&8TROKOMT. 



[MARS TUB AST i 



nmarkahle, u it has plainly been seen by most observer* 

 who Jure examined it ; and Maedler hu perceived the 

 quick change* which they undergo. Hersohel, however, 

 confirmed other appearance* detected by Schroeter viz. , 

 the brightaeM of the outer or circular edge of the 

 crescent, and the dimneM of the elliptical boundary. 

 Bench*! oonaidered that the atmucphere of Venus was 

 like our own, refracting and reflecting light ; and an we 

 view the circular part more obliquely than the elliptical 

 boundary, and consequently a greater thickness of the 

 atmosphere come* into view, this explanation would be 

 more in accordance with the appearances observed by 

 Sehroeter, where the outer bright portion gradually 

 melted into the interior dusky region that was observed 

 by Herschel himself the outer bright part being dis- 

 tinctly separated from the inner, and appearing as a 

 bright colour. 



In observing Venus when near its inferior conjuncti. >n, 

 Bchrooter perceived, in the very slender crescent seen i.n 

 such occasions, a very faint light stretching beyond the 

 pointed horns, which, as seen in an ordinary telescope, 

 would be the extreme points of the illuminated semi- 

 circle ; and Herschel confirmed this curious fact. This 

 twilight affords further proof of the atmosphere of Venus, 

 which Sehroeter supposed to be of about the same density 

 as that of the earth, having concluded that the horizontal 

 refraction was 30 J'. In the inferior conjunction of 1849, 

 Professor Maedler plainly perceived this faint extension 

 of the horns ; and he found, for the horizontal refractions, 

 quantities varying from 39 to 48 minutes, but whose 

 most probable value was 43' 7. It is consequently about 

 one-sixth greater than that of the earth's atmosphere. 

 From apparances noted during the transits of Venus 

 across the sun's disc, it would appear probable that the 

 atmosphere is very considerable. The two next tran- 

 sits of Venus will occur on December 8, 1874, and De- 

 cember 6, 1882. 



The apparent diameter of Venus, at its mean distance, 

 is 16* O ; but, at the time of inferior conjunction, this 

 increases to 62*, and at the time of superior conjunction, 

 decreases to 9J*. It resembles the earth, in volume and 

 density, more than any other of the planets. Its diameter 

 is 0-985 of that of the earth ; its volume 0-957, and its 

 density 0-923. A body which would weigh one pound on 

 the earth, would weigh 0'91 on the surface of Venus. 

 Light and heat would be nearly twice as great at Venus 

 as on the earth. No compression at the poles has been 

 perceived in Venus. 



MASS. This planet shines with a ruddy and dusky 

 light, quite different from any of the other heavenly 

 objects. When at favourable opposition, or when near 

 its perihelion and opposition at the same time, it is a very 

 bright object, and so different from its ordinary appear- 

 ance, that it has been frequently mistaken for a new star. 

 Its apparent diameter, at its mean distance, is 6" '8 ; but at 

 the time of opposition this can increase to 23", and at 

 conjunction decrease to 3* -3. The true diameter of this 

 planet U 0-619 of that of the earth, and consequently its 

 volume U only 0-140 of that of this earth. Its density, 

 compared with that of the earth, is 0.948. A body 

 which would weigh one pound on the earth, would only 

 weigh half a pound in Mars. The light and heat which 

 it receives at ita rnuan distance from the sun is -43 of 

 the earth. 



When examined with powerful telescopes, many dark 

 ipoU are perceived on the surface of this planet, winch 

 then low* much of that red colour so apparent to the 

 naked eye. These dusky portions, it has been found, 

 are quite fixed and constant in their position, and they 

 have thus been supposed to be the seas and continents of 

 this planet (see Fig. 07). But not only have the land 

 and wat*T become visible on its surface, but likewise its 

 climate ; fur wo perceive, at its north ami south poles, 

 bright white patches of light, which are naturally held 

 to bo the mow* of Mar* collected in an immense mass. 

 Ihis conjecture is considerably strcii"i]i. ne.l by the fact, 

 that when, by the position of the axis of Mars in respect 

 to the sun, those luminous spots are turned towards 

 that luminary, they diminish in size rapidly. In 1781, 



Sir W. Ilersohel noticed that the southern white ipofe 

 wore extremely brilliant aud extended ; this was after 



Flf . 67. 



winter of a year's duration on this part of the planet 

 In 1783 this same spot was very small, but the sun had 

 continued for nearly eight months alxive the horizon of 

 this part, and melted it away. The existence of an 

 atmosphere to this planet is apparent from other con- 

 siderations, as, at various times, dark, extraneous, cloudy 

 patches have been noticed, which have obscured the 

 planet and hid the spots on its surface. When the snow 

 melts away, a dusky and densely-clouded atmosphere 

 appears to hang over the planet at those parts. 



The rotation of the planet has been determined by 

 several observers ; and from the fixed nature of the spots 

 observed, and the good definition with which they appear 

 under favourable circumstances, it has been found with 

 considerable accuracy. Cassini, in 1GC6, determined it 

 to be performed in 24h. 40m. ; and, in 1704, Maraldi re- 

 peated the observation, and found the period of rotation 

 as 23h. 39m. Herschel made many observations relative 

 to the telescopic appearances of tliis planet. He deter- 

 mined the sidereal rotation as 24h. 39m. 21-7"., and the 

 synodical at 2th. 38m. 20 '3s. The equator of Mars he 

 found was inclined at an angle of 28 42* to his orbit, and 

 the node was directed to the constellation Sagittarius. 

 The seasons on this planet would not, therefore, bo much 

 ilill'erent from those on the earth ; but, on account of liis 

 great eccentricity, their duration is very different. 



The compression of Mars at the poles appears to be 

 considerably greater than that of the earth, which, con- 

 sidering his small diameter, and that the rotation and 

 density are nearly similar to those of the earth, is rather 

 remarkable. It does not appear, however, to be so con- 

 siderable, as determined by Sir W. Herschel, according 

 to whom the ratio of the polar and equatorial axes was 

 as 98 to 103, or as 15 to 16. Schroeter estimates them at 

 80 to 81. According to M. Arago, it is more than ,fo. 

 Harding has noticed that the equatorial sides of Mars 

 sometimes appear very bright ; and he thinks that tho 

 irradiation caused by this circumstance produces dis- 

 crepancies in the measurement of the equatorial and 

 polar diameters. 



THE ASTEROIDS. The present century opened with tho 

 discovery of four small planets Vesta, Juno, Ceres, and 

 1'allas, situate between Mars and Jupiter ; called asteroids 

 (the appearance of stars), because of their stellar aspect 

 under telescopic examination. The order of their ili<- 

 covery, with the namos of their discoverers, is as follows : 



Ceres . . January 1, 1801, by M. Piazzi, of 

 Palermo, in Sicily. 



Pallas . . March 28, 1802, by M. Gibers, of 

 Bremen, in Saxony. 



Juno. . SuptemW 2, isol, by M. Harding, 

 5f LiliL-iMial, in Hanover. 



Vesta . . March 29, 1807, by M. Olbers, of 



Bremen. 



These bodies are sometimes styled planetoids, as more 

 expressive of th-sir character, and extra-zodiacal planets, 

 because their orbits are not ciilinc<l within the y.oili.u- 

 like those of other planets. They are exclusively 

 scopic object*, and require the very best instruments to be 



