58 



KNOWLEDGE. 



[Mabch 1, 1895. 



To him and to Prof. Schaeberle a bright fan-shaped marking 

 covered the site of this gulf throughout August, 1894. 

 Herr Leo Brenner, it will be seen, saw it under its 

 accustomed form on September 14th, whilst on September 

 23rd Mr. Stanley Williams found it again covered, and Mr. 

 Cammell confirmed the observation four days later. The 

 obscuration then, whatever its cause, had cleared off in the 

 fortnight between August 31st and September 14th, and 

 had taken shape again nine days later. 



No. 5 shows a more extraordinary change still, but in 

 the other direction. Here M. Guiot records that the 

 Aonic Gulf has submerged the whole of loaria, so that the 

 Mare Sirenum and the Sinus Aonius have become merged 

 into one. This effect has often been represented, and 

 particularly in 1892. The difficulty of explaining it 

 arises from the fact that one observer will show Icaria 

 in its proper form and place, but a little while before 

 or after another observer fails to see it at all. Nor 

 is Icaria the only district on Mars which has this faculty 

 of vanishing. Compare Schiaparelli's drawing made 

 September 26th, 1877 (Fig. 2 of the plate), with Mr. 

 Green's drawing made three days later, and note how the 

 bright spike, Protei Eegio, which juts out from Thaumasia 

 towards the east on the former, is represented by a more 

 diffused marking on the latter. This region, well named 

 Proteus, was seen by Dawes in 1864, and was often observed 

 in 1892, amongst others by Stanley Williams and Keeler, 

 and always as of about the same size and in nearly the 

 same place, though the shape is apt to change a little. 

 Most drawings, however, fail to show it. 



There are in all some six or eight such areas on Mars, 

 which may be seen or not seen, apparently almost at 

 random. Either, then, these are districts which impress 

 differently the eyes of different observers, or they change 

 their appearance with great rapidity. 



It is easy to explain the disappearance of a dark marking, 

 even when it takes place in the course of a few minutes. 

 The formation of mist or cloud may readily hide a sea 

 from us. During the opposition of 1894, for instance, the 

 Maraldi Sea (Mare Sirenum and Mare Cimmerium), and 

 the whole region to the north, were completely hidden for 

 several days. But the temporary disappearance of a 

 district usually britiht is a harder matter to account for. 

 We can scarcely imagine that cloud masses habitually cover 

 certain water areas, preserving a definite and stable outline 

 above them, and then become dissipated for a while, soon 

 to reappear and to resume the old forms. 



Some changes appear to be manifest surface-changes, 

 and not mere meteorological effects. And here Thaumasia, 

 standing out as it does considerably nearer the pole than 

 the bulk of the land mass of Mars, is an object of special 

 interest. The observations of Lowell, made during the 

 opposition just past, confirming and completing similar 

 observations of other astronomers in former years, show 

 that when the long winter of the southern hemisphere 

 has come to an end, the polar cap shrinks with great 

 rapidity, and becomes ringed with a dark belt, evidently 

 the water from the melted snows. This area broadens 

 and spreads, and the " canals " begin to appear. In -June, 

 1894, midway between the spring equinox and summer 

 solstice for the southern hemisphere of Mars, the " canals " 

 were generally few and faint, and it was in Thaumasia 

 and the neighbourhood, where the great wave from the 

 melting of the polar snow might be expected first to 

 break, that they first showed up dark. The solstice fell 

 on August 31st, and the "canals" continued to be dark 

 throughout September ; afterwards they began to pale. 



The three chief "canals" entering the Lake of the 

 Sim are the Ambrosia on the south, the Nectar on the 



east, the Eosphorus on the west. Of these the Nectar 

 was seen in many comparatively early observations. 

 Thus, as already pointed out, Kaiser and Lockyer saw it 

 in 1862 and Dawes in 18G4, a broad sheet of water about 

 the length and breadth of the Adriatic. But in 1877 it 

 was not seen, whilst the Eosphorus was. The Ambrosia, 

 again, has likewise been seen, by itself and in company 

 with one or both of the other two. 



This circumstance renders it difficult to suppose, as one 

 otherwise might, that when the Erythrfean Sea rose 

 above a certain level it flowed by these three " canals " 

 into the Lake of the Sun, or that when the Lake of the 

 Sun attained a certain level it overflowed by these three 

 channels into the sea. For if one "canal" were full, all 

 three should be. We are driven to suppose, then, the 

 surface of Thaumasia to be exceedingly flat, so that a 

 very trifling change may block one channel or open 

 another. And seeing the apparent scale of these " canals," 

 as long, and often as wide, as the Adriatic, does it not 

 seem probable that each one of them, even the narrowest, is 

 really made up of many narrower channels, winding and 

 , turning after the manner of terrestrial rivers, and only seen 

 as straight, because what is seen by us is only the sum- 

 mation of a complexity of detail, far too minute to be ever 

 separately discerned '? The striking difference in tint 

 between the southern and northern halves of Thaumasia, 

 shown by Kaiser in Fig. 1 and confirmed in Fig. 7, may 

 well be due to a vast network of such minute canals in 

 the former district. 



Each optical improvement, each improvement in the 

 location of our telescopes, the continual improvement by 

 practice of observers, all uniformly tend to make the 

 " canal " system more complicated. Compare Mildler's 

 circular spot with no canals in 1830, Kaiser and Lockyer's 

 one canal in 1862, the two canals of Schiaparelli in 1877, 

 the three he saw radiating from the Lake of the Sim in 

 1879, the four M. Antoniadi shows, the five Mr. Hussey 

 saw at Lick, with the nine Mr. Douglass drew at the 

 Lowell Observatory last October, and has figured in his 

 sketch in Astronowi/ and Astrn-Physks for November. 



A very level surface intersected by shallow channels, 

 most of them probably too narrow to be separately defined 

 by any telescopic power we possess, is, then, the probable 

 explanation of the changes of Thaumasia, and if of Thau- 

 masia, no doubt of most of the land surface of Mars. The 

 changes in the size of the Lake of the Sun probably show 

 that the inner coast of Thaumasia, as one would expect 

 from so flat a country, shelves down into the lake very 

 gradually ; and this again may be taken as typical of 

 most Martial seas. The changes in Icaria, in the Aonic 

 Gulf, and in the region of Proteus seem to require, either 

 that these districts have some quality which causes cloud 

 to form above them more readily than is the case with 

 neighboiu-ing districts, or else some property which renders 

 the amount of cloud, which elsewhere would pass unnoticed, 

 readily visible when it does so form. The former we might 

 explain as indicating regions where warm moisture- 

 laden winds from the equator meet cold winds from the 

 pole ; but the regions seem too sharply limited for this to 

 be the case, and, as I have shown elsewhere, the meteoro- 

 logical circulation of Mars is necessarily very languid. To 

 my own mind it seems more probable that throughout the 

 planet in general the difference of level between land and 

 sea-bed is but shght, and the gradients very gentle, so that 

 the difference in the sea-level, consequent on the melting 

 of the winter snows, has a vast effect on the extent of the 

 area submerged. Probably, too, these are regions either 

 where the land is full of broad shallow lakes, or the sea full 

 of flat islands or slightly sunken shoals ; details too small 



