1873J The Planet Mars in 1873. 189 



region, where the evening deposition of hoar-frost was in 

 progress, should not appear so sharp and well defined as the 

 limit of the morning thaw. 



We come next to a rather sensational feature of the 

 theory, or rather of the consequences attributed to it : " At 

 the poles," says Mr. Williams, " and for some distance 

 around them, the annual amount of deposition must exceed 

 the annual amount of thawing and evaporation, and there- 

 fore a gigantic glacial mountain must there accumulate, 

 with a continual growth and tendency to assume a conical 

 form. As the deposition of ice-crystals would commence 

 before actual sunset, and would probably reach its maximum 

 or even be finished before reaching the boundary line of day 

 and night, in consequence of the thinness of the atmosphere 

 of Mars and the resulting rapidity of radiation, the building 

 up of this polar mountain would be very irregular. In 

 mid-winter, the lower slopes of its sides would receive the 

 greatest accessions. With the advancing line of daylight 

 the elevation of the zone of maximum deposition would in- 

 crease until it reached the summit. This coincidence of 

 maximum deposition with the summit would occur twice a 

 year, before and after midsummer. During the summer, 

 the only regions receiving any deposition at all would be the 

 summit and its immediate vicinity; while, at the same 

 time, its sides would be rapidly thawing by the powerful 

 action of the continuous sunshine of the long arctic summer 

 day. At this season, the slopes of the arctic mountain 

 would be riven by gigantic ice-floods and water-floods, ava- 

 lanches, glaciers, and torrents." 



While admitting as almost a necessary consequence of 

 the supposed condition of Mars that there would be an 

 accumulation of snow towards the poles of the planet, I 

 must confess I cannot follow Mr. Williams in assuming 

 that the snow-caps can attain a thickness sufficient to in- 

 crease perceptibly the apparent diameter of the planet. It 

 is true that the telescopist recognises an apparent projection 

 of the polar snows beyond the circular outline of the disc. 

 But irradiation affords so sufficient and satisfactory an 

 explanation of this circumstance as to leave in my opinion 

 little to be desired ; whereas the accumulation of snowy 

 masses to a depth of several miles appears difficult to 

 accept, when it is remembered how relatively small must be 

 the quantity of aqueous vapour which could be raised into 

 the tenuous Martial atmosphere. Nevertheless, as Mr. 

 Williams has advocated with some ingenuity the theory not 

 only that such masses exist, but that great glacial catastrophes 



