504 COSMOS. 



contrast colours, so-called seas ; M there are seen nporr tna 

 disc of Mars two white, brilliant, snow-like spots,* 7 either at 

 the poles which are determined by the axis of rotation, or ut 

 the poles of cold alternately. They were recognized, as early 

 as 1716, by Philip Maraldi, though their connection with cli- 

 matic changes upon the planet was first described by the elder 

 Herschel, in the seventy-fourth volume of the Philosophical 

 Transactions for 1784. The white spots become alternately 

 larger or smaller, according as the poles approach their 

 winter or summer. Arago has measured, by means of his 

 polariscope, the intensity of the light of these snow zones, 

 and found it twice as great as that of the remaining part of 

 the disc. The Physikalisch-astronomische Beitriige of Madler 

 and Beer, contain some excellent graphic representations* of 

 the north and south hemispheres of Mars ; and this remark- 

 able phenomenon, unparalleled throughout the whole plane- 

 tary system, is there investigated with reference to all the 

 changes of seasons, and the powerful action of the polar 

 summer upon the melting snow. Careful observations, during 

 a period of ten years, have also taught us that the dark spots 

 upon Mars preserve a constant form and relative position. 

 The periodical formation of snow-spots, as meteoric depositions 

 dependent upon change of temperature, and some optical 

 phenomena which the dark spots present as soon as they 

 have, by the rotation of the planet, reached the edge of the 

 disc, make the existence of an atmosphere upon Mars more 

 than probable. 



* Sir John Herschel, Outlines, 510. 



w Beer and Madler, Beitraye. pp. 117-125. 



* Madler, in Schumacher's Astr. Nadir, no. 192. 



