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Proceedings of the Royal Society of Edinburgh. [Sess. 
rising, as shown by the figures in Table I., and the radiation temperature 
on the grass is also rising quickly. The radiation temperature, of course, 
rises because the thickening of the air has made it less diathermanous. 
As the temperature of the air is rising it must be receiving its heat from 
the sun, since its temperature is above that of the surface of the ground, 
as shown by the temperature in the screen being always higher than on 
the grass during the formation of these fogs. Since the temperature of 
the air is rising, its relative humidity will be decreasing, and if some other 
influence were not at work the air ought to become clearer and not more 
hazed. But even if we suppose that radiation is in some mysterious way 
responsible for these fogs, yet it could not make them without the aid of 
the sulphur products, because these fogs form in air which is not saturated, 
and it is only with the aid of the sulphur products that fogs can be formed 
in unsaturated air. We may therefore dismiss the radiation theory, unless 
radiation can be shown to act in some way not at present known. Of 
course, it is not here contended that radiation cannot produce fogs, as it is 
well known that it can ; but in the absence of sulphur products it can only 
do so by cooling the air to the saturation point, a condition not necessary 
for the formation of these sun-fogs. 
From Dr Shaw’s recent book on Forecasting Weather, it appears that 
the tendency for fogs to form in the morning has been observed by others. 
At page 287 he says: “The morning is the most favourable time for the 
formation of anticyclonic fogs, because if the sky has been clear, radiation 
during the night will have reduced the temperature of objects on the 
surface of the ground, and a cold layer of air will be found at the surface.” 
This, however, gives no explanation of why these fogs do not form before 
sunrise, and only begin to form after the sun is up, and the temperature of 
the air and of the ground is rising ; nor does it explain why they are only 
formed in polluted air, or how they are formed in unsaturated air. 
There is a peculiarity of these sun-fogs which has been frequently 
observed ; and as it helps to confirm the explanation offered of their 
formation, it may be here referred to. During these sun-fogs, which often 
continue for days in succession, and persisted in a marked way last October, 
it has been observed that they frequently tend to get less thick as the 
night advances, and to be clearer the next morning, only to thicken again 
after sunrise. This clearing during the night would seem to be caused by 
the sun ceasing to shine and keep up the supply of nuclei, while the light 
winds tend to clear away those made during the previous day. 
It is not here contended that this investigation has gone into the whole 
question of the origin of these sun-fogs. We have only dealt with the 
