C. E. Burroy.— Physical Observations of Mars. 169 
90 Martian degrees, or included within it some 3,000 miles of the meridian. On 
more than one occasion, when both snow spots were visible, it was evident that no 
diameter of the disk would bisect both, and the greater eccentricity of the southern 
appeared to be established by the more striking changes of magnitude which it 
underwent in comparison with the northern snow spot, together with the far greater 
fluctuations in the brilliancy of the former. The northern snow spot did not visibly 
alter its position on the limb, or its magnitude, to any considerable extent, and its 
invisibility on Novy. 18, for instance, is to be ascribed to the interposition of 
immense masses of cloud. On the date mentioned all markings near the northern 
limb appeared dim and very ill-defined. On Nov. 22 the clouds had partially 
cleared away, and the snow was distinctly, though faintly, seen. The presence of 
cloud is also probably traceable in the appearance of Banks Cape and Hirst Island 
as a single marking, in No. 24 (Dec. 10). | Another instance is afforded by No. 22 
(Noy. 10), where the western extremity of Hooke Sea is cut off from the remainder 
by a bright narrow space not shown in any other of the drawings. 
It is singular that in many cases the clouds seem to be indistinguishable from the 
brighter parts of the disk, which are usually of an orange colour, inclining to red, 
by any perceptible difference of tint. Other observers who have inferred the exist- 
ence of Martian clouds, appear, like myself, rather to have inferred the presence of 
such obstacles to vision from the disappearance or enfeeblement of well-known mark- 
ings under otherwise favourable conditions for their visibility, than from any peculi- 
arity of tint or reflective power possessed by the clouds themselves. These veils 
have been seen by Lockyer to change their position as if borne by winds (23rd 
September, 1862). There can, therefore, be little doubt of their analogy at least to 
terrestrial clouds, or, more probably, to ground mists. In connexion with the pre- 
_ ceding remarks on the colour of the clouds of Mars, it may be mentioned that on 
Jan. 5, 1880, the southern snow region was much fainter and less definite than the 
northern, and appeared yellowish, while the northern was white. This observation 
seems to indicate that the southern snow had been covered, or possibly replaced by 
tinted clouds, as this region had been exposed continuously for several months to 
the sun’s rays. 
The presence of mist or hoar-frost for some time after sunrise on Mars seems to be 
indicated by the frequently observed dimness and indefiniteness, or even invisibility 
of the markings near the eastern limb, while they could often be followed to within 
less than a second of arc from the western limb ; e.g. Jan. 5, when Christie Bay was 
indistinct, though it was far within the eastern limb. But on many other occasions, 
the reverse held good, as on Oct. 24, when Christie Bay and Terby Sea were well 
seen near to the eastern limb, while Beer Bay and the regions west of it are lost in 
a diffused whitishness which extended to the west limb. So on Noy. 18 and Nov. 
22. Aninspection of the Plates VI. and VII. will readily furnish other instances of 
both modes of behaviour. This whitishness may be considered as undoubtedly 
