\vater|^vapour ox mars. 147 



The presence of water vapour near " a " in the red end of the 

 spectrum of Mars was demonstrated by Huggins and Miller as 

 early as 1864,* by Father Secchi in 1869.! by Janssen in 1867,$ 

 and finally by Vogel in 1872-3, § a quotation from whose work 

 reads almost identically with Lowell's recent report : 



" We may conclude with certainty that JNIars possesses an atmosphere 

 which in composition does not differ essentially from our own, and which 



is particularly rich in the vapour of water." 



Campbell, however, in 1894 and 1896. failed with the giant 

 Lick telescope and the perfected spectroscope at his disposal, 

 to find water vapour in Mars|| ; Keeler at, the same Observatory, 

 was equally unsuccessful^ ; while Jewel, of Johns Hopkins 

 University, maintains that the presence of water vapour could only 

 be perceived with the present appliances if the atmosphere of 

 Mars were much richer in water vapour than that of the Earth : 

 which, as the Flagstaff report shows, is the case.** 



The presence of water vapour was re-affirmed in 1895 and i8g6, 

 by Janssen, in reply to Campbell tf ; by Huggins, also in reply to 

 CampbellJJ ; by Vogel, in i894§§ ; by Scheiner and Wilsing, in 

 18941111 ; who find that the so-called " telluric " lines show far 

 more distinctly in the spectrum of Mars than in that of the Moon. 



I will now endeavour to show, in as few words as possible, how 

 the presence of water vapour in Mars can be concluded, 

 (i) From the principle of gravitation, 

 (2) From the kinetic theory of gases. 



(i) One effect of the law of gravitation is that the density of 

 the atmosphere is directly related to the gravitational pull ; and 

 this pull at the centre of Mars is but 38 of that of the Earth, 

 giving an atmosphere about -14 of the density of that of the Earth. 



On the Earth, the atmosphere, as every one knows, exercises, 

 in consequence of gravitation, a pressure on all that lies beneath 

 it, equal roughly to I4lbs. per square inch. Now, it is this pressure 

 that helps to keep water in its liquid state, and only when, by 

 means of heat, a contrary upward pressure is developed in. 

 the water is this atmospheric pressure counteracted. This 

 happens when the temperature of the water at sea-level — 760 mm. 

 (or 30 inches) pressure — reaches 100° Centigrade (212° Fahrenheit) 

 and this temperature is known as the boiling point of water. 

 Naturally the higher above sea-level one goes, the less theair- 



* Philosophical Transactions, 1864 ; Monthly Notices Royal Astronomical 

 Society, XXVII., p. 179. 



I Sugli Spettri prismatici di Corpi Celesti, Roma, 1872. . 



I Comptes Rendus, tome LXIV., 1867, p. 1304. 



§ Untersuchungen ueber die Spectra der Planeten, Leipzig, 1874. 



ii Publications, Astronomical Society of the Pacific, vol. VI., pp. 228, 273 ; 

 Astronomy and Astrophysics, vol. XIII., p. 752 ; Astrophysical Journal, 

 1895, vol. II., p. 28. 



^Astrophysical Journal, 1897, vol. V., p. 328. "• 



** Astrophysical Journal, vol. I., p. 311 ; vol. III., p. 254. 



ft Bulletin de la Societe Astronomique de France, 1895, P- 10. 



XI Astronomy and Astrophysics, XIII., 1894, P- 77'^ ', The Observatory, 

 1894, P- 353 ; Astrophysical Journal, 1895, vol. I., p. 193. 



§§ Astrophysical Journal, vol. I., p. 203. 



nil La Planete Mars, vol. II., p. 175. 



