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what indefinite in outline, extending through considerable distances across the planet.

3. In many cases the dark portions appear as if chained together to a greater or less extent, and thus give rise to the appearance of long dark channels.

The appearance on which this third phenomenon, which we may regard as identical with that observed by Cerulli, is based, may be well illustrated by looking, with a magnifying glass, at a stippled portrait engraved on steel. Nothing will then be seen but dots, arranged in various lines and curves. But take away the magnifying glass and the eye connects these dots into a well-defined collection of features representing the outlines of the human face. As the eye makes an assemblage of dots into a face, so may it make the minute markings on the planet Mars into the form of long, unbroken channels.

The features which we have hitherto described do not belong to the two polar regions of the planet. Even when the snowcaps have melted away, these regions are seen so obliquely that it would be difficult to trace any well-defined features upon them. The interesting question is whether the caps which cover them are really snow which falls during the Martian winter and melts again when the sun once more shines on the polar regions. To throw light on this question we have to consider some recent results as to the atmosphere of the planet.

All recent observers are agreed that, if Mars has any atmosphere at all, it is much rarer than our own, and