356 FROM NEBULA TO NEBULA 



of a whole hemisphere. This was the case in 1894, and again at 

 one of the more recent oppositions. Photography shows (Har- 

 vard Annals 53, 167) that an equatorial belt of cloud is often 

 present, but this must be very thin, since it has not as yet been 

 observed visually. As regards color, it may be noted that our 

 own terrestrial clouds are by no means as white- as snow. Some- 

 times when the sun is low and behind us, two clouds may be seen, 

 the nearer of which is much darker than the other, though it is 

 not possible for either of them to be in shadow. The cause of 

 this seems to be a matter of theoretical interest which has been 

 neglected by our meteorologists. 



The more conspicuous Martian clouds lie along the limb of 

 the planet, where their existence can be most readily detected by 

 their irregular distribution. They are also found upon the ter- 

 minator, especially near opposition, but on account of their wider 

 separation from one another they are not so conspicuous there 

 as upon the limb. Since they are found both at sunset and sun- 

 rise, it is believed that cloudy nights are not infrequent upon the 

 planet. On account of its comparatively low pressure, the at- 

 mosphere of Mars must at certain seasons contain a larger pro- 

 portion of water vapor than that of the earth. Some of this 

 would be precipitated at night by the cold, and cloudy nights are 

 therefore what we might naturally expect. 



It is believed that the annual circulation of moisture from 

 pole to pole takes place upon Mars as upon the Earth, chiefly by 

 floods and through the atmosphere. Upon the Earth our floods 

 are taken care of, except in a small way, cosmically speaking, by 

 our oceans, but on Mars they must be much more severe. Even 

 upon the Earth, however, where large masses of land occur and 

 the snow fall is heavy, as in Siberia, the spring floods offer a 

 startling demonstration of the intensity of our sunlight. Com- 

 pared to one of these huge Siberian freshets, our own western 

 floods sink into insignificance. 



On January 13, 1914, Professor Lowell reported that the 

 Martian southern cap was beginning in stippled patches of hoar- 

 frost, between longitudes 30 and 60. Our own next view of 

 this region was on February 7, when it was occupied by a cloud 

 1 200 miles in length. The cloudy area increased in size the next 

 night to 1800 miles, but two nights later was again smaller meas- 

 uring 1500 miles. February 12 it had entirely disappeared. The 

 following night all the south polar regions were recorded as very 

 dark, with no trace of cloud. February 14 a crescent of haze lay 

 along the whole length of the limb and part of the terminator, ex- 

 tending half-way to the center of the disk, entirely concealing the 

 polar regions. After this the area in question passed around to 

 the other side of the planet, and we saw no more of it. We are 

 quite clear however that there was no hoarfrost or permanent 



