 

 trays placed over soil always showed a greater amount of condensed vapour 

 inside them than those over grass. Sometimes there was a heavy deposit 

 of clew inside, while there was none outside. This would be owing to the soil 

 radiating directly to the trays, and to the amount of heat brought up and con- 

 veyed to the trays by the vapour. The temperature of the trays was thus in 

 some cases kept above the dew-point of the air outside. 



Experiments were also made by weighing a small area of the surface soil, 

 to see if it also lost weight like the grass-land during dewy nights. One of the 

 small pans was covered with a thin layer taken from the top of the 

 soil. The pan and its soil was then weighed and put on the surface of the 

 bare ground at the place where the soil had been taken out. It was left 

 exposed the same time as the other trays with the turfs. On weighing, the 

 soil was found to have lost 23 grains (1490 grammes) in five hours, or nearly 

 the same as the turf. Alongside this pan was placed another one of the 

 same area, and with the same weight of soil, but covered with a small tray, 

 to see whether the covering trays decreased the evaporation from soil as well as 

 from grass. The result was the same as was found with the turf — a decrease 

 in the evaporation. The protected soil lost only 8 grains (0'518 grammes). 



The following are the details of a few of the experiments on grass-land and 

 on bare soil made on different evenings, and show the temperatures and the 

 loss of moisture per 0'25 square foot, or 0023225 square metre, during the 

 experiments :* — 



* Throughout this investigation I have adhered to the Fahrenheit scale, as it is the one generally- 

 used for meteorological purposes in this country, and because it possesses what appears to me practical 

 advantages over the Centigrade scale. The degrees are of a more suitable size, and combine ease in 

 reading with accuracy. This scale also avoids a fruitful source of error, experienced by many, when 

 taking readings above and below zero. 



