I have seen it stated in a book on the "Solar System" by a well-known astronomer that the _axis_ of Mars "is inclined to the plane of the orbit" at an angle of 24° 50'! But this is quite erroneous. The angle given is the angle between _the plane of the planet's equator_ and the plane of its orbit, which is quite a different thing. This angle, which may be called the obliquity of Mars' ecliptic, does not differ much from that of the earth. Lowell finds it 23° 13' from observations in 1907.[99] The late Mr. Proctor thought that Mars is "far the reddest star in the heavens; Aldebaran and Antares are pale beside him."[100] But this does not agree with my experience. Antares is to my eye quite as red as Mars. Its name is derived from two Greek words implying "redder than Mars." The colour of Aldebaran is, I think, quite comparable with that of the "ruddy planet." In the telescope the colour of Mars is, I believe, more yellow than red, but I have not seen the planet very often in a telescope. Sir John Herschel suggested that the reddish colour of Mars may possibly be due to red rocks, like those of the Old Red Sandstone, and the red soil often associated with such rocks, as I have myself noticed near Torquay and other places in Devonshire. The ruddy colour of Mars was formerly thought to be due to the great density of its atmosphere. But modern observations seem to show that the planet's atmosphere is, on the contrary, much rarer than that of the earth. The persistent visibility of the markings on its surface shows that its atmosphere cannot be cloud-laden like ours; and the spectroscope shows that the water vapour present is--although perceptible--less than that of our terrestrial envelope. The existence of water vapour is clearly shown by photographs of the planet's spectrum taken by Mr. Slipher at the Lowell Observatory in 1908. These show that the water vapour bands _a_ and near D are stronger in the spectrum of Mars than in that of the moon at the same altitude.[101] The dark markings on Mars were formerly supposed to represent water and the light parts land. But this idea has now been abandoned. Light reflected from a water surface is polarized at certain angles. Prof. W. H. Pickering, in his observations on Mars, finds no trace of polarization in the light reflected from the dark parts of the planet. But under the same conditions he finds that the bluish-black ring surrounding the white polar cap shows a well-marked polarization of light, thus indicating that this dark ring is probably water.[102] Projections on the limb of the planet have frequently been observed in America. These are known _not_ to be mountains, as they do not reappear under similar conditions. They are supposed to be clouds, and one seen in December, 1900, has been explained as a cloud lying at a height of some 13 miles above the planet's surface and drifting at the rate of about 27 miles an hour. If there are any mountains on Mars they have not yet been discovered.