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34 Apart from their extension, the growth of the canals consists chiefly in depth of tint. They darken rather than broaden,--a fact which tends to corroborate their vegetal character; for that long tracts of country should be thus simultaneously flooded all over to a gradually deepening extent is highly unlikely, while a growth of vegetation would deepen in appearance in precisely the way that the darkening takes place. As for color, the lines would seem to be of the same tint as the blue-green areas. But, owing to their narrowness, this is only an inference. I have never chanced to see them of distinctive color. At this point it is probable that a certain obstacle to such wholesale construction of canals, however, will arise in the mind of the reader, namely, the thought of mountains; for mountains are by nature antagonistic to canals. Only the Czar of all the Russias--if we are to credit the account of the building of the Moscow railway--would be capable of running a canal regardless of topography. Nor will the doings at our own antipodes help us to conceive such construction; for though the Japanese irrigate hillsides, the water in the case comes from slopes higher yet, whereas on Mars it does not. Indeed, for the lines to contain canals we must suppose either that mountains prove no obstacles to Martians, or else that there are practically no mountains on Mars. For the system seems sublimely superior to possible obstructions in the way; the lines running, apparently, not where they may, but where they choose. The Eumenides-Orcus, for example, pursues the even tenor of its unswerving course for nearly 3500 miles. Now, it might be possible so to select one's country that one canal should be able to do this; but that every canal should be straight, and many of them fairly comparable in length, seems to be beyond the possibility of contrivance. In this dilemma between mountains on the one hand and canals on the other, a certain class of observations most opportunely comes to our aid; for, from observations which have nothing to do with the lines, it turns out that the surface of the planet is, in truth, most surprisingly flat. How this is known will most easily be understood from a word or two upon the manner in which astronomers have learnt the heights of the mountains in a the moon. The heights of the lunar mountains are found from measuring the lengths of the shadows they cast. As the moon makes her circuit of the earth, a varying amount of her illuminated surface is presented to our view. From a slender sickle she grows to a full moon, and then diminishes again to a crescent. The illuminated portion is bounded by a semi-circle on the outside, and by a semi-ellipse on the inner. The semicircle is called her limb, the semi- ellipse her terminator. The former is the edge we see because we can see no further; the latter, the line upon her surface where the sun is just rising or setting. Now, as we know, the shadows cast at sunrise or sunset are very long, much longer than the objects that cast them are high. This is due to the obliquity at which the light strikes them; the same effect being produced by any sufficiently oblique light, such as an electric light at a distance. Imperceptible in themselves, the heights become perceptible by their shadows. A road