194 The Planet Mars in 1873. [April, 



relatively small ocean surface (here I refer to the actual pro- 

 portion between the extent of land and water on Mars, and 

 not merely to the extent of water surface in square miles), 

 the atmosphere would be sufficiently vapour-laden to pro- 

 duce the required warmth. For it is to be remembered that 

 dry air is almost perfectly diathermanous, as well for the 

 luminous as for the obscure heat-rays, and that therefore 

 the heat of Mars would be freely radiated away into space, 

 unless the air were freely laden with aqueous vapour. It 

 seems difficult to believe that Mars has an atmosphere so 

 deep and dense as the conditions here considered appear to 

 require. 



On the whole, I cannot but think that the balance of 

 evidence is in favour of the theory that Mars has a relatively 

 rare atmosphere, and that the various phenomena pre- 

 sented by the planet are to be explained in the way suggested 

 by Mr. Williams. 



The reader will perceive that a considerable degree of 

 interest attaches to the study of Mars. We are by no means 

 dealing with a planet whose physical habitudes have been 

 thoroughly mastered and interpreted. 



But, apart from these considerations, the present opposi- 

 tion of the planet is one which is peculiarly favourable to the 

 investigation of the planet's condition. A reference to the 

 accompanying figure (and to the plate illustrating this essay) 

 will serve to show this. 



In the first place, let it be noticed that when Mars is in 

 opposition, on April 27, he is not far from the place where 

 he is at his mean distance from the sun ; for M' is the place 

 of his aphelion. So far, therefore, as the epoch of opposition 

 is concerned, the present return of the planet may be re- 

 garded as having a medium value.. 



Let it next be noticed that the midsummer of the planet's 

 northern hemisphere occurs when Mars is not far past his 

 aphelion, and that the period illustrated by the figure cor- 

 responds to the summer months of North Mars. Now a 

 peculiar interest attaches to this circumstance. Mars re- 

 sembles the earth (at the present time, and for many years 

 past and to come) in having his solstices near the aphelion 

 and perihelion of his orbit ; and the resemblance extends 

 even to the circumstance that the summer of North Mars 

 occurs when the planet is near aphelion, precisely as our 

 summer in the northern hemisphere occurs when the earth 

 is near aphelion. And precisely similar consequences 

 follow from the relation in both cases. Our northern sum- 

 mer is mitigated by increase of distance from the sun, while 





