[Illustration:  APPEARANCE OF MARS, 1860, JULY 6, 11 H. 33 M., Greenwich Mean Time.  Power of Telescope, 201; 8-1/4 inch object-glass.  Planet very low, yet pretty distinct.]

Like the earth, Mars has seasons, for his polar axis, like that of the earth, is aslant, and at one part of his year brings his northern regions more fully into sunlight, at which time summer prevails there and winter in his southern regions; while at the opposite part of his year his southern regions are turned more fully sunward and have their summer, while winter prevails over his northern regions.

Around his poles, as around the earth’s, there are great masses of ice, insomuch that it is very doubtful whether any inhabitants of Mars have been able to penetrate to his poles, any more than Kane or Hayes or Nares or Parry, despite their courage and endurance, have been able to reach our northern pole, or Cook or Wilkes or James Ross our antarctic pole.

In the summer of either hemisphere of Mars, the north polar snows become greatly reduced in extent, as is natural, while in winter they reach to low latitudes, showing that in parts of the planet corresponding to the United States, or mid-Europe, as to latitude, bitter cold must prevail for several weeks in succession.

The land regions of Mars can be distinguished from the seas by their ruddy color, the seas being greenish.  But here, perhaps, you will be disposed to ask how astronomers can be sure that the greenish regions are seas, the ruddy regions land, the white spots either snow or cloud.  Might not materials altogether unlike any we are acquainted with exist upon that remote planet?

The spectroscope answers this question in the clearest way.  You may remember what I told you in October, 1876, about Venus, how astronomers have learned that the vapor of water exists in her atmosphere.  The same method has been applied, even more satisfactorily, to the planet of war, and it has been found that he also has his atmosphere at times laden with moisture.  This being so, it is clear we have not to do with a planet made of materials utterly unlike those forming our earth.  To suppose so, when we find that the air of Mars, formed like our own (for if it contained other gases the spectroscope would tell us), contains often large quantities of the vapor of water, would be as absurd as to believe in the green cheese theory of the moon, or in another equally preposterous, advanced lately by an English artist—­Mr. J.T.  Brett—­to the effect that the atmosphere of Venus is formed of glass.

There is another theory about Mars, certainly not so absurd as either of those just named, but scarcely supported by evidence at present—­the idea, namely, advanced by a French astronomer, that the ruddy color of the lands and seas of Mars is due to red trees and a generally scarlet vegetation.  Your poet Holmes refers to this in those lines of his, “Star-clouds and Wind-clouds” (to my mind among the most charming of his many charming poems):