192 The Planet Mars in 1873. [April, 



of Mars. The one great difficulty which, as it seems to me, 

 would be fatal to Mr. Williams's theory, if demonstrably 

 shown to exist, is the darkening near the terminator of the 

 planet. It is possible, however, that this darkening may be 

 shown to be merely relative. It is to be remembered that, 

 assuming Mr. Williams's theory to be true, the region of 

 evening or morning whitening would be very much less 

 foreshortened at the time of corresponding quadrature 

 than as seen when the disc is full. The obvious consequence 

 of this would be that on the side towards the terminator 

 there would be a much broader whitened border, and not only 

 would the phenomenon be less noticeable on that account 

 (since the narrowness of the white bordering is what renders 

 it so remarkable), but the gradation of light would be much 

 slower. Then, from the obliquity with which the solar rays fall 

 on the parts towards the terminator, there is necessarily (what- 

 ever theory we adopt), a real defalcation of light there, and this 

 defalcation may probably be more easily recognisable than 

 the mere excess of light due to the whiteness of this part of 

 the disc. In fact, passing from the centre of the illuminated 

 half of Mars to the terminator, we have first the ruddy or 

 greenish tints of the lands or seas, then a gradually in- 

 creasing whiteness up to the absolute white of the hoar- 

 frost covered region, then a gradual defalcation of light 

 without change of colour; and the sole question is, Is the 

 latter defalcation likely to be more or less recognisable by the 

 telescopist than the deficiency of light in the middle of the disc 

 on account of the ruddiness or greenness there ? It is by 

 no means certain what answer is to be given to this question. 

 The subject has not, indeed, been specially studied by 

 telescopists. When it has been studied with due photo- 

 metric appliances, and under favourable circumstances 

 (for which the present opposition-period of Mars affords an 

 excellent opportunity) it may be possible to form a decided 

 opinion on the exceedingly interesting and important sugges- 

 tions made by Mr. Williams. 



I shall not make many remarks upon the ordinary theory 

 that the meteorological latitudes of Mars resemble those of 

 our own earth, because this theory has been discussed at con- 

 siderable length in my works referred to above. But there 

 is one point on which I must make a few remarks. If we 

 remember that the power of an atmosphere to increase the 

 mean temperature depends in the main on its density at 

 the mean level of the planet, we shall see that for Mars to 

 have a climate such as that of our earth, there must be 

 much more air above each square mile of the planet's 



