1 873.] The Planet Mars in 1873. 201 



applies to every part of the terminator, but the point has 

 been already considered for the middle parts). Now the 

 result of this is, that the southern parts of Mars, where 

 winter is in progress, are better seen than they would be if 

 the line of sight from the earth were coincident with the 

 line from the sun. Similar remarks apply to projections 2, 

 3, and 4, but to a gradually diminishing degree. 



After opposition the reverse holds, — a fact of more im- 

 portance, because it is the polar part of the planet which is 

 now more directly viewed than illuminated. It is seen from 

 the projections 6, 7, 8, and 9, how the terminator now 

 passes between the north pole and the northern edge of the 

 disc. It is obvious that the opportunity is thus an excellent 

 one for studying the behaviour of the north polar snow as 

 the summer months pass gradually on towards the autumnal 

 equinox. This opportunity ought not to be lost by those 

 who possess telescopes sufficiently powerful to distinguish 

 the shape and dimensions of the polar snow-caps. 



Lastly, it remains that I should make a few remarks on 

 the features of the surface of Mars. 



It will be understood that the projections in the illustrative 

 plate are not intended to resemble pictures of Mars.* The 

 land regions and oceans, for instance, are carried right to the 

 very edge of the disc, whereas in reality they are concealed 

 nearthe edge, under the white light already referred to. These 

 projections are, in fact, masses of Mars, but an orthographic 

 or natural projection, so that they show the various features 

 as they would be seen if Mars were like a terrestrial globe 

 and his aspect not affected by meteorological relations of 

 any sort. 



I may be permitted to point out that it was by means of 

 constructions resembling those in the illustrative plate that 



* The woodcut shows the method by which the areographic features of 

 Mars, for the epochs indicated in the plate, have been determined from an 

 observation of Mars made on February 23, 1867, at 6h. 45m. p.m., by Mr. 

 Browning. (The hour in each case is midnight, Greenwich mean time). The 

 picture of Mars then obtained is shown in Plate II. of my " Essays on 

 Astronomy." Between the date of that observation and April 27, 1873, mid- 

 night, there is an interval of 194,850,900 seconds. Taking the rotation period 

 of Mars as 8864273 seconds, I find that the number of rotations of Mars 

 amounts to 2198 + a rotation through 57 . I take the Kaiser Sea as 21 from 

 the central meridian in Mr. Browning's picture (approaching the meridian), 

 and the line joining the Earth and Mars on April 27 makes an angle of about 

 117 with the corresponding line on February 23, 1867. This obviously 

 amounts to setting Mars 117° back in rotation. Thus, instead of 2198 

 Rot. 4- 57 , we have 2198 Rot. — 6o°, or the Kaiser Sea 8i° from the central 

 meridian, instead of 21 as on Feb. 23, 1867, at 6h. 45m. The picture of Mars 

 for April 27, No. 5 of the plate, corresponds with this result. The others have 

 been obtained from similar considerations, account being taken in every case 

 of the changing bearing of Mars from the earth. 



VOL. III. (N.S.) 2 D 



