Observations made with a Black Bulb Thermometer. 89 



dew-points of Table V. the readings of the black bulb may be less, 

 they cannot well be greater, than they would have been if there were 

 not any radiation of dark heat. Therefore in any case the difference 

 of maxima should increase as the dew-point falls. So far this 

 accords with the conclusion formulated by Prof. Langley twenty 

 years ago, that " observations taken at different seasons of the year, 

 at different hours of the day, or at different altitudes above sea-level, 

 all point to the same conclusion, namely, that there is a large absorp- 

 tion of solar radiation which depends upon and increases with the 

 prevalence of atmospheric moisture." * 



If our conclusion be justified that the absorption of the sun's heat 

 is almost or quite independent of the humidity of the air, it follows 

 that the variation of the differences of maxima as shown by Table VI. 

 actually represents the variation of the radiation of dark heat from 

 the black bulb, and that on this account alone the black bulb will 

 read 4° or so lower in very dry air than in air half saturated with 

 aqueous vapour. 



But this difficulty is created : If it be acknowledged as a fact that 

 the sun emits rays of all orders of refrangibility, and that every 

 absorbing particle in the earth's atmosphere converts the energy 

 it receives into rays of a lower order, it would seem that a humid air 

 should have the same effect upon some parts of the extreme infra-red 

 of the solar spectrum as it has upon the rays of terrestrial dark heat. 

 Therefore direct solar rays of this class would be largely detained in 

 the middle reaches of the atmosphere because the high relative 

 humidity must retard their free passage. Consequently these rays 

 could never be easily discovered, unless at great altitudes, by direct 

 observation of the sun. 



I have not been able to ascertain whether the behaviour of the 

 black bulb thermometer in vacuo under a perfectly cloudless sky 

 has been previously discussed. Certainly there are not many places 

 in the world where the conditions are sufficiently favourable to make 

 such discussion profitable. So that I am not able to tell whether 

 Tables V. and VI. are in agreement with earlier work. But if the 

 suggestions in the three preceding paragraphs are correctly deduced 

 it seems that the solar heat rays capable of selective absorption are 



* S. P. Langley, in his classical "Researches on Solar Heat," p. 189. Since the 

 text above was written I have seen an even more definite statement by F. E. 

 Fowle, Jr., which is worth quoting in full : " The selective absorption of water 

 vapour within the range of densities observed seems to depend only on the amount 

 of the absorbent present, and is well expressed by Bouguer's formula. In other 

 words, the absorption produced by a given quantity of water in the form of vapour 

 is the same whether the path is great through a small density or vice versa."— 

 Smithsonian Mis, Coll., 1904, vol. ii., p. 11. 



