on Mr. Aitken's Theory of Dew. 271 



" the essential difference between the old and the new theories 

 is as to the source of the moisture which forms the dew. 

 Instead of being condensed from the air above by the cool 

 vegetation, Mr. Aitken maintains that it comes from the 

 ground.'"' 



Again the Chambers's article, referring to Wells, says : — 

 " The points of the grass, small twigs, and all other good 

 radiating surfaces are cooled the most; and accordingly we find 

 the dew-drops most abundant on these bodies ; whilst on metal, 

 or hard stone surfaces, which are poor radiators, we seldom or 

 never find any dew." This is the Wells picture ; the writer 

 now turns to the Aitken picture. " A closer observation 

 reveals the fact that these so-called ' dewdrops ' are formed at 

 the end of the minute veins of the leaves and grass, and are 

 not now recognised as dew at all, but moisture exuded from 

 the interior of the plants themselves.'''' 



And yet Mr. Aitken is angry with me for calling his theory 

 new, and for asserting that, if true, it will supersede the labours 

 of previous observers. He says : — " I do not find that he 

 [Mr. Tomlinson] adduces any results of previous observers 

 that are in any way rendered nugatory by the results set forth 

 in my paper." 



If Mr. Aitken would condescend to study the classical 

 memoir of Melloni (an abstract of which occupies the greater 

 part of my paper), I should be much surprised if he did not 

 become a convert to its experimental methods and conclusions. 

 But at present he sees through the spectacles of his own theory, 

 and therefore cannot appreciate the force of Melloni' s singular 

 care with which he protected his thermometers from sharing in 

 the radiation of the surrounding bodies, whose temperature 

 they had to indicate ; for while other observers get differences 

 of temperature between their two thermometers, amounting in 

 some cases to as much as 16° or 18° F., Melloni is satisfied 

 with a difference of only 2° or 3° C. and his theory of convec- 

 tion justifies this modest difference, and also accounts for 

 many other phenomena, including the inverted trays and other 

 objects which Mr. Aitken found wetted only on their under 

 surfaces. 



In like manner Mr. Aitken does not see the force of the 

 observations made in Persia and the African Desert, seeing 

 that his remarks " apply only to this climate." Surely the 

 great forces of Nature rule as impartially in Persia and in 

 Africa as in Scotland ; and where no aerial vapour exists, 

 there is no deposit of dew. The cases given were intended to 

 show that in the arid regions there was no dew ; but that long 



