Page:Mars - Lowell.djvu/226

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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 with the Eumenides-Orcus 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 height of the mountains in 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 full moon, and then diminishes again to a crescent. The illuminated portion is bounded by a semicircle on the one side, and by a semi-ellipse on the other. The semi-circle is called her limb, the semi-ellipse her terminator. The former is the edge we see because we can see no farther; the latter, the