MR JOHN AITKEN ON DEW. 43 



probable explanation now seems to me to be, that they are caused by the 

 uprising of the hot air and moisture from the ground, mixing with the colder 

 air above the grass, much in the same way as a fog is produced over a river in 

 sunny weather when the water is warm. So far as my observations go, these 

 fogs generally form over flat damp fields, after hot sunny clays, and they have 

 been seen where there was no high ground from which cold air could flow in. 



There almost seems reason for supposing that much of the moisture 

 collected on grass, and which looks like true dew, may under many conditions 

 be fine rain from fog formed in the manner above described. Because the hot 

 air and vapour rising through the grass will tend to form fog, where it mixes 

 with the cold air near the upper part of the grass, and if there is little wind 

 this fog will settle on the blades. Under most conditions this fog will not 

 form above the grass, and will not therefore be visible. It will often not form 

 above the blades, because the hot moist air may there meet with too much dry 

 air to supersaturate it, but there seems reason for supposing that it will be 

 often formed amongst the stems of the grass. 



During frosts we have excellent opportunities for studying the condensation 

 of the vapour of our atmosphere, because it remains in the position where it is 

 condensed and is easily seen, being neither absorbed by the ground nor dropped 

 from the plants, &c, on which it may be deposited. I took the opportunity 

 afforded by two nights of this kind for observing two opposite conditions of 

 the air, and I shall here describe the effects of the radiation on the nights of 

 the 14th and 15th November. During the afternoon the canopy of clouds that 

 had hung over the earth for some days was gradually drawn aside, and moved 

 away southwards. By 5 p.m. on the 14th the sky was cloudless. There was 

 only a very slight movement of the air from the north, the radiation was strong, 

 and the air dry. These conditions continued all night, and the minimum 

 thermometer in the screen fell to 25°. 



Next morning the ground and the grass were frozen. It was what is called 

 a black frost. There was no hoar-frost on the trees, and what little there was 

 on the grass was irregularly distributed. All the little hollows, of about a foot 

 square in area and under, had a deposit of hoar-frost, while the higher parts of 

 the grass had none. As there was no wind, and only a slow movement of the air, 

 this peculiar distribution would not be caused by the heating effect of the passing- 

 air on the higher and more exposed blades, but was probably owing to its 

 dryness. The small hollows being less freely exposed to the circulation, the 

 air in them became more moistened from the vapour rising out of the ground 

 than the air a little higher up, where it got mixed with a larger amount of the 

 dry air. The test surface on the ground was quite dry at 9 p.m., and also next 

 morning, showing that the ground had been giving off vapour all night. 



In contrast with this, let us now look at the condition of matters on the 



