49 o THE POPULAR SCIENCE MONTHLY. 



oped ; back again to the much earlier period when colossal reptiles 

 and birds were the chief inhabitants of the earth ; back again to those 

 still earlier ages when the luxuriant forests nourished that have siren 

 birth to the coal-fields ; back once more to the age of fishes ; back 

 finally to those earliest periods when the lowest forms of life began to 

 dawn in the Palaeozoic era. 



As we date remote ages astronomically by the distance of the moon, 

 so we date remote ages geologically by the prevailing organic life. It 

 is a great desideratum to harmonize these two chronological systems, 

 and to find out, if possible, what lunar distance corresponds to each 

 geological epoch. In the whole field of natural science there is no 

 more noble problem. Take, for examine, that earliest and most inter- 

 esting epoch when life perhaps commenced on the earth, and when 

 stratified rocks were deposited five or ten miles thick, which seem to 

 have contained no living forms higher than the humble Eozoun, if 

 even that were an organized being. Let us ask what the distance of 

 the moon was at the time when those stupendous beds of sediment 

 were deposited in the primeval ocean. We have in this comparison 

 every element of uncertainty except one. The exception is, however, 

 all-important. We know that the moon must have been nearer to the 

 earth than it is at present. There are many very weighty reasons for 

 supposing that the moon must have been very much nearer than it is 

 now. It is not at all unlikely that the moon may then have been situ- 

 ated at only a small fraction of its present distance. My argument is 

 only modified, but not destroyed, whatever fraction we may take. 

 We must take some estimate for the pu-rpose of illustration. I have 

 had considerable doubts what estimate to adopt. I am desirous of 

 making my argument strong enough, but I do not want to make it 

 seem exaggerated. At present the moon is 240,000 miles away ; but 

 there was a time when the moon was only one sixth part of this, or, 

 say, 40,000 miles away. That time must have corresponded to some 

 geological epoch. It may have been earlier than the time when the 

 Eozoon lived. It is more likely to have been later. I want to point 

 out that, when the moon was only 40,000 miles away, we had in it a 

 geological engine of transcendent power. 



On the primitive oceans the moon raised tides as it does at present ; 

 but the 40,000-mile moon was a far more efficient tide-producer than 

 our 240,000-mile moon. The nearer the moon the greater the tide. 

 To express the relation accurately we say that the efficiency of the 

 moon in producing tides varies inversely as the cube of its distance. 

 E< re, then, we have the means of calculating the tidal efficiency for 

 any moon-distance. The 40,000-mile moon being at a distance of only 

 one sixth of our present moon's distance, its tidal efficiency would be 

 increased 6 X X G fold. In other words, when our moon was only 

 40,000 miles away, it was 21G times as good a tide-producer as it is at 

 present. 



