VENUS AND MERCURY. 147 



the sun than we are. Schiaparelli's attention lias also been 

 given to Venus. He has certainly shown that the period 

 of rotation of this planet is very much longer than has 

 been hitherto supposed. However, the difficulties of the 

 observations are so great that the results are not so defi- 

 nite as in the case of Mercury. It seems, however, hard 

 to resist the conclusion that Venus, like Mercury, revolves 

 so as always to show the same face to the sun. 



But to my mind the significance of Schiaparelli's 

 investigation does not lie so much in the mere perception 

 of certain features in the planetary movements. It is the 

 interpretation of these phenomena which offers the chief 

 points of interest. Indeed, there has never been any 

 time at which these discoveries would have been more 

 acceptable in the scientific world than at the present 

 moment. They furnish us with a vivid and unexpected 

 illustration of the doctrine of tidal evolution, to which we 

 have elsewhere (p. 60) given attention. 



We revert to the case of the moon for a suggestion as 

 to the cause of the remarkable peculiarities shown in the 

 movement of Mercury, and presumably true in the case 

 of Venus. We have explained that the constant face of 

 the moon is a monument of tidal action. It would be 

 an infinitely improbable coincidence if there were no 

 physical cause to account for it, and the tides afford, a 

 perfectly satisfactory explanation. 



In a precisely similar way we can account for the fact 

 that Mercury always bends the same face to the sun. 

 The matter is fortunately not a little simplified by the 

 fact that Mercury is certainly devoid of any cosider- 

 able satellite, and very likely has no satellite what- 



