CONDITIONS OP HABITABILITY OF A PLANET. 83 



tioM of its entire population. Accepting the argument that the 

 regularity of the " canals " and " oases " proves that they are 

 artificial, we reach the conclusion not only that there are 

 intelligent beings on Mars, but that they must have achieved 

 a complete political unity, and have developed intellectual 

 powers and a command over the forces of nature which far 

 outstrip anything that we as yet have been able to accomplish 

 here. 



The study of the surface of Mars goes back almost to the 

 time of the invention of the telescope, the earliest drawing 

 extant having been made in the year 1636. In 1666, Eobert 

 Hooke, the Gresham Professor of Astronomy, and Secretary to 

 the Koyal Society, detected several dark spots on the planet, 

 and in the same year Cassini discovered that Mars rotated upon 

 it axis in a period of about twenty-four hours forty minutes. 

 The next great advance was made by Sir William Herschel, 

 who during the oppositions of 1777, 1779, 1781, and 1783, 

 determined the inclination of the axis of Mars to the plane of 

 its orbit, measured its polar and equatorial diameters, and ascer- 

 tained the amount of the polar flattening. He paid also special 

 attention to two bright white spots upon the planet, and he 

 showed that these formed round the planet's poles, and increased 

 in size as the winter of each several hemisphere drew on, and 

 diminished again with the advance of summer, behaving there- 

 fore as the snow does in our own polar regions. 



The next stage in the development of our knowledge of 

 Mars must be ascribed to the two German astronomers, Beer 

 and Madler, who made a series of drawings in the years 1830, 

 1832, and 1837, by means of a telescope of four inches aperture, 

 from which they were able to construct a chart of the entire 

 globe. This chart may be considered classic, for the features 

 which it represents have been observed afresh at each 

 succeeding opposition. The surface of Mars therefore possesses 

 permanent features, and some of the markings in question can 

 be identified not only in the rough sketches of Sir William 

 Herschel, but even in those of the year 1666, made by Hooke 

 and Cassini. In the forty years that followed, the planet was 

 studied by many of the most skilled observers, and in 1877 the 

 late Mr. E. Green, Drawing Master to Queen Victoria, and 

 a painter in water-colours with a most delicate appreciation 

 of colouring, made a series of sketches of the planet from a 

 station in the island of Madeira, 2,000 feet above sea level. 

 When the opposition was over, Mr. Green collected together a 

 large number of drawings and formed a chart of the planet 



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