THE MOON'S HISTORY. 75 



those who lived on this side of our satellite would always 

 be able to see the earth apparently fixed in the same part 

 of the sky. An observatory located at the middle of the 

 moon's disc, say near the crater Ptolemy, would always 

 have the earth in its zenith or very near thereto, while the 

 astronomer, let us say, in the Mare Crisium, would always 

 find the earth low down near his horizon. 



In order to facilitate our reasoning I have assumed that 

 the moon is the only tide-producing agent ; this is, how- 

 ever, not the case. No doubt the ebb and the flow 

 around our coasts is generated mainly by the attraction 

 of the moon. It must not, however, be forgotten that a 

 portion of the tide is originated by the attraction of the 

 sun. These solar tides will still continue to ebb and flow 

 quite independently of the lunar tides, so that even if 

 the accommodation between the earth and the moon had 

 been completed some further tidal disturbance would not 

 be wanting. The effect of the solar tides will be to abate 

 still further the velocity with which the earth turns 

 round on its axis, and consequently a time must ultimately 

 arrive when the length of the day will be longer than 

 the time which the moon takes to revolve around our 

 earth. 



It is here interesting to note that an adaptation of 

 a somewhat similar kind has already been detected in 

 another part of the solar system. Our neighbouring 

 planet Mars is attended in its revolution around the sun by 

 a pair of small satellites. The inner of these little moons 

 presents features unlike those to be seen in any other part 

 of our system at present, but resembling in a marked 

 degree the future which we may venture to prognosticate 



