190 The Planet Mars in 1873. [April, 



occur which are recognisable by the terrestrial tele- 

 scopist, I shall venture to quote some observations by the 

 late Gen. Mitchel (the American observer), which seem to 

 accord singularly well with that rather startling theory. 



First, let us examine what in Mr. Williams's opinion 

 would happen and be seen : — " The tendency of the summer 

 growth of the summit and undermining of the sides would 

 be," he remarks, " to bring about periodical catastrophes, 

 by the more or less complete toppling over of the mountain 

 cone in the form of a gigantic avalanche. The occurrence 

 of such a catastrophe would be most sensibly indicated to a 

 terrestrial observer by an irregular and temporary extension 

 of the polar whiteness ; where the debris of the great ava- 

 lanche had been hurled beyond the general glacial boundary, 

 and had usurped the region of the summer thaw." The 

 evidence quoted by Mr. Williams himself is an observation 

 made by Prof. Phillips, of Oxford, and two practised 

 observers — Messrs. Luff and Blorridge, working with him. 

 "We noticed," says Prof. Phillips, "a gleaming mass of 

 snow very distinct, so much so, that as happened with the 

 south polar snow of 1862, it seemed to project beyond the 

 circular outline, an optical effect no doubt due to the bright 

 irradiation." On this Mr. Williams remarks that, although 

 Prof. Phillips attributes this appearance to irradiation, it 

 may have been due to the actual heaping of the avalanche 

 material of the overthrown polar ice-cone. But the follow- 

 ing observations by Mitchel seem far more strikingly to 

 favour Mr. Williams's bold and ingenious hypothesis : — 



" I will here record," says Mitchel, at p. 89 of his " Popular 

 Astronomy," "some singular phenomena connected with the 

 ' snow-zone,' which, so far as I know, have not been noticed 

 elsewhere. On the night of July 12, 1845, the bright polar 

 spot presented an appearance never exhibited at any pre- 

 ceding or succeeding observation. In the very centre of the 

 white surface was a dark spot, which retained its position 

 during several hours, and was distinctly seen by two friends 

 who passed the night with me in the observatory. It was 

 much darker and better defined than any spot previously or 

 subsequently observed here ; and, indeed, after an exa- 

 mination of more than eighty drawings at previous oppo- 

 sitions, I find no notice of a dark spot ever having been 

 seen in the bright snow-zone. On the following evening no 

 trace of a dark spot was to be seen, and it has never after 

 been visible." This is singularly suggestive of the falling 

 away of a great portion of the snow-cone, followed very 

 soon (as would naturally happen) by the snowing over of 



