436 [Feb. 12, 



bright and almost silvery in other parts, to Professor Phillips, looking 

 through his achromatic, which also showed the ' water ' of a grey or 

 greenish tint. No redness appeared in Mr. Lockyer's instrument, 

 which, like many others of excellent quality for astronomical re- 

 search, is intentionally ' over-corrected/ 



Mr. Nasmyth saw the snow-patch on the south pole so distinctly 

 bordered, as to give him the impression of its having a cliff- 

 boundary. The south snow-patch did not appear to him to agree 

 with the south pole of the planet, but, on the contrary, to be con- 

 siderably excentric to it; and he supposed this to be due to the 

 relative distribution of land and water, influencing the position of 

 the centre of greatest cold. Only a faint glimmering of the snowy 

 surfaces round the north pole was seen by any observer. 



On the whole, the author of this paper concluded that, over a per- 

 manent basis of bright and dusky tracts on the surface of Mars, a 

 variable envelope gathers and fluctuates, partially modifying the 

 aspect of the fundamental features, and even in some cases disguising 

 them under new lights and shades, which present no constancy, a 

 thin vaporous atmosphere probably resting on a surface of land, 

 snow, and water. 



Addendum. 



Since the reading of the paper the author has been enabled, by the 

 kindness of the Earl of Rosse, to examine a series of sketches of 

 Mars during the late opposition, from the great telescopes at Birr. 

 These drawings, six in number, were made on July 22, Sept. 14, 

 Sept. 16, Oct. 6, Oct. 29, and Nov. 6. They confirm in a remark- 

 able manner the conclusions already presented by the author, and 

 suggest some interesting questions for further observation and study. 

 On the 22nd of July the southern snow was a large patch, meeting 

 the limb by its diametral line. It must then have had a radius of 

 500 miles at least : in the later observations it was reduced to less 

 than half this measure. 



One of the drawings nearly corresponds to longitude 180 on 

 the author's scale, and represents the specially dark short band 

 which distinguishes that aspect of the planet (fig. 3). Two corre- 

 spond nearly to fig. 1, and contain the remarkable deep angular bay 

 which extends so far towards the north pole. In these and the re- 



