We study Mars under much more favourable conditions than either Jupiter or Saturn. And yet, at a first view, the telescopic aspect of this interesting planet is exceedingly disappointing. Galileo, who quite easily discovered the moons of Jupiter with his largest telescope, could barely detect with it the fact that Mars is not quite round at all times, but is seen sometimes in the shape of the moon two or three days before or after full. "I dare not affirm," he wrote on December 30, 1610, to his friend Castelli, "that I can observe the phases of Mars; yet, unless I mistake, I think I already perceive that he is not perfectly round." But even in a large telescope one can see very little except under very favourable conditions. It has only been by long and careful study, and piecing together the information obtained at various times, that astronomers have obtained a knowledge of the facts which appear in our text-books of astronomy. The possessor of a telescope who should expect, on turning the instrument towards Mars, to perceive what he has read in descriptions of the planet, would be considerably disappointed. First noticed among the features of the planet were two white spots of light occupying the northern and southern parts of his disc. These are now known to be regions of snow and ice, like those which surround the poles of our own earth. But how different the reality must be from what we seem to see in the telescope! These two tiny white specks represent hundreds of thousands of square miles covered over with great masses of snow and ice, which doubtless are moved by disturbing forces similar to those which make our arctic regions for the most part impassable even for the most daring of our seamen. The snow-caps of Mars change in size as the planet circuits round the sun, completing his year of seasons (which lasts 687 of our days). They are largest in the winter of Mars, smallest in the Martian summer; so that, as it is winter for one hemisphere when it is summer for the other, one of the snow-caps is larger than the other at the winter and summer seasons. In the same way, our arctic snows extend more widely during our winter, while the antarctic snows then retreat; whereas, during our summer, when it is winter in the southern hemisphere, the antarctic snows advance and our arctic snows retreat. But we have still to learn why these white spots are _known_ to be masses of snow. They might well from analogy be considered to be snows, since they behave like the snows of our polar regions. Yet that would be very different from proving them to be snow masses. I shall now show how this has been done, and afterwards describe the lands and seas of the planet, and give a short account of the recent interesting discovery of two moons attending on the planet which Tennyson had called the "moonless Mars." * * * * *