2 : 2 The Planet Mars in 1873. A p ril . 



I succeeded in interpreting the telescopic pictures of Mars 

 obtained by Dawes, and in forming from them the stereo - 

 graphic and Mercator's charts of Mars which appear in my 

 " Other Worlds " and "Essays :r. Astronomy" respectively. 

 For every picture which he lent me or had published I con- 

 structed the proper orthographic projection, of suitable size, 

 and carefully timed with reference to the actual rotation 

 progress of Mars. It need hardly be said that the results 

 were not found to be in strict accordance, simply because 

 Mr. Dawes in drawing was not able to represent the features 

 of Mars precisely as the}' were. Eye-judgments must 

 always be, to some slight extent, faulty; and though some 

 of his pictures must have been remarkably accurate (espe- 

 cially those taken in his later years), yet some slight dis- 

 cordances neverthelr:: 7 ::ed. The charts, as finally drawn 

 by me, present the features so that their shapes form a sort 

 of mean between the various shapes which result from the 

 separate drawings. 



I believe it will be found that the telescopic study of the 

 planet during the approaching opposition, with continual 

 and careful reference to the accompanying projections, will 

 enable any tolerably good draughtsman, possessed of ade- 

 quate telescopic means, to improve our knowledge of the 

 planet's features. It need hardly be said that, from the 

 various projections given in the plate, the aspect of the 

 planet at any time may be readily determined. The meri- 

 dians marked on the planet are 30 degrees apart, and. since 

 the planet rotates once upon its axis in 24b. 37m. 22f s.. it 

 follows that he rotates so as to carry one of these meridians 

 to the place occupied by the next forwards in a period of 

 2h. 3m. 7s. very nearly. This would be strictly correct if it 

 were not for the circumstance that, as we see Mars from the 

 e?-7th, account has to be taken of the varying direction in 

 which he is seen. For instance, comparing the position of 

 Mars at M, in the figure with his position at M 9 , it is mani- 

 fest that it would be insufficient merely to consider so many 

 rotations and parts of a rotation, in order to deduce his 

 asreft at any given hour when he is near M s . For the line 

 from M g to the earth at E 9 is, as it were, swayed round from 

 the direction occupied by the line from Mars at M, to the 

 earth a: E : . and in a direction contrary to that of the 

 planet's rotation ; see the globe of the planet in the lower 

 left-hand corner of the picture. And it is plain that this 

 has precisely the same effect as though the planet had 

 rotated so much farther forward. 



But although in long intervals this is an important 



