146 M STARRY REALMS. 



Schiaparelli has arrived are so striking and full of inte- 

 rest that they constitute one of the most important addi- 

 tions to our knowledge of the solar system that has been 

 brought to light for many a long day. It ought, however, 

 to be added that some competent observers are unable to 

 convince themselves of the accuracy of Schiaparelli' s in- 

 vestigation. 



Now, however, Schiaparelli has shown that the constant 

 face which the moon turns to the earth seems to .have a 

 parallel in the revolution of Mercury around the sun. It 

 is not that Mercury always directs the same face to the 

 earth. That would be utterly inconceivable on any 

 rational hypothesis, and would not be at all analogous 

 to the behaviour of the moon. It is to be remembered 

 that Mercury revolves around the sun. This planet, there- 

 fore, stands in the same relation to the sun that the moon 

 stands to the earth. The singular fact to which I desire 

 to call attention is that Schiaparelli's researches show 

 that Mercury constantly keeps the same face directed to- 

 wards the sun. The period of rotation of Mercury on its 

 axis is, accordingly, of the same length as the period of its 

 revolution around the sun. We never see the other side of 

 the moon, because the moon always moves so as to keep the 

 same face towards us. In a similar manner, only one 

 side of Mercury is ever visible from the sun. The other 

 side is sedulously averted from that luminary. 



The side of Mercury which is turned away from the sun 

 is, therefore, eternally in dark night, while the favoured 

 side is perennially suffused with the glory of an uninter- 

 mittent day, much brighter and hotter than any day we 

 know, inasmuch as Mercury is a planet so much nearer te 



