146 WAT£R VAPOUR ON MARS. 



length 542 at the other end of the spectrum. Of, these seven 

 groups, the two from wave-length 660 to 646, around Ho, and 

 597 to 585 around D have until now been the most conspicuous, 

 though the lines around B and a in the red are easily seen. A 

 great difftculty in the way has been the fact that, owing to the 

 weakness of the Hght reflected from the planets, the dispersion 

 employed could be but small, and thus the individual lines due 

 to the planet's atmosphere are in danger of becoming mixed up, 

 not only with one another, but occasionally with the metallic 

 lines due to the sun's light. Particularly is this the case with 

 regard to wave-lengths 548 to 542. 



Although, as I shall mention later, many eminent observers 

 were satisfied that they had proved the existence of water vapour 

 on Mars, the intrinsic difficulties inherent in the investigation were 

 so great that grave doubts were expressed as to the accuracy 

 of the results announced, and it was very desirable for the sup- 

 porters of the theory that the matter be rendered indisputable 

 and indubitable. With the larger amount of light collected 

 by the modern giant telescopes, it became practicable to use a 

 greater number of prisms thus giving to the planet's light a larger 

 dispersion than hitherto. Then Lowell conceived the idea of 

 photographing the spectrum of Mars, the photographic plate 

 ■causing the fugitive and faint impressions received b}' the eye 

 to accumulate on a permanent record. This method has further 

 the distinct advantage of allowing a leisurely and more accurate 

 verification and check of the results to be obtained. V. ]M. Slipher, 

 who is in charge of the photographic department at Flagstaff 

 Observatory, by rendering the photographic plates sensitive far 

 into the red, was enabled to obtain definite proof of the existence 

 •of water vapour in the atmosphere of the planet,* And the 

 photographs obtained by this process are so very clear and definite 

 that F. A. Very was enabled to make quantitative measurements, 

 and the results of the comparisons, repeated many times and 

 in different orders, show that the water vapour bands at " a " 

 in the Martian spectrum are 22 per cent, stronger than in our 

 ■own atmosphere. 



Now what I want to point out is that while many astronomers 

 liave contended that there could not be water vapour on ^Nlarsf 

 a simple application of the law of gravitation and of the kinetic 

 theory of gases would have sufficed to demonstrate at least the 

 possibility of the existence of water vapour in the Martian at- 

 mosphere, and in large quantities. 



I have several times in lectures dwelt on tliis point, and have 

 always maintained the conviction, based on the arguments referred 

 to above, that water vapour must of necessity be present in Mars 

 .and in large quantities, and the spectroscope has once more 

 vindicated this point of view. 



* Astronomische Nachrichten, Band 179, Nr. 4290. 



t e.g., Johnstone Stoney, Atmospheres upon Planets and SateUites, Scien- 

 .tilic Transactions of the Royal Duljlin Society, November, 1897 ; Seward, 

 Fortuightly Reviciv, Augu-;t, 1907 ; C. Lane Poor, The Solar System. London, 

 1908. 



