488 TEE POPULAR SCIENCE MONTHLY. 



but they would overshoot the mark, and the globe would thus oscillate 

 to and fro. Now, it has been shown that the period of such oscillations 

 in our primitive globe is about an hour and a half, or very close to half 

 the supposed length of the day at that time. The solar tides, however, 

 also have a period half the length of the day. Here, then, we have a 

 case precisely analogous to the fourteen-pound weight I have just ex- 

 perimented on. We have a succession of small impulses given which 

 are timed to harmonize with the natural vibrations. Just as the small- 

 timed impulses raised a large vibration in the weight, so the small 

 solar tides on the earth threw the earth into a large vibration. At first 

 these vibrations were small, but at each succeeding impulse the ampli- 

 tude was augmented until at length the cohesion of the molten matter 

 could no longer resist : a separation took j>lace : one portion consoli- 

 dated to form our present earth ; the other portion consolidated to 

 form the moon. 



There is no doubt whatever that the moon was once quite close to 

 the earth ; but we have to speculate as to what brought the moon into 

 that position. I have given you what I believe to be the most reason- 

 able explanation, and I commend it to your attention. There are diffi- 

 culties about it, no doubt : let me glance at one of them. 



I can easily imagine an objector to say : " If the moon were merely 

 a fragment torn off, how can we conceive that it should have that beau- 

 tiful globular form which we now see ? Ought not the moon to have 

 rugged corners and an irregular shape ? and ought not the earth to show 

 a frightful scar at the spot where so large a portion of its mass was rent 

 off?" 



You must remember that in those early times the earth was not the 

 rigid, solid mass on which we now stand. The earth was then so hot 

 as to be partially soft, if not actually molten. If, then, a fragment 

 were detached from the earth, that fragment would be a soft yielding 

 mass. Not for long would that fragment retain an irregular form ; the 

 mutual attraction of the particles would draw the mass together. By 

 the same gentle ministrations the wound on the earth would soon be 

 healed. In the lapse of time the earth would become as whole as 

 ever, and at last it would not retain even a scar to testify to the mighty 

 catastrophe. 



I am quite sure that, in so large and so cultivated an audience as 

 that which I am now addressing, there are many persons who take a 

 deep interest in the great science of geology. I believe, however, that 

 the geologist who has studied all the text-books in existence might 

 si ill be unacquainted with the very modern researches which I am at- 

 tempting to set forth. Yet it seems to me that the geologists must 

 quickly take heed of these researches. They have the most startling 

 and important bearing on the prevailing creeds in geology. One of 

 the principal creeds they absolutely demolish. 



I suppose the most-read book that has ever been written on geology 



