THE MOON'S HISTORY. 6? 



pression of what tides are able to accomplish if we 

 merely contemplated this change in the length of the day, 

 striking and significant though it doubtless is. The 

 student of natural philosophy is well aware that there is 

 no action without a corresponding reaction, and it is 

 instructive to examine in this case the form which the re- 

 action assumes. Our reasoning has been founded on the 

 supposition that it is the attraction of the moon on the 

 waters of our globe that gives rise to the tides. It is, 

 therefore, the influence of the moon which checks the 

 speed of the earth's rotation and adds to the length of the 

 day. 



As the moon acts in this fashion on the earth, so, by 

 the general law that I have mentioned, the earth reacts 

 upon the moon. The form which this reaction assumes 

 expresses itself in a tendency to allow the moon gradually 

 to move farther and farther away from the earth than 

 the earth's attraction would permit if our globe were 

 a solid mass void of all liquid capable of being dis- 

 tracted by tides. It is, therefore, certain that the distance 

 of the moon, which is at present about two hundred and 

 forty thousand miles, must be gradually increasing ; but 

 we need not look for any appreciable change in the moon's 

 distance arising from this cause when only an interval of 

 a few centuries is considered. We need not expect to 

 measure the difference due to tides between the size of the 

 moon's orbit this month and the size of the orbit last 

 month. In fact, there are so many periodic causes of 

 change in the dimensions of the moon's orbit, that it 

 becomes impossible to detect the tidal influence even in the 

 course of centuries. Here, again, we have to remember 



