870 SPECIAL RESULTS IN THE URANOLOGICAL 



pression is somewhat greater than either of the two last- 

 named quantities. 



If the study of the surface of the Moon reminds us of 

 many geognostical relations in the surface of our own planet, 

 the analogies with the Earth which Mars presents are, on the 

 other hand, entirely of a meteorological kind. On the disk 

 of Mars (besides the dark spots, of which some are 

 blackish, others very few in number, however orange 

 ( 599 ), and surrounded by what are called "seas" ( 60 ) 

 of the contrasted colour, green) at the two poles of 

 the axis of rotation, or it may be at the poles of 

 cold in their vicinity, are two white snow -bright patches 

 ( 601 ). These were noticed as early as 1716 by Philip 

 Maraldi ; but their connection with climatic variations in 

 the planet was first described by the elder Herschelin 1784, 

 in the 74th volume of the Philosophical Transactions. The 

 white spots become alternately larger or smaller as the pole 

 approaches its winter or its summer. Arago has measured 

 with his polariscope the intensity of the light of these snowy 

 zones, and has found it twice as great as that of the light of 

 the remainder of the disk. The " Physico-astronomical 

 Contributions" of Madler and Beer contain some excellent 

 graphical representations ( 603 ) of the northern and southern 

 hemispheres of Mars, in which this remarkable phaeno- 

 menon, unique so far as our knowledge extends in the entire 

 planetary system, is, shown in its relations of measure, in all 

 the variations through which it passes under the influence 

 of the change of seasons and the powerful action of the 

 polar summer in melting the snow. Careful observations 

 continued for ten years have also taught us that the dark 

 spots of Mars retain constantly both their forms and their 

 relative positions on the planet. The periodical enlarge- 



