486 TEE POPULAR SCIENCE MONTHLY. 



months ago. But the evidence is unimpeachable, and it is, indeed, 

 wonderful to see how such information has been gained by merely 

 looking at the ripples of the tide. 



Everybody knows that the moon revolves now around the earth in 

 a period of twenty-seven days. The period depends upon the distance 

 between the earth and the moon. The time and the distance are con- 

 nected together by one of Kepler's celebrated laws, so that, as the 

 distance shortens, so must the time of revolution shorten. In earlier 

 times the month must have been shorter than our present month. 

 Some millions of years ago the moon completed its journey in a week 

 instead of taking twenty-eight days as at present. Looking back 

 earlier still, we find the month has dwindled down to a day, then down 

 to a few hours, until, at that wondrous epoch when the moon was al- 

 most touching the earth, the moon spun round the earth once every 

 three hours. 



It would require the combined powers of a poet and a mathemati- 

 cian to portray the scene with becoming dignity. I have only prom- 

 ised to give you that glimpse along the Corridors of Time which I 

 have myself been able to obtain. The scene is laid in the abyss of 

 space ; the time is more than 50,000,000 years ago ; the dramatis per- 

 sonal are the earth and the moon. 



In those ancient times I see our earth to be a noble globe, as it is 

 at present. Yet it is not partly covered with oceans and partly clothed 

 with verdure. The primeval earth seems rather a fiery and half- 

 molten mass, where no organic life can dwell. Instead of the atmos- 

 phere which we now have I see a dense mass of vapors, in which per- 

 haps all the oceans of the earth are suspended as clouds. I see that 

 the sun still rises and sets to give the succession of day and of night, 

 but the day and the night together only amount to three hours instead 

 of twenty-four. Almost touching the chaotic mass of the earth is 

 another much smaller and equally chaotic body. Around the earth I 

 see this small body rapidly rotating. The two revolve together as if 

 they were bound by invisible bands. This smaller body is the moon. 

 Such is the picture which I wish to present to you as a glimpse through 

 the Corridors of Time. 



I have hitherto refrained from introducing any merely speculative 

 matters. If we can believe anything of mathematics, anything of 

 dynamics, we must admit that the picture I have attempted to outline 

 is a faithful portrait. The only uncertain elements are the date and 

 the periodic time. I do, however, now propose to venture on one 

 speculation in which Mr. Darwin has indulged. I propose to offer a 

 suggestion as to how a small body came into this most remarkable 

 position close by the earth, and how its motion was produced. 



We have hitherto been guided by the unerring light of dynamics, 

 but at this momentous epoch dynamics deserts us, and we have only 

 probability to guide our faltering steps. One hint, however, dynam- 



