THE INNER PLANETS. 83 



and was enabled to follow it hourly instead of 

 looking at it for a short period when near the 

 horizon. At length, after seven years' observa- 

 tion, he announced, on December 8, 1889, that 

 Mercury performs only one rotation during its 

 revolution round the Sun in fact, that its day 

 and year coincide. As a consequence, the planet 

 keeps the same face towards the Sun, one side 

 having everlasting day and the other perpetual 

 night ; but owing to the libratory movement of 

 Mercury the result of uniform motion on its 

 axis and irregular motion in its orbit the Sun 

 rises and sets on a small zone of the planet's 

 surface. Schiaparelli's observations indicated that 

 Mercury is a much spotted globe, with a moder- 

 ately dense atmosphere, and he w r as enabled to 

 form a chart of its surface-markings. 



Schiaparelli's conclusions remained until 1896 

 unconfirmed and yet not denied, although most 

 astronomers were sceptical on the subject. In 

 1896 the subject was taken up by the American 

 astronomer, Percival Lowell (born 1855), who, in 

 the clear air of Arizona, confirmed Schiaparelli's 

 conclusions, fixing 88 days as the period of 

 rotation. He remarked, however, that no signs 

 of an atmosphere or clouds were visible to him. 

 The surface of Mercury, he says, is colourless, 

 " a geography in black and white." The deter- 



