About each pole, as it happens to be turned earthward, is to be seen a round white patch (supposed to be snow), and this gradually disappears as the summer advances in that hemisphere of the planet—for Mars has seasons very closely resembling our seasons, except that they are about twice as long. The inclination of the axis of Mars to the plane of its orbit is about 24° 50′, which is not very different from the inclination of the earth's axis. Moreover, Mars rotates in a period of 24 hours, 37 min., 22 sec., so that the length of day and night upon its surface is very nearly the same as upon the earth. The surface of the planet is marked by broad irregular areas of contrasting colour, or tone, some of them being of a slightly reddish, or yellowish, hue, and others of a neutral dusky tint. The general resemblance to a globe of the earth, with differently shaped seas and oceans, is striking. On account of the many likenesses between Mars and the earth, some astronomers are disposed to think that Mars may be a habitable planet. The terms “seas” and “continents” were formerly applied to the contrasted areas just spoken of, but now it is believed that there are no large bodies of water on Mars. Crossing the light, or reddish-coloured, areas there are sometimes seen great numbers of intersecting lines, very narrow and faint, which have received the name of “canals.” Some speculative minds find in these ground for believing that they are of artificial origin, and a theory has been built up, according to which the so-called canals are “irrigated bands,” the result of the labours of the inhabitants. The argument of the advocates of this theory is put about as follows: Mars is evidently a nearly dried-up planet, and most of the water left upon it is periodically locked up in the polar snows. As these snows melt away in the summer time, now in one hemisphere and now in the other, the water thus formed is conducted off toward the tropical and equatorial zones by innumerable canals, too small to be seen from the earth. The lands irrigated by these canals are narrow strips, whose situation is determined by local circumstances, and which cross one another in all directions. Within these bands, which enlarge into rounded “oases” where many of them intersect, vegetation pushes, and its colour causes them to appear as dark lines and patches on the surface of the planet. The fact that the lines make their appearance gradually, after the polar caps begin to disappear, is regarded as strongly corroborative of the theory. In answer to the objection that works so extensive as this theory of irrigation calls for would be practically impossible, it is replied that the relatively small force of gravity on Mars not only immensely diminishes the weight of all bodies there, but also renders it possible for animal forms to attain a greater size, with corresponding increase of muscular power. It is likewise argued that Mars may have been longer