446 



NATURE 



[September 8, 1892 



features. The effect of a cloudless and perfectly pure sky, both 

 •here and on Mars, appeals to be that the dark portions of the 

 planet become darkest and most distinctly visible ; the coast- 

 lines (if I may so call them) being at such times so hard and 

 sharp, that (as has been mentioned by Mr. Lassell) it is quite 

 impossible to represent the outlines faithfully ; and this effect, 

 be it observed, is completely distinct from the way in which the 

 features grow upon one. MM. Beer and Madler remark : 

 ' Generally some time elapsed before the undefined mass of 

 spots seen upon first looking into the telescope resolved 

 itself into recognizable parts.' This observation will commend 

 itself to all who have observed such a delicate object. 



"The effect of clouds, on the contrary, will be, I think, to 

 make the dark portions less dark in proportion to the density of 

 the clouds, and the light portions lighter \n the same proportion. 

 It can never 7?iake a light portion dark. If this be so, when we 

 see a dark spot well defined, we can be sure that no clouds are 

 above it, and that we actually see the planet itself; we cannot 

 be sure, however (unless we are acquainted with the locality 

 from previous observation), that dark spots do not underlie any 

 of the lighter portions. Some instances of cloud-transit were 

 suspected by Father Secchi in 1858. Several unmistakable 

 instances occurred during my observations. . . . 



" But besides the cloud-ndasses, which, as we have seen, 

 obliterate the dark portions either partly or wholly, giving rise 

 to different contours and tones, and rendering the actual 

 features of the planet undistinguishable, the dense atmosphere 

 of Mars, with its fogs and mjsts, appears to go for very much. 

 I mention this more especially to point out that— -although 

 its effect was evident in the southern hemisphere in mid-summer, 

 upon the spots as they came on, and left the disc, as remarked 

 by previous observers— it \vas much more evident in the northern 

 hemisphere in mid-winter, blotting out, as before remarked, 

 even on the central meridian, all features north of + 30° lati- 

 tude. This would appear to furnish another proof of extreme 

 seasons on Mars, in addition to that supplied by the rapid melt- 

 ing and great extent of the polar snows, and to point out the 

 desirability of taking advantage of all oppositions which happen, 

 as did those last year and in 1830, in the full summei--time of 

 . the southern hemisphere, when the atmospheric conditions of 

 the planet may be considered the best possible. With regard 

 to this last point, it may be remarked that the southern hemi- 

 sphere is the one which we shall ever be able to study best, in 

 consequence of the great distance of the planet from us at 

 those oppositions which occur when the northern one is turned 

 to us. 



" With regard to the green and red tints so often noticed on 

 Mars, my observations have led me to hold the same opinions 

 as to their nature as those arrived at by Father Secchi in his 

 study of the planet in 1858. Nor do I think that it can any 

 longer be doubted that — as he considered probable — the green 

 and red portions do actually represent seas and continents, and 

 are not the effect of contrast. 



" The dark portions were noticed to be decidedly green in 

 my instrument, both by myself and others who observed Mars 

 from time to time with me, the colour being especially marked 

 in_ Beer and Miidler's spot / n (Drawings Nos. 7 and 8). In 

 spite of the over-correction of my object-glass, which should 

 have ' reinforced ' the red tinge, it was never sufficiently deci- 

 ded, I think, to suggest a contrast ; and, indeed, the green was 

 sometimes unmistakable when the red was not noticed, and 

 when therefore there was no contrast to mislead the eye. 



" Another point of agreement between the two series of 

 drawings is not a little remarkable : the spots which were ob- 

 served to be of a most decidedly dark tint in 1830 were darkest 

 last year ; and supposing the dark portions to be water, the 

 darkest spots are those which are nearly, if not quite, land- 

 locked. Passing on from the consideration of the general 

 features of the planet, the snow-zone next demands our atten- 

 tion Last year the solstice occurred on August 30, on 



the 23rd of which month the snow-zone was estimated to be ^ 

 of the apparent diameter ; by the 25th of the next month, Sep- 

 tember, this was reduced to about ^V. and again to -^g by Octo- 

 ber II, when it was at times scarcely discernible ; after which 

 it began apparently to increase again. 



" To the great eccentricity of the orbit of Mars, and the fact 

 that the summer of the southern hemisphere occurs when the 

 planet is near perihelion, is doubtless to be ascribed this very 

 rapid melting of the southern snow-zone, an observation con- 

 iirmed by the much slighter'variatfon in the dimensions of the 



opposite one. It appears to follow from my drawings, and I 

 think also from thos-e of Messrs. Beer and Madler, although 

 they make no mention of the fact, that even at its minimum the 

 centre of the snow-zone was not absolutely coincident with the 

 planet's pole, being situated in somewhere about 20° of areocen- 

 tric longitude (using Beer and Miidler's start-point), and in a 



latitude probably only a very few degrees from it The 



snow-zone was at times so bright that, like the crescent of the 

 young moon, it appeared to project beyond the planet's limb. 

 This effect of irradiation was frequently visible ; on one occa- 

 sion the snow-spot was observed to shine like a nebulous star 

 when the planet itself was obscured by clouds, a phenomenon 

 noticed by Messrs. Beer and Madler, recorded in their valuable 



Fig. 3. Mars September 25, 

 locked water surface and : 



186?, showing the darker shading of a land- 

 is projection into the open water beyond. 



NO. II 93. VOL. 46] 



Fig. 4. Mars September 25, 1862, showing bright appearance of : 

 and the details of one of the chief coast lines. 



work. Fragments stir les Corps Celestes. The brightness, how- 

 ever, seemed to vary very considerably, and at times, especially 

 when the snow-zone was near its minimum, it was by no means 

 the prominent object it generally is upon the planet's disc." 



We owe it to the illustrious Italian astronomer 

 Schiaparelli that a world of wonders undreamt of thirty 

 years ago now forms the chief subject of inquiry. His 

 work was begun at the opposition of 1877, which, as we 

 have seen, was as favourable as the present one, and con- 

 tinued during that of 1879-80. He showed that those 



