262 Mr. J. M. Wilson on the Diminution of Direct 



or less delusive, or that we must be careful not to misinterpret 

 its readings ; but the special failure of the black bulb which he 

 suggests as possible would not explain this perplexing pheno- 

 menon. 



For if the black bulb were opake to the visible and transpa- 

 rent to the invisible rays, the case would be as follows : — There 

 issue from the sun calorific rays, as is commonly supposed, of 

 various qualities, which we may class as visible and invisible 

 rays. Both fall on our atmosphere and pass through it, suffer- 

 ing loss as they descend, but the invisible rays suffering most, 

 from their greater absorption (as is known) by the aqueous 

 vapour in the atmosphere. Now if the black bulb were opake 

 to, and therefore absorbed, the visible rays, but were transpa- 

 rent to, and therefore, we will suppose, totally reflected the in- 

 visible raySj the two thermometers at great heights would differ 

 only by the amount due to the direct action of the visible rays. 

 Now they differ, as Mr. Glaisher assures us, by nil; therefore 

 the effect of the visible rays on the black bulb at great heights, 

 where the visible rays are at a maximum, is nil; and therefore, 

 of course, if they suffer no change in their transit through the 

 atmosphere, their effect at all points in their path down to the 

 surface of the earth is nil. The invisible rays, moreover, as they 

 descend are more and more absorbed, and therefore affect both 

 bulbs equally, and have less and less effect by direct action on 

 the exposed bulb, if by this hypothesis they have any effect at all. 

 Why, then, the two thermometers should differ so much on the 

 earth^s surface it is not easy to see. It cannot be in conse- 

 quence of one of them being exposed to the visible rays, for 

 they produced no effect above ; nor in consequence of the in- 

 visible rays, on this hypothesis, for, if they affected it at all, they 

 would affect it most where they themselves are greatest, at great 

 heights. 



I am obviously not supposing that Professor Tyndall made 

 this hypothesis with a view to explain the phenomenon in ques- 

 tion; but it is as well to point out that his remarks, though very 

 valuable as indicating to meteorologists a possible limitation of 

 the powers of the black bulb which may have escaped them 

 hitherto, do not at all set this question at rest. But his hypo- 

 thesis is somewhat startling, for one imagines glass transpa- 

 rent to brilliant, and opake to obscure heat; and it would be 

 strange if blackening the glass reversed both these properties at 

 once. 



I am aware that the pyrheliometers of Herschel and Pouillet 

 have given grounds from which it has been concluded that if 

 the atmosphere were removed we should receive two-thirds as 



