10 MR JOHN AITKEN ON DEW. 



with it, while trees were dry. M. Du Fay followed up these observations with 

 experiments, made by placing sheets of glass at different heights from the ground. 

 He found that dew formed on the lowest pane first, and only appeared on the 

 highest at a later hour; he also found that the lowest pane collected most 

 moisture. Other observers gave somewhat different explanations of the pheno- 

 mena connected with dew; but owing to a want of clearness, the subject did not 

 advance much till the masterly Essay on Deiv by Dr Wells made its appearance. 



Dr Wells' experiments were so simple, and his interpretation of the different 

 phenomena connected with dew so clear, that he has been justly considered the 

 great master of this subject. In his Essay he struck a medium between the 

 two previous theories as to the source of the moisture that forms dew. He 

 did not think with the ancients that it fell from heaven, nor with Gersten that 

 it rose from the earth, but that it was simply condensed out of the air in 

 contact with the surfaces of bodies cooled by radiation below the dew-point of 

 the air at the place. This opinion has, so far as I am aware, been generally 

 received up to the present time. 



Some experiments I have recently made on this subject have caused me to 

 differ entirely from Dr Wells as to the source of the vapour that forms clew. 

 As everything written by Dr Wells is, so to speak, stereotyped and final, there 

 seems to be the greater reason that any of his conclusions that seem doubtful 

 should be carefully criticised and fully investigated; I shall therefore give an 

 account of the experiments that have caused me to differ from so great an 

 authority. 



Dr Wells thought that almost all the moisture deposited as dew at night 

 was taken up by the air during the heat of the day ; so that, according to his 

 idea, vapour ascended from the earth during the day, and again descended and 

 became condensed as dew on the surface of the earth at night. My observa- 

 tions have led me to the very opposite conclusion. All my experiments indicate 

 that dew, on bodies near the surface of the earth, is almost entirely formed 

 from the vapour rising at the time from the ground; at least this would appear to 

 be the case generally in this climate, to which my experiments have been confined. 



After Gersten gave his reasons for supposing that dew rose from the 

 ground, and Du Fay extended the subject, Dr Wells combated their conclu- 

 sions, and successfully showed that their experiments did not prove that 

 vapour rose from the ground, and that all the phenomena adduced in favour of 

 their theory could be equally well explained according to his own. With 

 regard to Du Fay's reason for thinking that dew rises from the ground — 

 namely, that it appears on bodies near the earth earlier than on those at a 

 greater height — he says :* " But this fact readily admits of an explanation on 

 other grounds, that have already been mentioned. 1. The lower air, on a 



*An Essay on Dew, by William Charles Wells, p. 109. 



