The chief advocate of this theory is LOWELL, an American observer, who has given very great attention {64} to the study of the planet during the last seventeen years. His argument is that the straight lines, the canals, which he sees on the planet, and the round dots, the "+oases+," which he finds at their intersections, form a system so obviously _un_natural, that it must be the work of design--of intelligent beings. The canals are to him absolutely regular and straight, like lines drawn with ruler and pen-and-ink, and the oases are exactly round. But, on the one hand, the best observers, armed with the most powerful telescopes, have often been able to perceive that markings were really full of irregular detail, which Lowell has represented as mere hard straight lines and circular dots, and, on the other hand, the straight line and the round dot are the two geometric forms which all very minute objects must approach in appearance. That we cannot see irregularities in very small and distant objects is no proof at all that irregularities do not exist in them, and it has often happened that a marking which appeared a typical "canal" when Mars was at a great distance lost that appearance when the planet was nearer. Astronomers, therefore, are almost unanimous that there is no reason for supposing that any of the details that we see on the surface of Mars are artificial in their origin. And indeed the numerical facts that we know about the planet render it almost impossible that there should be any life upon it. If we turn to the table, we see that in size, volume, density, and force of gravity at its surface, Mars lies between the Moon and the Earth, but is nearer the Moon. This has an important bearing as to the question of the planet's atmosphere. On the Earth we pass through half the atmosphere by ascending a mountain {65} that is three and a third miles in height; on Mars we should have to ascend nearly nine miles. If the atmospheric pressure at the surface of Mars were as great as it is at the surface of the Earth, his atmosphere would be far deeper than ours and would veil the planet more effectively. But we see the surface of Mars with remarkable distinctness, almost as clearly, when its greater distance is allowed for, as we see the Moon. It is therefore accepted that the atmospheric pressure at the surface of Mars must be very slight, probably much less than at the top of our very highest mountains, where there is eternal snow, and life is completely absent.