1865.] Carter on a Plurality of Worlds. 231 



ditions of habitability is by no means impossible, for Prof. Hansen, 

 as a result of bis inquiries into tbe causes of the secular variation of the 

 Moon's mean motion, has arrived at the conclusion that her centre of 

 gravity and the centre of her figure do not coincide, the former being 

 so much farther from the Earth, as to give to the proximate hemisphere 

 a bulging of twenty-nine miles over the remote one ; so that it would 

 not be impossible for a considerable quantity of water to be collected 

 on this without there being evidence of it to us, something like a 

 parallel to such a condition being found, says Sir John Herschel, in 

 the immense preponderance of water in our own southern hemisphere. 

 On turning to the primary planets, we are met by positive evidence 

 of a very remarkable character. Combined observations on the best 

 known of these, namely Mars, and especially the recent ones of Mr. 

 Phillips,* show clearly that the climatal variations " are comprised 

 within nearly the same thermal limits as those of the Earth," rendering 

 certain the operation of some adapting cause ; inasmuch as the relative 

 distances of the two from the sun being as 100 to 152, and the solar in- 

 fluence therefore as 231 to 100, without it, the planet's surface, instead 

 of possessing the genial temperature which it does of 40° to 60°, should 

 be fixed in perpetual frost. This conclusion as to climate is deduced 

 from tracing the changes which take place with variation of season in 

 certain white glittering masses in the neighbourhood of the poles. 

 The existence of these was first pointed out by Maraldi, in 1716, and 

 the phenomenon explained by Sir William Herschel as due to the 

 alternate accumulation and melting of large quantities of snow during 

 winter and summer respectively. All subsequent observations have 

 served but to confirm this opinion. Thus, Lord Eosse watched the 

 planet for several months in succession, and observed the following 

 changes. On the 22nd of July, 1862, when his first observation was 

 made, the South pole, which, owing to the high inclination of the axis, 

 could at the time receive but little solar heat, was in winter, and sur- 

 rounded by a large white patch, having a radius of at least 500 miles. 

 In the course of three months, however, during which it had been 

 brought more under the sun's influence, this was reduced to less than 

 a half, and appeared only as a small glittering ellipse. What renders 

 the analogy with the Earth even more striking is, that in the same 

 seasons of different Martial years, the amount of snow appears nearly 

 equal ; that it in no case is regular in its limits, but is collected along 

 one or two principal tracts as far as 40° or 45° from the pole ; and that 

 it is invariably very excentric round the Southern pole, the amount of 

 excentricity amounting, according to Secchi, to 17° 42'. The only 

 probable explanation of these latter phenomena being the production 

 of local elevations and depressions of temperature by the relative dis- 

 tribution of land and water, the constant set of warm aerial or oceanic 

 currents, and so on, which we know cause the most marked inequality 

 in any of the isothermal lines in our own Northern hemisphere. 

 Local patches, similar to those about the poles, have at different times 

 been seen in other parts, though of far less extent. The geographical 

 limit of the isothermal, 32°, has been shown by Mr. Phillips to be 



* 'Phil. Proc' vol. xii. p. 431 et seq.; and No. 71. 



