1873.] The Planet Mars in 1873. 197 



the northern winter is mitigated by the reduced distance of 

 the sun ; and, on the contrary, the summer heat and winter 

 cold of the southern hemisphere are both intensified. Just 

 so it necessarily happens in the case of Mars, but the 

 effects are more marked, because of the greater eccentricity 

 of the orbit of Mars. The heat received by Mars at mid- 

 summer of his northern hemisphere is less than that 

 received at mid-winter, in the proportion of about 7 to 10. 

 Of course this is more than compensated, in north latitudes 

 resembling our mid-temperate and subarctic zones, by the 

 greater length of the summer's day and the greater height 

 of the midday sun in summer. Nevertheless, the contrast 

 between summer and winter must be most importantly re- 

 duced by the relation. On the other hand, the summer of 

 the southern and winter of the southern hemisphere of 

 Mars are intensified by the circumstance that more heat is 

 received directly from the sun at the time of southern mid- 

 summer than at the time of southern mid-winter, in the 

 proportion of 10 to 7. The northern summer is also longer 

 than the southern, to the following extent : — Counting from 

 the vernal to the autumnal equinox the northern summer 

 contains 371J days, while the southern contains only 296^ 

 days. Thus we have, in the northern hemishere, a long 

 mild summer and a short mild winter; in the southern 

 hemisphere, we have a short but (relatively) hot summer 

 and a long and bitter winter. 



It is manifest that, under these circumstances, we may 

 fairly look for a great difference in the aspect of the northern 

 half of the planet during the present opposition period, 

 when the effects of the northern summer (counting still 

 from equinox to equinox) are nearly at a maximum, and 

 that presented during the corresponding opposition-period 

 for the southern half of the planet, — the period, namely, 

 including the opposition of 1864. Then the southern half 

 had passed through its relatively intense summer, and there 

 was a relatively rapid diminution of heat towards the mean 

 heat at the equinox. Now the northern half has passed 

 through its mitigated summer, and a relatively slow diminu- 

 tion of heat is taking place. As several excellent pictures 

 of Mars were taken by Mr. Dawes, in 1864-65,* it will be 

 possible to institute a comparison between the phenomena 

 then observed and those which may be recognised on the 

 present occasion. 



It is next to be noticed that the present opposition-period 



* The four best appear among the coloured illustrations of my " Other 

 Worlds." 



