160 COSMOS. 



tainty. It was stated by William Herschel to be T 'g- ; accord- 

 ing to Arago's more accurate measurement,* with one of Ro- 

 chon's prismatic telescopes, in the first instance (before 1824), 

 only in the proportion of 189 : 194, i. e., ^i. T ; by a subsequent 

 measurement (1847), ^', still, Arago is inclined to consider 

 the flattening somewhat greater. 



If the study of the Moon's surface calls to mind many ge- 

 ognostic relations of the surface of the Earth, so, on the con- 

 trary, the analogies which Mars presents with the Earth are 

 entirely of a meteorological nature. Besides the dark spots 

 some of which are blackish ; others, though in very small 

 numbers, yellowish-red,! and surrounded by the greenish con- 

 trast colors, so-called seas$ there are seen upon the disk of 

 Mars two white, brilliant, snow-like spots, either at the poles 

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

 of cold alternately. They were recognized as early as 1716 

 by Philip Maraldi, though their connection with climatic 

 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 win- 

 ter or summer. Arago has measured, by means of his polari- 

 scope, the intensity of the light of these snoiv zones, and found 

 it twice as great as that of the remaining part of the disk. 

 The Physikalisch-astronomischen Beitrdgen of Madler and 

 Beer contain some excellent graphic representationsll of the 

 north and south hemispheres of Mars ; and this remarkable 

 phenomenon, unparalleled throughout the whole planetary 

 system, is there investigated with reference to all the changes 

 o'f 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 period- 

 ical formation of snow-spots, as meteoric depositions depend- 

 ent upon change of temperature, and some optical phenom- 

 ena which the dark spots present as soon as they have, by the 

 rotation of the planet, reached the edge of the disk, make the 

 existence of an atmosphere upon Mars more than probable. 



* Laplace, Expos, du Syst. du Monde, p. 36. Schroter's very imper- 

 fect measurement of the diameter of the planet gave Mars a flattening 

 of only ^. t Beer and Madler, Beitrdge, p. 111. 



t Sir John Herschel, Outlines, $ 510. 



$ Beer and Madler, Beitrdge, p. 117-125. 



j Madler, in Schumacher's Astr. Nachr., No. 192. 



