September 1, 1905.] 



SCIENCE. 



263 



may also have been Avidely different. The 

 theory of which I shall have next to speak 

 claims to trace the gradual departure of 

 the moon from an original position not far 

 removed from the present surface of the 

 earth. If this view is correct we may sup- 

 pose that the detachment of the moon from 

 the earth occurred as a single portion of 

 matter, and not as a concentration of a 

 Laplacian ring. 



If a planet is covered with oceans of 

 water and air, or if it is formed of plastic 

 molten rock, tidal oscillations must be gen- 

 erated in its mobile parts by the attractions 

 of its satellites and of the sun. Such 

 movements must be subject to frictional 

 resistance, and the planet's rotation will 

 be slowly retarded by tidal friction in 

 much the same way that a fly-wheel is 

 gradually stopped by any external cause 

 of friction. Since action and reaction are 

 equal and opposite, the action of the satel- 

 lites on the planet, which causes the tidal 

 friction of which I speak, must correspond 

 to a reaction of the planet on the motion 

 of the satellites. 



At any moment of time we may regard 

 the system composed of the rotating planet 

 with its attendant satellite as a stable 

 species of motion, but the friction of the 

 tides introduces forces which produce a 

 continuous, although slow, transformation 

 in the configuration. It is, then, clearly of 

 interest to trace backwards in time the 

 changes produced by such a continuously 

 acting cause, and to determine the initial 

 condition from w^hich the system of planet 

 and satellite must have been slowly de- 

 grading. We might also look forward, and 

 discover whither--the transformation tends. 



Let us consider, then, the motion of the 

 earth and moon revolving in company 

 round the sun, on the supposition that the 

 friction of the tides in the earth is the 

 only effective cause of change. We are, 

 in fact, to discuss a working model of the 



system, analogous to those of which I have 

 so often spoken before ; and it must suffice 

 to set forth the result in its main outline 

 as referring only to the past. 



If we take the 'day,' regarding it as a 

 period of variable length, to mean the time 

 occupied by a single rotation of the earth 

 on its axis; and the 'month,' likewise vari- 

 able in absolute length, to mean the time 

 occupied by the moon in a single revolution 

 round the earth, the number of days in the 

 month expresses the speed of the earth's 

 rotation relatively to the speed of the 

 moon's revolution. 



Now in our retrospect both day and 

 month are found continuously shortening; 

 but as on the whole the month shortens 

 much more quickly than the day, the num- 

 ber of days in the month also falls. We 

 may, then, ask at once. What is the initial 

 stage to which the gradual transformation 

 points ? I say, that on following the argu- 

 ment to its end the system may be traced 

 back to a time when the day and month 

 were identical in length, and were both 

 only about four or five of our present 

 hours. The identity of day and month 

 means that the moon was always opposite 

 to the same side of the earth; thus at the 

 beginning the earth always presented the 

 same face to the moon, just as the moon 

 now always shows the same face to us. 

 Moreover, when the month was only some 

 four or five of our present hours in length 

 the moon must have been only a few thou- 

 sand miles from the earth's surface— a 

 great contrast with the present distance of 

 240,000 miles. 



It might well be argued from this con- 

 clusion alone that the moon separated from 

 the earth more or less as a single portion of 

 matter at a time immediately antecedent 

 to the initial stage to which she has been 

 traced. But there exists a yet more 

 weighty argument favorable to this view, 

 for it appears that the initial stage is one 



