492 THE POPULAR SCIENCE MONTHLY. 



up their phenomena." The mathematicians had other reasons for be- 

 lieving that the earth could not have been so old as the geologists 

 demanded. Now, however, the mathematicians have discovered the 

 new and stupendous tidal grinding-engine. With this powerful aid 

 the geologists can get through their work in a reasonable period of 

 time, and the geologists and the mathematicians may be reconciled. 



I have here a large globe to represent the earth, and a small globe 

 suspended by a string to represent the moon. At the commencement 

 of the history the two globes were quite close ; they were revolving 

 rapidly, and the moon was constantly over the same locality on the 

 primeval earth. I do not know where that locality was ; it was prob- 

 ably the part of the earth from which the moon had been detached. 

 No doubt it was somewhere near the equator, but the distinction of 

 land and water had not then arisen. Around the primeval earth the 

 moon revolved in three hours ; the earth also revolved in three hours, 

 so that the moon constantly remained over the red region. This I can 

 illustrate by holding the small globe which represents the moon in one 

 hand, and making the large globe which represents the earth revolve 

 by the other. 



This state of things formed what is known as unstable dynamical 

 equilibrium. It could not last. Either the moon must fall back again 

 on the earth, and be reabsorbed into its mass, or the moon must com- 

 mence to move away from the earth. Which of these two courses 

 was the moon to take ? The case is analogous to that of a needle 

 balanced on its point. The needle must fall some way, but what is to 

 decide whether it shall fall to the right or to the left ? I do not know 

 what decided the moon, but what the decision was is perfectly plain. 

 The fact that the moon exists shows that it did not return to the 

 earth, but that the moon adopted the other course, and commenced its 

 outward journey. 



As the moon recedes, the period which it requires for a journey 

 round the earth increases also. Initially that period was but three 

 hours, and it has increased up until our present month of six hundred 

 and fifty-six hours. 



The rotation of the earth has been modified by the retreat of the 

 moon. Directly the moon began to retreat, the earth was no longer 

 under an obligation to keep the same face thereto. When the moon 

 was at a certain distance, the earth made two rotations for every revo- 

 lution that the moon made. Thus, as I carry the small globe round 

 the large globe, the latter makes two revolutions for one revolution of 

 the small globe. Still the moon gets farther and farther away, until 

 the earth performs three, four, or more rotations for each of the moon's 

 revolutions. Do not infer that the rate of the earth's rotation is in- 

 creasing ; the contrary is the fact. The earth's rotation is getting 

 slower, and so is that of the moon ; but the retardation of the moon 

 is much greater than that of the earth. Even though the rotation of 



