MR JOHN AITKEN ON DEW. 33 



I feel that the dissecting hand of science has here dope an injury to our 

 poetic feelings. Every poet who has sung of the beauties of nature has added 

 his tribute to the sparkling dew-drop, and Ballantine in his widely-known 

 song has taught a comforting lesson from the thought that " ilka blade o' grass 

 keps its ain drap o' clew." No doubt the drop of clew to which the poets refer 

 is the large sparkling diamoncl-like gem that tips the blades of grass, and which 

 we now know is not dew at all. While, however, our interpretation of nature 

 has changed, the teaching of the poet remains, and the sparkling dew-drop may 

 still teach the same comforting lesson. We must, however, change our views 

 regarding the source of the refreshing influence. We may no longer look upon 

 it as showered down from without, but as welling up from within — no longer as 

 taken by the chill hand of night and given to refresh and invigorate exhausted 

 nature ; we must rather look upon it as suggesting that we are provided with 

 an internal vitality more than sufficient to restore our exhausted powers, after 

 the heat and toil of the clay are past. 



Radiation. 



I have said in a previous part of this paper that the surface of bare soil and 

 of roads will radiate at night as much heat as grass. It may be thought I have 

 said this simply because we do not now require that grass should be the more 

 powerful radiator to enable us to explain its greater wetness on dewy nights. 

 Though it is not now necessary to suppose that grass is a powerful radiator, yet 

 there is nothing in the above experiments to prove it either a good or a bad one. 

 It therefore seemed desirable that some definite experiments be made on this 

 point, and also to determine the radiating powers of different substances at 

 night, as this is always an interesting and important point in questions con- 

 nected with the deposition of dew ; and the radiating power of grass, though 

 not the principal cause of its wetness at night, might be still considered to play 

 a subordinate part. 



We have already a great number of experiments on the radiating powers of 

 different substances. Unfortunately most of the accurate measurements of this 

 kind are from laboratory experiments, and do not appear to bear very directly 

 on our subject. Franklin's early experiments, made with different coloured 

 cloths placed on snow, seem to have given our ideas an unfortunate bias on 

 this subject. From observing the different depths to which cloths of dif- 

 ferent colours sunk in snow, when exposed to solar radiation, he came to 

 the conclusion that the dark colours absorb most heat, and this conclu- 

 sion seems for long to have influenced our ideas. If the heat radiated and 

 absorbed by a surface was composed entirely of visible rays, then no doubt 

 the colour of a body would be an index of its radiating and absorbing powers. 



VOL. XXXIII. PART I. E 



