1865.] Phillips on the Planet Mars. 373 



1862 the opposition of Mars drew near, one of the first objects, with 

 many observers, was to estimate the difference of his polar and equato- 

 rial diameters, in fact, to measure his ellipticity. The compression due 

 to his rotation is very small, not greater than that of the earth, about 

 •g^oth. Now as the diameter of Mars is about 4,000 miles, and the 

 difference of the diameters would be about 13 miles, and as the appa- 

 rent diameter of the planet was nearly 22", it is obvious that the com- 

 pression would be 3 2 o 2 o or t? of a second ; a quantity too small to be 

 observable with certainty by any but the finest instruments. Our 

 observations with the ordinary position micrometer failed to determine 

 any sure difference of the diameters ; and this is perhaps the most 

 general conclusion of other late observers. Yet there is the high 

 authority of Mr. Main, and a large discussion of measures with the 

 famous Oxford heliometer for assuming the compression to be not 

 less than -grth. Arago had previously stated it at ^th. Herschel 

 indeed stated it at T Vth, but this was probably some accidental error. 

 At the time of the opposition, the south pole of Mars was visible from 

 the earth, the axis being then tilted toward the observer about 25£. 



Around this south pole there was seen a coronet of bright, nay, 

 splendid and glittering snow, elliptical in figure, and rather unsym- 

 metrical to the axis. This mass of snow had been observed by Lord 

 Eosse in July, three months before the opposition, and was then 

 estimated to have a diameter of 1,000 miles ; at the beginning of the 

 opposition in October it was estimated again not to exceed 560 miles; 

 it diminished sensibly and continually during two months of observa- 

 tion. The north pole was not visible, nor was any snow positively 

 remarked in that region by any observer. 



From this it appears that snows which surrounded the south pole 

 to a distance of 16 degrees or to S. lat. 74° in the month of July, had 

 retreated to about 8° or S. lat. 82 3 in the month of October, the pole 

 having been all that time in sunshine. In 1864 the opposition was 

 later in the year by about seven weeks (end of November). The south 

 pole was tilted towards the observer only about 6°, and the snows had 

 undergone the longer action of the sunshine. Accordingly it was 

 only a very small ellipse of snow which could be detected, and that 

 not always by the most careful observer round that only just visible 

 pole. 



On the other hand there were sure indications of snows in two 

 situations very brilliant and prominent on the northern surfaces whose 

 places, as represented on a drawing, have been transferred to the ac- 

 companying chart. These northern snows then appeared to reach a 

 latitude of about 50°, and to constitute an interrupted arctic zone in 

 the latter part of the south Martial summer, and towards the close of 

 the north Martial winter. At this epoch then, not less than 50° of 

 northern latitude were free from at least great tracts of snow, a result 

 which fairly matches the average climate of the northern zones of the 

 earth towards the end of our winter quarter in the month of February. 

 In April, 1856, a careful drawing made by De la Eue,* represents two 



* See ' Quarterly Journal of Science,' No. vi. p. 232. 



