MARS AND ITS SATELLITES. 707 



those of 1830, gave 24'' 37"" 23.7'. In regard to tlie exact period of 

 rotation and the slight discrepancies between the results obtained by- 

 different observers, Prof. O. M. Mitchel remarks as follows : 



" In 1839 Madler reviewed Herschel's observations, wJaence his first results 

 were deduced, and discovered that, after introducing the necessary reduc- 

 tions, the discrepancy of two minutes might be reduced to two seconds, by 

 giving to Mars one more rotation on its axis, between the observations of 1777 

 and 1779, than Ilerschel had employed. 



" By combining Madler's observation, made at Berlin, 1830, September 14th, 

 12'' SO"", with one made at the Cincinnati Observatory, 1845, August 30th, 

 S"" 55"", making the corrections due to geocentric longitude, phase, and aberra- 

 tion, I find the period of rotation to be 24'' 37" 20.6', differing by only two 

 seconds from Madler's period as last corrected." ' 



Finally, Richard A. Proctor, Esq., by an exhaustive discussion of 

 all the observations, has determined the period to be 24'' 37 22.735'. 



The diameter of Mars is about 4,700 miles. Its surface is rather 

 more than one-third that of the earth, while its volume is to that of 

 our planet in the ratio of two to nine. 



The persevering labors of Beer and Madler proved beyond ques- 

 tion that many of the lineaments observed in the aspect of this planet 

 are permanent in their character, and not merely atmospheric. Tiie 

 same spots, with the same general outlines, and the same varieties of 

 color, have been noticed at successive oppositions ; not always, it is 

 true, with precisely the same distinctness, but without any other 

 changes than such as might be attributed to atmospheric variations. 

 Two white circular spots are observed in the polar regions, which in- 

 crease during the winter, and decrease in the summer, of each hemi- 

 sphere respectively, and which may, therefore, be regarded as polar 

 snows. These spots were noticed by Maraldi as early as 1716;. their 

 connection, however, with the change of seasons was first shown by 

 Sir William Herschel. The same astronomer found the inclination of 

 the axis of Mars to the plane of its orbit to be 61 18'. The Martial 

 tropics are therefore 28 42' from the equator, making the torrid zone 

 10 wider than that of the earth. In so far, then, as climatic changes 

 are dependent on the obliquity of the planets, the seasons of Mars 

 may not differ, perhaps, except in their duration, very greatly from 

 our own. 



. The Satellites of Maks. We come now to the history of one 

 of the most interesting discoveries of the nineteenth century. With 

 the single exception of our own moon, Mars is the most favorably 

 situated of all the heavenly bodies for telescopic observation. The 

 most careful scrutiny, however, for more than two centuries, had 

 failed to furnish any indication of the existence of a satellite. The 

 opposition of Mars in August, 1877, occurred when the planet was 



' Sidereal Messenger^ p. 101. 



