76 A CENTURY'S PROGRESS IN ASTRONOMY. 



plete photographic lunar atlas ever published, in 

 his work 'The Moon' (1903), in which he sums 

 up all his observations since 1891, and concludes 

 that "the evidence in favour of the idea that 

 volcanic activity upon the Moon has not yet 

 ceased is pretty strong, if not fairly conclusive." 



Pickering points out that the density of the 

 lunar atmosphere is not greater than one ten- 

 thousandth of that at the Earth's surface, and, 

 under these circumstances, water cannot exist 

 above freezing-point, which of course brings us 

 to the subject'of snow. He considers that snow 

 is observed on the mountain peaks and near the 

 poles of the Moon, and he believes his conclusion 

 to be verified by observations on the well-known 

 crater, Linnd He brings forward evidence of 

 the probable existence on the Moon of organic 

 life, pointing out that the difference between 

 the conditions of the Earth and the Moon is 

 not so great as that above and below the ocean 

 on our own planet. He has collected evidence 

 of the existence of something resembling vege- 

 tation on the Moon " coming up, flourishing, and 

 dying, just as vegetation springs and withers 

 on the Earth." 



The first successful attempt to measure the 

 heating power of moonlight was made in 1846 on 

 Mount Vesuvius by Melloni, an Italian physicist, 



