Mr. John Aitken on Dew. 365 



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 pic- 

 ture. ' A closer observation reveals the fact that these so-called 

 1 dew-drops ' 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 them- 

 selves/ And yet Mr. Aitken is angry with me for calling 

 his theory neiv, and for asserting that, if true, it will supersede 

 the labours of previous observers." My critic here shifts his 

 ground ; when he finds the contents of the first part of my 

 paper, regarding the rising of vapour at night, does not justify 

 his title of New Theory, he cleverly seems to put me in oppo- 

 sition to recognized authority on another point. T^e above 

 quotations certainly are in opposition to each other, and great 

 credit is due for the very ingenious manner in which the case 

 is put. But, unfortunately for the critic, the words are not 

 the words of Dr. Wells, but are those of the writer in ' Cham- 

 bers's Journal/ and can scarcely even be said to be founded 

 on the teaching of Wells ; so that, further than affording Mr. 

 Tomlinson a little mental gymnastics, his efforts are here 

 entirely thrown away. 



It does seem strange, considering how much is indirectly 

 attributed to Dr. Wells regarding the deposition of dew on 

 grass, that his ' Essay ' really contains very little that is definite 

 either about the radiation from grass, compared with that from 

 other bodies, or about the amount of dew deposited upon it. 

 Towards the end of the first part of his ' Essay,' where he 

 gives "the results of some experiments which were made for 

 the purpose of ascertaining the tendencies of various bodies 

 to become cold upon exposure to the sky at night," he 

 says : — "In the observations hitherto given by me o a the cold 

 connected with dew, the temperature of the grass has been 

 chiefly considered, partly because my first experiments had 

 been made upon it, and partly from a wish, which arose after- 

 wards, to compair my own experiments with those of M. Six, 

 which had been confined to that substance. I found it, how- 

 ever, very unfit to furnish the means of compairing the degrees 

 of cold produced at night on the surface of the earth, at dif- 

 ferent times and places ; as its state on different nights, on 

 the same parts of the plat I commonly made use of, and in 

 different parts of the plat on the same nights, was often very 

 unequal in point of height, thickness, and fineness ; all of which 

 circumstances influenced the degree of cold produced by it." 



