Observations made with a Black Bulb Thermometer. 95 



the ground and the lower and warmer strata of the atmosphere. 

 Owing to this the carbonic acid of the atmosphere acts as the glass 

 of a greenhouse, letting through the solar rays, but partly retaining 

 the dark rays emitted from the ground. Thus if the quantity of 

 carbonic acid in the atmosphere increases, the temperature of the 

 ground and the lower atmospheric strata will be raised, till the 

 increase of radiation into space caused by the increase of tempera- 

 ture has restored the equilibrium between gain and loss of heat. 

 But to this is added a circumstance which considerably adds to the 

 influence of the carbonic acid. Aqueous vapour possesses the same 

 remarkable property as carbonic acid, and is nearly transparent to 

 solar heat, and nearly opaque to terrestrial heat. Aqueous vapour 

 alone is, however, unable to produce any radical change of climate. 

 For the quantity of aqueous vapour in the atmosphere is itself 

 depending upon the temperature of the air ; if this be lowered by 

 some cause, for instance by radiation, the aqueous vapour is partly 

 condensed and separated from the atmosphere, whereby its protecting 

 influence is diminished, and then the increased radiation causes a 

 new condensation of vapour, and so on. It is, therefore, only in 

 regions and seasons already favoured by nature with a warm and 

 damp climate that aqueous vapour alone is able to play the part of 

 greenhouse glass ; whereas in cold and dry regions, where the 

 protection is most needed, aqueous vapour fails." * 



These two descriptions are fairly typical, but differ in the 

 important particular that whereas Tyndall's remarks are based 

 upon some very high-class experimental facts, Ekholm's are based 

 upon sheer assumption.! But in either case it is difficult to see 

 where the great protection comes in ; for at the best, especially on 

 a rotating globe, the good absorber, and therefore good radiator, 

 can only delay somewhat, and chiefly by absorbing its own radia- 

 tion,! the final emission of heat into space. For it is to be noted 



* N. Ekholm, " On the Variations of the Climate of the Geological and Historical 

 Past, and their Causes," Q. J. R. Met. S., January, 1901. Some authors suggest 

 that carbonic acid is an important constituent of the atmosphere of the planet Mars. 



f See inter alia a notice by Cleveland Abbe and F. W. Very, of a paper by 

 K. Angstrom, in M. W. R. y 1901, p. 268 ; J. Hann, " Handbook of Climatology," 

 1903, p. 399. There is an interesting " Eeport on Carbonic Acid," by W. C. Day, 

 in Langley's " Eesearches on Solar Heat," p. 202. Incidentally the author 

 mentions the theory of M. H. Schloesing that the ocean acts as a reservoir and 

 regulator of atmospheric carbonic acid, confining its variations between very narrow 

 limits. 



\ According to C. C. Hutchins and J. C. Pearson, a column of ordinary damp air, 

 with a relative humidity of 78 per cent., 245 cm. thick, absorbs 60 per cent, of its 

 own radiation, the other 40 per cent, being freely transmitted. " Air Eadiation," 

 M. W. R., July, 1904, p. 314. 



7 



