1873O The Planet Mars in 1873. 187 



the moon if any water exists on that satellite, and which he 

 compares to the cryophorus experiment. There should, 

 however, be some difference between the case of Mars and 

 the moon. The vacuum of Mars being only comparative, 

 the action w r ould be much slower and less decided than in 

 Sir J. Herschel's supposed case; and the mean temperature 

 of Mars being so much lower, the freezing-point and con- 

 sequent precipitation of a haze of hoar-frost must com- 

 mence considerably before reaching the actual boundary 

 between light and darkness ; at that angular distance, in 

 fact, from solar vertically, where the cooling influences of 

 the planetary radiation, — aided by those of the remaining 

 ground-ice, — must reduce the surface temperature to the 

 freezing-point." "Thus," proceeds Mr. Williams, " there 

 would be no great well-defined masses of vesicular vapour 

 floating irregularly, like our clouds, in the atmosphere of 

 Mars, — no cumulus, no cumulo-stratus, nor even cirro- 

 cumulus clouds ; and, excepting at the borders of the Polar 

 ice, nothing denser than a thin veil of stratus or cirro- 

 stratus cloud, formed of ice-crystals, — the kind of cloud or 

 mist which in our atmosphere makes halo round the moon, 

 and only hides her face sufficiently to exaggerate her beauty, 

 like the gauze ' complexion-veil ' of the coquette. The 

 mid-day region, and a certain distance round it, would but 

 rarely be subject to this small degree of obscuration, as the 

 sun's heat there should under ordinary circumstances hold 

 all the vapours it had raised in complete and transparent 

 solution. " 



It will be gathered, from what has been already stated, 

 that while the results thus indicated accord well with the 

 general features of Mars, they do not agree with the 

 observed appearance of the terminator, when Mars is 

 gibbous. I pause to note this circumstance, because it is 

 manifestly important that observation should be specially 

 directed to the examination of the actual brightness near 

 the terminator of Mars ; and it chances that, as will pre- 

 sently be more particularly indicated, the present opposition- 

 period of Mars is particularly well suited for the observation 

 of this feature. But it may be also well to note in this 

 place, that in one circumstance the aspect of Mars cor- 

 responds well with Mr. Williams's theory. Mr. Dawes 

 makes the following remarks, in describing the appearances 

 presented by Mars during the opposition of 1865, when the 

 planet was particularly well placed for observation : — " On 

 the whole," he says, " my impression has been that Mars 

 has not usually a very cloudy atmosphere. During the last 



