A GLIMPSE THROUGH THE CORRIDORS OF TIME. 485 



Here, then, we have a range of some 1,500 years for the date of the 

 tombs, and no dates between these two are possible. I am sure I do 

 not pretend to decide between them, or even to have an opinion on 

 the subject ; but I can not help saying that in one respect the astron- 

 omers are better off than the historians. The historians can not even 

 agree whether Schliemann's gold ornaments are b. c. or a. d. Astron- 

 omers are, at all events, certain that the date of the moon's birth was 

 before the present era. 



At the critical epoch to which our retrospect extends, the length 

 of the day was only a very few hours. I can not tell you exactly how 

 many hours. It seems, however, to have been more than two and less 

 than four. If we call it three hours we shall not be far from the 

 truth. Perhaps you may think that, if we looked back to a still earlier 

 epoch, the day would become still less and finally disappear altogether ! 

 This is, however, not the case. The day can never have been much 

 less than three hours in the present order of things. Everybody 

 knows that the earth is not a sphere, but that there is a protuberance 

 at the equator, so that, as our school-books tell us, the earth is shaped 

 like an orange. It is well known that this protuberance is due to the 

 rotation of the earth on its axis, by which the equatorial parts bulge 

 out by centrifugal force. The quicker the earth rotates the greater is 

 the protuberance. If, however, the rate of rotation exceeds a certain 

 limit, the equatorial portions of the earth could no longer cling to- 

 gethei-. The attraction which unites them would be overcome by 

 centrifugal force, and a general break-up would occur. It can be 

 shown that the rotation of the earth when on the point of rupture 

 corresponds to a length of the day somewhere about the critical value 

 of three hours, which we have already adopted. It is therefore im- 

 possible for us to suppose a day much shorter than three hours. What 

 occurred prior to this I do not here discuss. 



Let us leave the earth for a few minutes, and examine the past 

 history of the moon. We have seen that the moon revolves around 

 the earth in an ever-widening orbit, and consequently the moon must 

 in ancient times have been nearer the earth than it is now. No doubt 

 the change is slow. There is not much difference between the orbit 

 of the moon a thousand years ago and the orbit in which the moon is 

 now moving. 



But when we rise to millions of years the difference becomes very 

 appreciable. Thirty or forty millions of years ago the moon was 

 much closer to the earth than it is at present ; very possibly the moon 

 was then only half its present distance. We must, however, look still 

 earlier, to a certain epoch not less than fifty millions of years ago. 

 At that epoch the moon must have been so close to the earth that the 

 two bodies were almost touching. I dare say this striking result will 

 come upon many with surprise when they hear it for the first time. 

 It was, I know, with great surprise that I myself read of it not many 



