i873«] The Planet Mars in 1873. igi 



the cavity thus formed. The other observation is fully as 

 singular: — "On the evening of August 25, 1845, the snow- 

 zone, which for several weeks had presented a regular out- 

 line nearly circular in appearance, was found to be some- 

 what flattened at the under part, and extended east and 

 west, so as to show a figure like a rectangle with its corners 

 rounded. On the evening of the 30th August, I observed 

 for the first time a small bright spot, nearly or quite round, 

 projecting out of the lower side of the polar spot. 1 In the 

 early part of the evening the small bright spot seemed to be 

 partly buried in the large one. After the lapse of an hour 

 or more my attention was again directed to the planet, 

 when I was astonished to find a manifest change in the 

 position of this small bright spot. It had apparently sepa- 

 rated from the large spot, and the edges of the two were 

 now in contact, whereas when first seen they overlapped by 

 an amount quite equal to one-third of the diameter of the 

 small spot. On the following evening I found a recurrence 

 of the same phenomenon" (in other words, the phenomenon 

 was shown to be optical, and depending on the relative 

 positions of two great snow-masses). " In the course of a 

 few days," proceeds Mitchel, " the small spot gradually 

 faded from the sight and was not seen at any subsequent 

 observation." 



Certainly these observations accord remarkably well with 

 Mr. Williams's theory respecting the polar snows of Mars. 

 The objections to the theory are found mainly in facts 

 already mentioned. Thus it is difficult to understand how 

 a sufficient quantity of the vapour of water should be pre- 

 sent in the Martial atmosphere to produce the dark bands 

 seen by Dr. Huggins, if the atmosphere itself (that is the 

 permanent atmosphere) were so tenuous as the theory 

 implies. It must, however, be noted that the tenuity of the 

 atmosphere would encourage evaporation ; in fact, the boil- 

 ing point at the surface of Mars would be so low as 138° 

 with Mr. Wiliams's assumption as to the atmospheric 

 pressure, and lower still with mine. Nor does so great a 

 difficulty arise as at first sight might be supposed from the 

 fact that large Martial regions have at times seemed to be 

 clouded over, since, in the first place, clouds would not be 

 an unfrequent phenomenon in tenuous atmosphere ; and 

 under certain circumstances, as for example great atmo- 

 spheric disturbances or the effects of such arctic catas- 

 trophes as Mr. Williams has described, there might be 

 occasional extensions of dense, though perhaps shallow, 

 cloud-layers, or heavy mists, over wide tracts of the surface 



