The controversy about the habitableness of Mars. Humidity on Mars. Early observations. The spectra of Mars and of the Moon compared. Investigations by Campbell and Marchand. The work of Lowell. Measurements by Slipher. Calculations by Very. The temperature on Mars according to these sources. Campbell’s expedition to Mount Whitney in California. Oxygen on Mars. The cold on Mars detrimental to anything but the lowest forms of life. Cause of different results by Slipher and Campbell. Very’s answer to Campbell’s criticism. New measurements by Slipher. Campbell’s new method of measurement of 1910. Christiansen calculates the temperature on Mars from intensity of the Sun’s radiation. The sun-constant. Average temperature of Mars about forty degrees Centigrade. Possibly low plant life around the poles during summer. The canals on Mars are probably fissures in the crust. The length of the canals compared with that of the fissures in Earth’s crust. The double canals on Mars compared with the parallel fissures in Calabria. Emanations along the fissures. The canals as affected by increasing cold or heat. The polar snow. Thawing of the canals. Travel of the water vapour independent of the topography. The desert sand on Mars. Clouds and mists. Highlands and mountains on Mars. Sand filling of the canals. The seas on Mars. The straightness and uniform breadth of the canals an illusion. Light and dark spots. “New” canals. The fancies of Lowell. CHAPTER VII MERCURY, THE MOON, AND VENUS 228 Fissures on Mercury. Lowell’s drawing. Centres of collapse. Absence of atmosphere. The climate on the Moon. W. Pickering’s belief in frost formations on the Moon. The mountains on the Moon. Volcanoes. Circular elevated rings. “Seas” on the Moon. The Crater “Linné.” “Sinuses” and “Streaks” on the Moon. The light matter of the streaks probably lava-scum. The colour of the Moon and of the Earth. Comparison between the Moon and Mars. Changes on the Moon. “Snow” and “Vegetation” on the Moon according to W. Pickering. The fate of Mars and of the Earth. Falling meteoric dust. The climate on Venus. Swamps like those of the carboniferous age. Abundant vegetation. Low organisms. “Culture” on Venus will proceed from the poles. The future of Venus. The claims of astrology in modern light. Tycho Brahe. The dreams of Giordano Bruno probably true. ILLUSTRATIONS PAGE FIG. 1.--THE MILKY WAY 46 Photo by Easton. FIG. 2.--PLANETARY NEBULA NO. 7009, NEW GENERAL CATALOGUE 52 FIG. 3.--NEBULA NO. 4594, NEW GENERAL CATALOGUE 66