372 Original Articles. [Juty} 



remarkable adjustment appears not to be explained by the hypothesis, 

 but not to offer a positive contradiction to it. 



The orbit of Mars is more excentric than that of any of the other 

 planets except Mercury : his distance from the sun varies from about 

 154 to about 130 millions of miles, the mean being about 140 millions. 

 The earth being about 93 millions from the sun, the interval between 

 us and Mars varies from about 40 millions at the least to about 240 at 

 the most. The apparent magnitude of the planet as seen from the 

 earth is sometimes as little as four seconds of space, and sometimes as 

 great as twenty-two seconds. 



When Mars is in opposition, that is to say, on the same side of 

 the sun, and nearly in the same straight line as the earth, the best 

 opportunity is afforded for telescopic observation. Mars revolves 

 round the sun in 687 days, the earth in 365 ; consequently once in 

 two years and seven weeks, roughly speaking, Mars is in opposition. 

 As the points of opposition thus circle round the ecliptic, the decli- 

 nations of the planet vary. Thus oppositions are more or less 

 favourable, as the distance of Mars is less and his declination more to 

 the .northward. In 1862 Mars was in opposition on the 9th of 

 October, in 1864 on the 30th of November; in the former case he 

 was nearer to us, and had a visible diameter of 22" ; in the latter he 

 was farther off, having only a diameter of 16", but his declination was 

 more to the northward, and on this account he was well seen. 



No occasion so favourable for observing the planet in opposition 

 has occurred since 1836, or will occur again for twelve or fourteen 

 years to come ; but every opportunity should be seized of watching 

 every change of appearance ; for by reason of the considerable angle 

 between the equator of Mars and the plane of his orbit, the planet is 

 presented in various aspects ; at one time the north pole, at another 

 the south pole is more visible, while at particular epochs the axis is 

 at right angles to the line of sight. Hence the extremely diverse and 

 at first view perplexing appearances, which have been faithfully por- 

 trayed by Madler, Herschel, De la Rue, and other observers. 



The telescopes employed by Galileo were powerful enough to 

 detect the phases of Mars ; those in the hands of his successors 

 revealed the more considerable spots. Hooke and Campanus were 

 among the first to observe them, and Cassini, as might be expected 

 from so great an astronomer, employed them in determining the daily 

 period of rotation, which he found to be a little longer than that of the 

 earth, viz. 24h. 40m., a result which differs (by excess) only 2m. 38s. 

 from the period now generally recognized. Cassini's nephew, Maraldi, 

 repeated these observations in 1704, and found the rotation to be 

 24h. 39m. ; Herschel in 1784 assigned to this same period 24h. 39m. 

 21s. '67. These nearly agreeing determinations assure us, among 

 other things, of the goodness of the long refractors employed in the 

 seventeenth century before the introduction of achromatic telescopes. 

 By observations of the same kind the inclination of the axis of the 

 planet to the plane of its orbit is ascertained to be about 61° 18', or, to 

 use the ordinary expression, the obliquity of his ecliptic is 28° 42'. 

 This ecliptic of Mars is oblique to the earth's orbit, 1° 51'. When in 



