day, and falling to about the temperature of liquid air during the lunar night. So wide a range of temperature must be fatal to living organisms, particularly when the range is repeated at short, regular intervals of time. But this range of temperature comes directly from the length of the Moon's rotation period; for the longer the day of the Moon, the higher the temperature which may be attained in it; the longer the night, the greater the cold which will in turn be experienced. We learn, therefore, that the time of rotation of a planet is an important factor in its habitability. CHAPTER VI THE CANALS OF MARS Both of the two worlds best placed for our study are thus, for different reasons, ruled out of court as worlds for habitation. The Sun by its vastness, its intolerable heat and the violence of its changes, has to be rejected on the one hand, while the Moon, so small, and therefore so rigid, unchanging and bare, is rejected on the other. Of the other heavenly bodies, the planet Mars is the one that we see to best advantage. Two other planets, Eros and Venus, at times come nearer to us, but neither offers us on such occasions equal facilities for their examination. But of Mars it has been asserted not only that it is inhabited, but that we know it to be the case, since the evidence of the handiwork of intelligent beings is manifest to us, even across the tremendous gulf of forty or more million miles of space. A claim so remarkable almost captures the position by its audacity. There is a natural desire among men to believe the marvellous, and the very boldness of the assertion goes no small way to overcome incredulity. And when we consider how puny are men as we see them on this our planet, how minute their greatest works, how superhuman any undertaking would be which could demonstrate our existence to observers on another planet, we must admit that it is a marvel that there should be any evidence forthcoming that could bear one way or another on the solution of a problem so difficult.