420 



• KNOWLEDGE • 



[Maiicu 17, Ifct:;. 



Add to this tlic coiisidnrntion that aftor Oxford hod won 



njiio successive races, from ISOl to M*C>0 iiiclusivo, one of 

 tl>o best oarsmen Oxford has over produced, Mr. CSnorgo 

 Morrison, ;{ave nnich time nnd care to couchinfj tho Cam- 

 bridge crew into a bett<'r style tlmn tlii'V Iiad before 

 followed, so that there is ;;oiid reason for l)plicvin',' that 

 in some of the races which followed (from 1870 to 1H73, 

 p«?rhaps) tho inlluenct- of Oxford training was at work in 

 the Caniliridfjo crews. 



These points considered suggest a strong probability 

 tliat there has been a radical difference for matiy years 

 between the Cambridge and tlie Oxford style, the latter being 

 the better. As it is well known that for many years since 

 the old-fashioned racing-boats went out of use, the olil- 

 fiushioned principles of rowing have been in vogue at Cain- 

 bridge, we miglit fairly assume, apart from all dynamical 

 evidence, that thi- old-fashioned stroke does not suit racing- 

 lioats of the present fashion. 



Let us see what theory suggests as likely to be the best 

 kind of stroke (for racing purposes) in these light boats, 

 and then let us intpiirc what evidence we have to show 

 that such a stroke really is rowed by the most successful 

 crews. 



(To be coiilinned.) 



FUTURE OF THE ICAUTU AND MOON. 



By Dr. Hall, AsTuoxoMEii-PtOYAL for Ireland. 



IT^VERYONE knows that the moon always turas the 

 \i same face towards the earth ; this has been shown to 

 be a consequence of the tides which were anciently raised 

 in the moon. The tides in the moon were produced by the 

 attraction of the earth, just as the tides on the earth arc 

 produced by the attraction of the moon. There is, how- 

 ever, an important diHerence ; the earth is so much heavier 

 than the moon, that the tides which the earth raised on the 

 moon must have been much greater than the tides which 

 the moon can raise on the earth. It matters not that the 

 moon now contains no li([uid ocean. All that is necessary 

 is that the moon shall once have been soft enough to admit 

 of being distorted by tidal inllucnce. It must be remem- 

 bered that it is not the mere presence of a high tide or a low- 

 tide that does the work. It is the rising and falling of the 

 tide which produces the currents, and it is the tidal currents 

 whicli do the work. The mighty tides which once acted on 

 the moon have long si:ice ceased, but they have forced the 

 moon always to turn the same face to the earth, as this is 

 the only attitude in which tides do no work on the moon. 

 In the distant future the small mass of the moon will 

 .achieve the sitmc result on the earth. At the final stage 

 the earth and moon will move as if they were fixed rigidly 

 together by movable bars, and were revolving around their 

 common centre of gravity in 1,400 hours. 



If the earth and the moon could be isolated from all 

 external interference, there is no reason why this state of 

 things should not continue indefinitely, but there is another 

 disturbing cause with which we must reckon. We have 

 seen tli.it it was probably the sun which originally broke 

 off the moon as a fragment from the earth. It seems, also, 

 that the sun is destined to derange the harmonious compact 

 in which the earth and the moon would have otherwise 

 agreed. Once the 1,400 hour day and the 1,400 hour 

 month have been reached, the earth will no longer be affected 

 by tides ])roduced by the moon. No doubt there will be a 

 liigh tide on the earth and there will lie a low tide, but as 

 the earth will then always regard the mooii with the same 



aspect, thego tides will not rise or fall, they will noteViband 

 flow. There can then be no lunar tidal currents, and the 

 tide.t will always remain at the same height at each point 

 on our coasts. The sun, however, will still continue to 

 produce tides on the earth. These tiiles will no doubt V 

 small, us the solar tides are small at present ; they will also 

 ris(! and fall with extreme slowness. At present one high 

 tide follows another in a little over six hours. At the 

 final stage one solar high tide will follow another solar high 

 tide only after an interval of about five weeks. These tides 

 are small, and the currents they produce are very wt^ak, 

 but by incessant perseverance even these small tides cannot 

 fail of producing an appreciable effect. The solar tidal 

 currents act always in one direction, they always ten<l to 

 retard the earth and to make the earth revolve more slowly. 

 Here, then, we are conducted to a very remarkable con- 

 dition of things in the distant future. ITie month will 

 remain at 1,400 hour.s, while the day is lengthened still 

 more. We thus have for the first time in the history of 

 our earth-moon system the da\' actually longer than the 

 month. A few years ago we knew of no analogy in the 

 solar system to the state of things hei-e foreshadowed. But 

 the splendid discovery of the satellites of Mars has enabled 

 us to give an illustration. The interior satellite of Mars 

 moves round the planet in aViout seven hours, while the 

 planet itself takes more than three times as long. At the 

 time of its discovery this seemed a most anomalous circum- 

 stance, but now the ditiiculty has been to a great extent 

 removed. It seems likely that Mars himself once rotated 

 more rapidly than at present, and that by the intervention 

 of the solar tides the present state of things has been 

 brought about. It must not be overlooked that Mars and 

 his satellites are much smaller than the earth and the moon. 

 We might therefore expect to find that the process of 

 evolution has proceeded much further in the case of Mars 

 than in the case of the earth. Once the solar tides have 

 acted on the earth sufficiently to abate its velocity below 

 that of the moon, a new reaction on the moon will be 

 manifested. This new influence is not a little curious. As 

 the earth ceases to turn the same face to the moon, the 

 lunar action will again commence to develope tides on the 

 earth. The tidal currents produced in this way w-ill tend 

 to drag the earth on faster, instead of to retard it as Viefore, 

 but the moon can exert no action on the earth without a 

 corresponding reaction. In this case the reaction will take 

 the form of a force tending to draw the moon in again 

 towards the earth. The matter is, however, too coniplicat«l 

 for us to pursue it any further with advantage. 



It is, indeed, remarkable that so striking a period in the 

 earth-moon history can be traced out merely as limiting 

 the influence of the tide. It would seem, from one 

 aspect of the question, that at the present moment we are 

 near the centre of the period, inasmuch as the ratio of the 

 month to the day has but recently passed its maximum. 

 From another point of view, however, w-e seem to be vastly 

 nearer to the first stage, ancient as that is, than we are to 

 the last. The day has lengthened from .'? hours to 24 

 hours, but the lengthening has to go on until the day lasts 

 1,400 hours, and the rate at which the change proceeds is 

 now extremely slow, and is getting still slower. We can- 

 not estimate the countless myriads of years that must 

 elapse before the moon has attained its greatest distance 

 and the day has become 1,400 hours in length. 



We ha\e in this paper merely touched on one depai"t- 

 ment of the great problem of tidal evolution. The subject 

 is yet in its infancy, Imt it seems to have before it a most 

 noble future. By a series of most splendid discoveries, 

 Lagrange had shown that the solar system contained the 

 elements of stability and of permanence. Lagrange showe<l 



