From this point of view the permanent dark spots are the low-lying parts of the planet's surface, in which at all times there is a sufficient accumulation of water to support vegetable life. [Illustration: FIG. 98.--The same face of Mars at three different seasons.--LOWELL.] 154. THE CANALS.--In Fig. 98 the lower part of the disk of Mars shows certain faint dark lines which are generally called canals, and in Plate III there is given a map of Mars showing many of these canals running in narrow, dusky streaks across the face of the planet according to a pattern almost as geometrical as that of a spider's web. This must not be taken for a picture of the planet's appearance in a telescope. No man ever saw Mars look like this, but the map is useful as a plain representation of things dimly seen. Some of the regions of this map are marked Mare (sea), in accordance with the older view which regarded the darker parts of the planet--and of the moon--as bodies of water, but this is now known to be an error in both cases. The curved surface of a planet can not be accurately reproduced upon the flat surface of paper, but is always more or less distorted by the various methods of "projecting" it which are in use. Compare the map of Mars in Plate III with Fig. 99, in which the projection represents very well the equatorial parts of the planet, but enormously exaggerates the region around the poles. It is a remarkable feature of the canals that they all begin and end in one of these dark parts of the planet's surface; they show no loose ends lying on the bright parts of the planet. Another even more remarkable feature is that while the larger canals are permanent features of the planet's surface, they at times appear "doubled"--i. e., in place of one canal two parallel ones side by side, lasting for a time and then giving place again to a single canal. It is exceedingly difficult to frame any reasonable explanation of these canals and the varied appearances which they present. The source of the wild speculations about Mars, to which reference is made above, is to be found in the suggestion frequently made, half in jest and half in earnest, that the canals are artificial water courses constructed upon a scale vastly exceeding any public works upon the earth, and testifying to the presence in Mars of an advanced civilization. The distinguished Italian astronomer, Schiaparelli, who has studied these formations longer than any one else, seems inclined to regard them as water courses lined on either side by vegetation, which flourishes as far back from the central channel as water can be supplied from it--a plausible enough explanation if the fundamental difficulty about temperature can be overcome. [Illustration: FIG. 99.--A chart of Mars, 1898-'99.--CERULLI.] [Illustration: PLATE III. MAP OF MARS (AFTER SCHIAPARELLI)]