186 The Planet Mars in 1873. [April, 



consider certain features suggesting a different theory, I 

 must point out that the balance of evidence appears to me 

 to be decidedly in favour of his theory. Meantime I will 

 follow the line of reasoning pursued by Mr. Williams, 

 noting that much of what he says must be regarded as 

 following obviously from the theory on which it is based. 



In the first place, it is clear that with so shallow an 

 atmosphere and so small a direct supply of solar heat, the 

 cold in Mars would be intense. The mean temperature 

 would be below the freezing-point. Nevertheless in the 

 day time, especially in low latitudes, the heating power of 

 the solar rays would be considerable. It would not be so 

 intense as on the summits of our loftier mountains, when a 

 mid-day sun is pouring his rays on the snow-masses there, 

 but would correspond rather to the heat of the sun at about 

 ten or eleven on a summer's morning in Switzerland. It 

 would certainly suffice to melt any surface snows, and also 

 the surface ice of the Martial oceans, which on the theory 

 we are considering must be regarded as frozen throughout 

 their depth. 



Now, in considering what would follow as the day pro- 

 ceeded, we find some difficulty in deciding whether there 

 would be an inflow toward the warmed mid-day regions or 

 an air-current flowing outwards (we are speaking now of 

 surface-currents). On earth there is a flow of air towards 

 the region where evaporation is taking place, and it has 

 been urged that this is due to the fact that the aqueous 

 vapour, rising by reason of its relative lightness, causes 

 upward currents in the permanent atmosphere, and that 

 thus an indraught is produced. On the other hand, where 

 evaporation proceeds rapidly, there is a great addition to 

 the atmosphere and consequently an increase of pressure, 

 which would tend to occasion an outflow. In the case we 

 are dealing with, the latter effect might prevail ; but in any 

 case it is not perhaps very important to consider the ques- 

 tion ; because, whether the surface-flow were towards or 

 from the region of evaporation, there would be a flow of 

 moisture-laden air from that region. In one case it would 

 be a surface-flow, in the other it would be an upper-air 

 current ; but it is immaterial, so far as our present purpose 

 is concerned, whether the outward flow took place in the 

 upper or lower regions of the air. 



Then as the day proceeded, and some considerable time 

 before sunset, " a feathery hoar-frost" would begin to fall. 

 " There would," in fact, " be the same kind of action which 

 Sir J. Herschel has described as necessarily taking place in 



