DEW. 365 



temporary blindness. This had been ascribed to the moon's 

 influence, and the term moon-blindness had therefore been 

 given to the affection. In reality, the moon has no more to 

 do with this form of blindness than the stars have to do with 

 the formation of dew. The absence of clouds from the air 

 is the true, cause of the mischief. There is no sufficient 

 check to the radiation of heat from the eyeballs, and the 

 consequent chill results in temporary loss of sight, and some- 

 times even in permanent injury. 



Since clouds possess this important power, it is clear that 

 while they are present in the air there can never be a copious 

 formation of dew, which requires, as we have seen, a con 

 siderable fall in the temperature of the air around the place 

 of deposition. When the air is clear, however, radiation 

 proceeds rapidly, and therefore dew is freely formed. 



But it might seem that since objects in the upper regions 

 of the air part with their radiant heat more freely than 

 objects on the ground, the former should be more copiously 

 moistened with dew than the latter. That the fact is exactly 

 the reverse is thus explained. The cold which is produced 

 by the radiation of heat from objects high in the air is com- 

 municated to the surrounding air, which, growing heavier, 

 descends towards the ground, its place being supplied by 

 warmer air. Thus the object is prevented from reducing the 

 air in its immediate neighbourhood to so low a temperature 

 as would be attained if this process of circulation were 

 checked. Hence, a concave vessel placed below an object 

 high in air, would serve to increase the deposition of dew by 

 preventing the transfer of the refrigerated air. We are not 

 aware that the experiment has ever been tried, but undoubt- 

 edly it would have the effect we have described. An object 

 on the ground grows cold more rapidly, because the neigh- 

 bouring air cannot descend after being chilled, but continues 

 in contact with the object; also cold air is continually 

 descending from- the neighbourhood of objects higher in air 

 which are parting with their radiant heat, and the cold air 

 thus descending takes the place of warmer air, whose neigh- 



