SCIENCE. 



409 



The Great Primordial force owes its genesis to the in- 

 itial impulse which set all spheres in motion in vacuous 

 space. To this universal principle, not only all physical 

 force, but new life itself is due. 



Humboldt says: "It is indeed a brilliant effort, 

 worthy of the human mind, to comprise in one organic 

 whole the entire science of nature, from the law of grav- 

 ity to the formative impulse in animated bodies." 



That the earth, and the sun, and all the heavenly 

 bodies, are possessed of the mysterious magnetic energy, 

 and consequently exert a powerful magnetic influence 

 over each other, has long ago been conceived by such 

 men as Herschel, Humboldt, Faraday ; and is the 

 faith of scientists to-day. But when we have arrived at 

 such a conclusion, it is impossible for us to stop short, 

 and not make the necessary deductions therefrom. 

 Mighty magnets, when involved in mighty motions, must 

 produce mighty currents and mighty effects. It is not 

 for nothing that these powers and conditions exist. If 

 we admit the premises, we must not ignore the conclu- 

 sions that are necessitated. Provision must be made for 

 the outcome of every admitted fact in science. 



Therefore, it is with assurance that we urge the elec- 

 tric theory, and maintain that the burden of proof rests 

 with those, who, admitting the elements of motion and 

 magnetism, have yet made no provision whatever for 

 their keeping. 



Besides, there are two other principles already alluded 

 to — the conservation of force, and the unity of all the 

 forces — with respect to which it may be demanded, to 

 what other result do they lead, and can they lead, in all 

 reason and logic, than to the admission of the grand fact 

 of a Great Primordial Force. 



THE TIDAL EVOLUTION OF THE MOON. 



On Saturday, June 4th, in the Museum Buildings, 

 Trinity College, Dr. Ball, Astronomer Royal for Ireland, 

 delivered an interesting and instructive lecture on recent 

 discoveries in astronomical science. Dr. Ball said that 

 from the variety of topics which might fairly be dealt with 

 in his lecture he would select three, and in making this 

 selection he had been mainly guided by the relative im- 

 portance of different astronomical problems. He had 

 also endeavored to exercise his choice so that his lecture 

 should, as far as possible, refer to the various branches of 

 astronomy. Having dealt with two branches of his sub- 

 ject, Dr. Ball described " Darwin's Theory of the Tidal 

 E\ olution of the Moon." It had, he said, been the triumph 

 of modern gravitational astronomers to indicate the 

 changes which must be going forward in a system devoid 

 of rigidity. It was at all events easy to show that the 

 tendency of these changes lay in one direction, and this 

 was the most important point for consideration. Every- 

 one was aware of the daily movements of the sea, which 

 were called the tides. Most people were aware that the 

 movements of the waters were caused by the attraction 

 of the sun and the moon. Let them ponder therefore on 

 the tides, as they seemed to give a clue to some of the 

 profoundest of nature's secrets. He had heard that the 

 port of Dublin was gradually being improved by the 

 deepening of the bar. He had heard that the deepening 

 of the bar had been attributed to the judicious action of 

 the Port and Docks Board. But what the board had 

 chiefly done was to call into requisition the scouring 

 power of the tide, which, as he was informed, was gradu- 

 ally reducing or bearing away the bar. The tide was 

 therfore accomplishing, at the bar of Dublin, the same 

 kind of work as could be accomplished by men or by 

 steam-engines. In other words, the tide was here doing 

 a useful work that could otherwise only be done by the 

 expenditure of energy. It was the same elsewhere. The 

 tides were doing work useful or the reverse, and expend- 

 ing energy in so doing. Where did the energy come 



from? It could not be created. It could only come 

 from the store of energy available for such purposes in 

 the solar system. The reserve energy whence the tides 

 drew the supplies they were daily consuming consisted 

 partly in the daily rotation of the earth on its axis. The 

 earth was like a mighty flywheel which would absorb a 

 prodigious amount of energy in setting it in motion, and 

 which would give out that energy before it would be 

 brought to rest. The rotation of the earth on its axis was 

 a vast but not inexhaustible storehouse of energy, on 

 which the tides could draw lor thousands of years. En- 

 ergy also existed in the solar system in many other forms, 

 some of which could also be rendered available for the 

 tides. So far as was known, the total amount of energy 

 could not be increased. The important question was — 

 Can that total ever be diminished ? The tides were di- 

 minishing it every day. The small oceanic tides were not 

 the sole source ot the expenditure. The solid body of the 

 earth itself must be subject to tides; still more must the 

 fluid or gaseous members of our system be subjected to 

 tides. All tides involved friction, and all friction involved 

 "loss of energy. Here, then, was the great discrepancy 

 between the theory of Lagrange and the actual condition 

 of our system. Lagrange's calculations assumed that the 

 total energy of the solar system was constant, but the 

 actual fact was that the energy was slowly diminishing. 

 The tracing of tidal evolution was chiefly due to the 

 labors of Mr. G. H. Darwin, son of the celebrated natural- 

 ist. The influence of the tides had already been 

 recognized as the cause of the same face of the 

 moon being always bent on the earth. Whether the 

 tides were merely oceanic, or whether they were 

 actual bodily tides, the results remained much 

 the same. At the present time the moon revolved around 

 the earth in a month : the earth revolved on its axis in a 

 day. The tides produced in the earth by the moon must 

 act to reduce the rate of the earth's rotation. The effect 

 of the tides on the earth was to lengthen the day. The 

 day was gradually lengthening, but this change could not 

 take place without a reactionary change on the moon. 

 The change undergone by the moon was perhaps a little 

 difficult to understand, as it depended on some by no 

 means simple dynamical principles. The friction of the 

 tides consumed the energy of the system. It turned a 

 large portion of that energy into heat, which was then 

 radiated off into space to be forever lost. But the fric- 

 tion of the tides could not alter the moment of moment- 

 um of the system. As the earth became grad-ially 

 slower and slower in its rotation its moment of moment- 

 um decreased, yet for this to happen the moment of mo- 

 mentum of the moon should increase. It followed math- 

 ematically that as the tides gradually made the earth 

 rotate more and more slowly, the moon must be getting 

 farther and farther away from us. At the end of a 

 million years from the present time the day will be more 

 than one day ot twenty-four is now ; and in one million 

 years hence the moon will move round the earth at a 

 greater distance than she does now, and the length of 

 the month will be correspondingly increased. In the far 

 distant fu'.ure therefore, we are to look for an increased 

 length of month. The length of day will, however, in- 

 crease much faster than the length of the month, until at 

 length the duration of the day equals that of the month. 

 When this time arrives the moon w.ll have moved out to 

 a distance half as great again as it is at present, and the 

 length of the month will have increased to two months. 

 Our day will then have increased from twenty-four hours 

 up to nearly two months, and as the moon continues to 

 to show the same face to us, we are destined to turn the 

 same face on the moon. Were the earth and the moon 

 the only bodies in the universe, such a state of things 

 might go on forever. The sun, however, will produce 

 tides in the earth which will again modify their move- 

 ments. He had said that the moon was gradually reced- 

 ing farther and farther from the earth, and that the 



