[[Categoria:Pagine che usano RigaIntestazione|Scientia - Vol. VII.djvu{{padleft:272|3|0]]that small photographs of our satellite about 1 centimetre in diameter, if held at the proper distance, reveal, after prolonged staring, single and double «canals» on the Moon as on Mars.

In 1907 Prof. Newcomb made some experiments in the same directions as those by Dr Cerulli and myself, and reached the same general conclusion. He further drew attention to the extent to which the «canal» system as then known, — Mr Lowell had named and catalogued 398, — covered the surface of the planet. «Making due allowance for the aberration of the best achromatic telescope, the total area of the entire system of 400», as depicted on the retina of the terrestrial eye, can scarcely fall much below one half the total area of the planet, and may be greater». Since then the number of «canals» has been increased, so that now their area as depicted on the retina, must be fully two-thirds the area of the planet.

Whilst there is still controversy as to the true nature of these straight lines and round spots on the surface of Mars it is well to bear in mind that there are many facts about the planet that are not in dispute.

The length of the year of Mars is 669 days as reckoned in Martian time; or 687 as reckoned in terrestrial time. Of these 199 days belong to the spring of the northern hemisphere, 183 to summer, 147 to autumn, and 158 to winter; or the summer half includes 382, and the winter 305 days. The mean distance from the Sun, that of the Earth being unity, is 1.5237, and in consequence the light and heat received from the Sun, per unit of surface, is only 0.43 of that received by the Earth, or about three-sevenths. The diameter surface, volume, mass and density of Mars are respectively 0.53, 0.281, 0.1489, 0.107, and 0.72 of the same values for the Earth, and the force of gravity at its surface is 0.381. It follows that the atmosphere of Mars is arranged in a very different manner from that of the Earth; the barometric gradient is much less steep. From this circumstance, and from the feebler force of gravity, the atmospheric circulation must be relatively languid and slow. The level of half-surface-pressure is reached at three and a third miles on the Earth, but not until eight and three-quarter miles on Mars. Assuming that the mass of the atmosphere bears the same relation