A GLIMPSE THROUGH THE CORRIDORS OF TIME. 483 



therefore follows that the energy of the earth's rotation must he de- 

 creasing. This leads to a consequence of the most wonderful impor- 

 tance. It tells us that the speed with which the earth rotates on its 

 axis is diminishing. We can state the result in a maimer which has 

 the merits of simplicity and brevity: 



" The tides are increasing the length of the day." 



This statement is the text of the discourse which I am to give you 

 this evening. From this simple fact the new and wondrous theory of 

 tidal evolution is deduced. A great scientific theory is generally the 

 outcome of many minds. To a certain extent this is true of the the- 

 ory of tidal evolution. It was Professor Helmholtz who first appealed 

 to what tides had already done on the moon. It was Professor Purser 

 who took an important step in the analytical theory. It was Sir Will- 

 iam Thomson's mathematical genius which laid the hroad and deep 

 foundations of the fabric. These are the pioneers in this splendid 

 research. But they were only the pioneers. The great theory itself 

 is chiefly the work of one man. You are all familiar with the name 

 he bears. The discoverer of tidal evolution is Mr. G. II. Darwin, Fel- 

 low of Trinity College, Cambridge. 



It would be impracticable for me now to go into the actual mathe- 

 matical calculations. I shall rather endeavor to give you an outline 

 of this theory, shorn of its technical symbols. I think this can be 

 done, even though we attempt to retain the accuracy of mathematical 

 language. Nor would it be fair to throw on Mr. Darwin or the other 

 mathematicians I have named the responsibility for all I am going to 

 say. I must be myself responsible for the way in which those theories 

 are set forth, as well as for some of the deductions made from them. 



At present, no doubt, the effect of the tides in changing the length 

 of the day is very small. A day now is not appreciably longer than 

 a day a hundred years ago. Even in a thousand years the change in 

 the length of the day is only a fraction of a second. But the impor- 

 tance arises from the fact that the change, slow though it is, lies 

 always in one direction. The day is continually increasing. In mill- 

 ions of yeai's the accumulated effect becomes not only appreciable 

 but even of startling magnitude. 



The change in the length of the day must involve a corresponding 

 change in the motion of the moon. This is by no means obvious. It 

 depends upon an elaborate mathematical theorem. I can not attempt 

 to prove this for you, but I think I can state the result so that it can 

 be understood without the proof. If the moon acts on the earth and 

 retards the rotation of the earth, so, conversely, does the earth react 

 upon the moon. The earth is tormented by the moon, so it strives to 

 drive away its persecutor. At present the moon revolves round the 

 earth at a distance of about 240,000 miles. The reaction of the earth 

 tends to increase that distance, and to force the moon to revolve in an 

 orbit which is continually getting larger and larger. 



