I873-J The Planet Mars in 1873. 181 



speculate too confidently, since the observed colour is 

 probably produced by the blending together of a great 

 number of different tints. That the bluish tracts on Mars 

 are oceanic may be inferred almost with absolute certainty, 

 simply because we know that the atmosphere of Mars is at 

 times loaded with considerable quantities of the vapour of 

 water. This the spectroscope has told us ; for it need 

 hardly be remarked that for the lines due to water vapour 

 to be seen at all in the spectrum of the planet, the quantity 

 of aqueous vapour then present in the Martial atmosphere 

 must be very great. I remark in passing, to remove possible 

 misapprehensions on the part of those who are unfamiliar 

 with Dr. Huggins's researches on Mars, that he has shown 

 beyond dispute that the water-lines in the spectrum were 

 not due (at the time of observation) to our own atmosphere. 

 Then this result agrees excellently with the observations 

 which had been made for many years on the white spots 

 near the poles of Mars. Sir W. Herschel had come to 

 the conclusion that these spots are the polar snows of Mars, 

 partly because this conclusion seemed justified by terrestrial 

 analysis, and partly because the white spots waxed and 

 waned in magnitude, in accordance with the theory that 

 they are snowy regions waxing in winter and waning in 

 summer. Snows cannot be produced without large water- 

 covered regions ; and the bluish tracts have precisely the 

 appearance which we should expect Martial oceans to 

 present. Then, we have already seen that the spectroscope 

 gives evidence of aqueous vapour in the Martial atmosphere. 

 There is then, therefore, a permanent atmosphere (for no 

 physicist can entertain for an instant the belief that the 

 Martial atmosphere consists solely of aqueous vapour). 

 There must, moreover, be winds, and probably clouds and 

 rain, besides those other meteorological phenomena depend- 

 ing on the increase and diminution of the quantity of aqueous 

 vapour present in the atmosphere. Accordingly, the tele- 

 scopist finds ample evidence of such phenomena in the 

 appearance of Mars. He finds that known Martial lands 

 and seas are often concealed from view, as if under a layer 

 of clouds ; he has been even able to watch the gradual 

 dissipation of such cloud-layers, as if under the rays of the 

 sun as it rose higher in the Martial skies. He sees all 

 round the disc of Mars a whitish light, which can be 

 explained as due to rounded or cumulus clouds in the 

 Martial atmosphere.* He notes the greater distinctness of 



* In my " Essays on Astronomy," I show that this explanation is available ; 

 and I add as another explanation, the possibility that the morning and evening 



