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II. — On Dew. By Mr John Aitken. 



(Read 21st December 1885.) 



The immense amount that has been written on the subject of dew renders it 

 extremely difficult for one to state anything regarding it which has not been 

 previously expressed in some form. It has been examined over and over by 

 minds of every type, and from every point of view; so that every possible 

 explanation of the different phenomena seems to have been given, and so many 

 passing thoughts recorded, that from the literary point of view the whole 

 subject seems exhausted. As a necessary result, these different treatises 

 are in many respects contradictory; and it would be quite impossible to 

 construct anything like a consistent explanation and account of our subject^ 

 from the very voluminous writings of those who have treated it from the purely 

 literary point of view, and whose ideas have been evolved from their inner 

 consciousness, according to what seemed to them the fitness of things, and 

 without questioning nature as to the truth of their conclusions. On the 

 scientific side of the subject, however, the writings are not so voluminous, and 

 additions to it are still required to enable us to determine which of the many 

 conflicting opinions are correct. 



In ancient times it was thought that the moon and stars had an important 

 influence on dew, probably because there is most dew on those nights when 

 these orbs shine brightly on the earth; thus confusing two things which have a 

 common cause, and making one the effect of the other. Aristotle placed the 

 knowledge of this subject far in advance of his time. He defines dew to be 

 humidity detached in minute particles from the clear chill atmosphere. The 

 Romans, led by the writings of Pliny, returned again to the primitive idea that 

 dew fell from the heavens. This idea retained its position during the course of 

 the Middle Ages. Then began an endless variety of theories, such as, that the air 

 is condensed into water by the cold, that the moon's rays caused it, and so on. 



In the beginning of the eighteenth century clearer ideas began to be formed, 

 and a reformation took place, in which, as in most reformations, the swing of 

 the pendulum went to the extreme on the opposite side. Dew was no longer 

 believed to descend from the heavens, for Gersten advanced the idea that it 

 rose from the earth ; and in this opinion he was followed by M. Du Fay and 

 Professor Musschenbroek, the latter, however, afterwards made some observa- 

 tions which caused him to change his opinion. Gersten was led to think that 

 dew rose from the ground, because he often found grass and low shrubs moist 



VOL. XXXIII. part i. B 



