18 MR JOHN AITKEN ON DEW. 



at the tops of the blades ; and as the air and vapour are warmer, they tend to 

 rise and diffuse themselves, and so come into contact with the colder blades at 

 the surface, where the moisture gets deposited as dew. 



Having proved that, under the conditions existing during the experiments, 

 the ground was giving off vapour during the night, I then proceeded to 

 test the value of the observations previously described, and which were made 

 by placing shallow trays over the grass, in order to see if those experiments 

 were of any value. A small tray, similar to those used in the earlier 

 experiments, was prepared. It was made to fit tightly into one of the shallow 

 pans, in which, as before, was placed a small turf cut from the lawn. After the 

 turf and its pan was weighed, the tray was placed over it, and the whole 

 removed, and put in its place in the lawn. This was done at the same time 

 as the other experiment previously described, in which the turf and pan, 

 after being weighed, was freely exposed to radiation and evaporation. The 

 result was that the tray was found to check the evaporation. The inside 

 of the covering tray was dewed very much like another one placed over un- 

 disturbed grass. The turf covered by the tray lost only 6 grains (0*388 

 grammes) during the five hours, or about \ of the amount lost by the one freely 

 exposed to the air. This shows that the trays check the evaporation; we may 

 therefore conclude that the amount collected by them is less than would be 

 given off by the exposed parts of the grass. 



There seems to be reason for supposing that the amount lost per unit of 

 area in these experiments, with the freely exposed turfs, is too low an estimate 

 for the loss of the lawn at the parts where it was undisturbed, because the 

 under sides of the pans were not in good contact with the ground beneath them. 

 The experimental turf would not therefore be so warm as the rest of the 

 ground, and its evaporation would therefore be less. Most of the heat was 

 conveyed upwards towards the experimental turfs by the rising vapour, which 

 condensed on the under sides of the pans on which the turfs rested, as they 

 were always found to be dripping wet underneath when removed from the soil. 



The question now comes to be, Does this evaporation take place from grass- 

 land on all nights and in all weathers ? So far as my observations at present 

 go, evaporation is constantly going on, however strong the radiation. On all 

 nights on which the inverted trays have been exposed, dew has collected 

 on their inner surfaces. There is, however, an indirect way of testing this 

 point which may be noticed here, as it is specially applicable to observations on 

 giass land. As soil capable of supporting vegetation tends to saturate the air 

 in contact with it, it will be admitted that so long as the soil is hotter than the 

 air in contact with the grass, vapour will tend to diffuse upwards. Now I find 

 1 ij placing a minimum registering thermometer on the grass, and another on the 

 top of the soil among the stems of the grass, that there is always a difference 



