272 Further Remarks on Mr. Aitken's Theory of Dew, 



before the travellers reached any considerable body of water, 

 nocturnal dews were abundant, and they were deposited from 

 the air, and did not rise out of the ground. 



Mr. Aitken also remarks that my notice of the Florentine 

 Academicians, of Robert Boyle, and Le Roi have no bearing 

 on the subject. The bearing is that these early observers 

 proved that the moisture which forms dew and hoar frost 

 exists in the air, and does not exhale from the ground. 



Mr. Aitken is also "puzzled to understand" what bearing 

 Pictet's observation has on the subject. In the abstract in 

 ( Nature ' of Mr. Aitken's memoir, it appears as an original 

 discovery that " these observations made at night showed the 

 ground at a short distance below the surface to be always 

 hotter than the air over it." Pictet observed the same fact in 

 1779. So also in my account of the weighed turf, I certainly 

 did not wilfully form a " misconception of the essential features 

 of the experiment," when I compared it to objects which, when 

 exposed on Patrick Wilson's scale-board, gained weight, while 

 in Mr. Aitken' s case the turf lost weight. It is true that my 

 observations were founded on the abstract of the memoir 

 contained in ' Nature/ In January last I wrote for a copy 

 of the memoir, which was promised as soon as the i Edinburgh 

 Transactions ' were published. I waited until May and did 

 not receive it. I inquired for it at the Royal Society in June, 

 but it had not arrived, nor have I yet had the privilege of 

 reading it. Mr. Aitken is therefore entitled to any advantage 

 that may arise from my use of the abstract instead of the 

 original memoir. 



As I do not intend to write again on this subject, I conclude 

 by assuring Mr. Aitken that I have no unfriendly feeling 

 towards him ; but on the contrary freely admit that he has 

 achieved much good scientific work, which I cannot but 

 admire ; but as regards his new theory of Dew I think he has 

 gone astray, and in the interests of scientific truth I have 

 ventured to criticise it. The subject is one that has occupied 

 a great deal of my attention, and there is no doubt in my mind 

 that, if this theory be accepted, a large amount of excellent 

 work on the part of first-rate observers must be set aside as 

 false. 



Highgate, N., August 9, 1886. 



