148 IN STARRY REALMS. 



ever. The sun is the only tide-raising globe that can 

 affect Mercury. We, of course, also experience sun- 

 raised tides on this earth, but to us they seem of com- 

 paratively little importance, because they are much less 

 than the tides raised by the moon. There is, however, 

 a double reason for regarding the sun -raised tides on 

 Mercury as of special importance to that globe. In the 

 first place, as there are no perceptible satellite-raised tides, 

 there the solar tides are the determining agents in tidal 

 phenomena, and, secondly, as Mercury is closer to the sun 

 than we are, the sun-raised tides will be much larger and 

 stronger than those which we experience. It seems, there- 

 fore, certain that the constant face which Mercury directs 

 towards the sun is a consequence of tidal action. In fact, 

 it might almost have been anticipated from tidal pheno- 

 mena alone that Mercury would have been found to move 

 in this manner. The solar tidal control on Mercury must 

 be much more vigorous than that on Venus, but any 

 attempt on either planet to escape from the thraldom that 

 requires it to keep the same face constantly to the sun 

 would be checked with exemplary energy. 



It will thus be seen that the results of the delicate 

 observations of an Italian astronomer seem to illustrate in 

 a striking manner one of the most interesting of modern 

 astronomical doctrine*. 



