increase the difference to such, an extent that the seasonal changes might be fatal to all higher forms of life. We have only to recollect how powerful the effect of the comparatively moderate variations between the seasons of our own planet is upon the human organism in order to understand what must be the condition of things in the southern hemisphere of Mars, where the passage from one season to the other presents the succession of violent winter cold, accompanied by days of gloom and faint sunshine, followed by a blazing summer, with the sun hanging overhead, visibly increased in apparent size by its approach. Telescopic observations show clearly by the great variation in the extent of the polar snows how extensive is the effect of these changes upon the surface of the planet. In the hot summer the snows rapidly retreat toward the pole, and even leave the actual pole itself bare of snow, showing that upon Mars, as upon the Earth, the center, or pole, of greatest cold (at least in the southern hemisphere) does not coincide with the geographical pole of the planet. Then, with the on-coming of winter, the march of the snows begins and they rapidly advance further and further toward the equator, spreading over the antarctic regions until another change of season brings back a flaming sun to melt them away. It should be added that, as Prof. Young has remarked, the climate of Mars, upon the whole, appears to be much milder than we should naturally have expected in view of its distance from the sun.
Bearing in mind these general facts about the size of Mars and its position in the solar system, we shall now proceed to the discussion of its surface phenomena as revealed by the telescope, merely pausing to remark that the atmosphere of Mars is apparently less dense than that of the Earth, and that the spectroscope has demonstrated the presence of watery vapor in it.
The little telescope of Galileo, which had enabled him to discover the phases of Venus, the satellites of Jupiter, the mountains of the moon, the existence of Saturn's ring, and "vast crowds of stars" in the Milky Way, was not powerful enough to show him the markings that diversified the disk of Mars. The earliest drawings of Mars that have come down to us were made by Fontana, in Italy, in 1636 and 1638. They contain very little detail, the best representing the planet simply with a darkish spot in the center of the disk. Twenty odd years later Huygens made much better drawings, and then the work was taken up by Cassini, Maraldi, and others, with the cumbersome telescopes of the time, the most powerful of which consisted of an object-glass suspended high in the air by means of a long pole or other support, while the eye-piece in the hand of the observer on the ground was, with infinite difficulty, brought and kept in line with the optical axis of the instrument. One of these telescopes was no less than three