282 Professor S. P. Langley on Sunlight, &c. [April 17, 



We deduce in connection with it a new value of the solar heat, so 

 far altering the old estimates that we now find it capable of melting a 

 shell of ice sixty yards thick annually over the whole earth, or, what 

 may seem more intelligible on its practical bearings, of exerting over 

 one horse-power for each square yard of the normally exposed surface. 

 We have studied the distribution of this heat in a spectrum whose 

 limits on the normal scale our explorations have carried to an extent 

 of rather more than twice what was previously known, and we have 

 found that the total loss by absorption from atmosphere is nearly 

 double what Las been heretofore supposed. 



We have found it probable that the human race owes its existence 

 and preservation even more to the heat-storing action of the atmosphere 

 than has been believed. 



The direct determination of the effect of water vapour in this did 

 not come within our scope ; but that the importance of the blanketing 

 action of our atmospheric constituents has been in no way overstated, 

 may be inferred when I add that we have found by our experiments 

 that if the planet were allowed to radiate freely into space without 

 any protecting veil, its sunlit surface would probably fall, even in the 

 tropics, below the temperature of freezing mercury. 



I will not go on enumerating the results of these investigations, 

 but they all flow from the fact, which they in turn confirm, that this 

 apparently limpid sea above our head, and about us, is carrying on a 

 wonderfully intricate work on the sunbeam, and on the heat returned 

 from the soil, picking out selected parts in hundreds of places, sorting 

 out incessantly at a task which would keej) the sorting demons of 

 Maxwell busy, and as one result, changing the sunbeam on its way 

 down to us in the way we have seen. 



I have alluded to the practical utilities of these researches, but 

 practical or not, I hope we may feel that such facts as we have been 

 considering about sunlight and the earth's atmosphere may be stones 

 useful in the future edifice of science, and that if not in our own 

 hands then in those of others, when our day is over, they may find 

 the best justification for the trouble of their search, in the fact that 

 they prove of some use to man. 



May I add an expression of my personal gratification in the oppor- 

 tunity with which you have honoured me of bringing these researches 

 before the Eoyal Institution, and of my thanks for the kindness with 

 which you have associated yourselves for an hour, in retrospect at 

 least, with that climb toward the stars which we have made together, 

 to find, from light in its fulness, what unsuspected agencies are at 

 work to produce for us the light of common day. 



[S. p. L.] 



