332 



• KNOWLEDGE 



[Feu. 17, 1882. 



whicli I refer, the day was very mach shorter than oar 

 present day. It was, indeed, only a small fraction of 24 

 hours. We cannot bo sure of the precise numlicr of hours 

 in the day at that time ; it sfN^nis to have l)ee.n more than 

 two hours luid less than five liours. For simplicity, w-e 

 may spexik of the Iciigtli of the ancient day as al>out 

 three hours, but it must Ik- carefully remcnilx-red that this 

 estimate is to be regarded merely as provisional, though 

 perhaps approximately correct 



It had long l>eeji known that the tides were increasing 

 the length of the day, but the very remarkable researches 

 now to be de-scril)ed have only l)een made quite rec-ently. 

 They arc the work of Mr. G. H. Darwin, Fellow of Trinity 

 College, Cambridge Mr. Darwin's labours are contained 

 in a series of memoirs of a very abstruse nature, and we 

 here propose to give a general sketch of the principal 

 results to which he has \>ecn conducted, so far as the earth- 

 moon system is concerned. We shall endeavour a.s clearly 

 as possible to indicate what portions of tJie theory are to 

 be regarded as absolutely established and what portions are 

 still more or le.ss speculative. We have halted in our 

 retrospect at a day of 3 hours. Why is it that we caimot 

 look much earlier — to a day of one hour, for instance 1 There 

 is a very good reason why we cannot do so. In those 

 exceedingly early times, our earth was not the hard rigid 

 body with which we are familiar. It was in those days so 

 heated as to be quite soft, even if not actually molten. A 

 body like our earth in a molten state will not remain in a 

 spherical form when it is rotating on its axis. It will bulge 

 out at the equator ; it will become llattened at the poles. 

 The greater the velocity, the greater will be the protuber- 

 ance at the equator. If, indeed, a certain critical velocity 

 be attained, it will be impossible for the body to hold 

 together ; the centrifugal force would be too great, and a 

 rupture of the body must ensue. It is not practicable to 

 calculate what that critical velocity may be. The critical 

 velocity depends upon circumstances not within our present 

 knowledge, but it can be shown that the \elocity does not 

 differ, perhaps, very much from a rotation once every 3 

 hours. We thus see that a rotation of this amount is 

 about the greatest that our earth could ever have had in 

 the present order of things. What occurred prior to this 

 is not to be discussed at present. 



We now turn our attention to the moon, which, in con- 

 sequence of the tides, must be describing an orbit of 

 gradually-increasing dimensions. It may be well to remind 

 the reader that the orbit of the moon is at any moment a 

 nearly circular ellipse, and that this ellipse is subject to 

 many disturbing influences well known to astronomers. 

 But these disturbances are all periodic. They increase and 

 they decrease. They may, in the course of ages, be 

 entirely overlooked in comparison with the tidal changes, 

 which constantly act in the same direction. In very 

 ancient days, the moon must, therefore, have been nearer 

 to the earth than it is at present The further we look 

 back, the nearer must the moon be. There is no reason 

 why we should not look back to an excessively remote 

 time, when the moon was as near as possible to the earth. 

 'I'he most extreme case would arise when the moon was so 

 near the earth that the two bodies were almost touching, 

 and we are bound to believe that, at some inconceivably 

 remote epoch, this did actually happen. It is easy to cal- 

 culate what must then have been tlu; length of the month, 

 or the time which the moon occupied in completing one 

 revolution around the earth. Kepler's law shows tliat when 

 the moon complet<>d one revolution around the earth in three 

 hours, the two bodies must have been veiy close together. 

 There was thus a certain very critical epoch in the eartli- 

 moon history. At that time the earth and the moon were 



close together ; the earth was spinning round on its axis in 

 three hours, and the moon was revolving around the earth 

 in the same time. The three hours is, as already remarked, 

 open to some uncertainty ; but there can be no doubt that 

 at this critical epoch the earth was turning round in the 

 same time as the moon, whether that time l)e three hours 

 or some otlier amount of alxjut the same magnitude. At 

 tliis interesting epoch the earth kept the same face directed 

 towards the moon, and the moon kept the .same face 

 towards the earth. In fact, the two IxKlies revolved just 

 OS if they were bound to each other by invisible bands. 



MICROSCOPIC VISION AND MINUTE 

 LIFE. 



By Henry J. Slack, F.G.S., F.R.M.S. 



IN the early days of the microscope, wonderful reports 

 were made of its revelations, and in 174") Baker com- 

 plained that "some people made false pretences and ridi- 

 culous boasts of seeing by their gla.sses the atoms of 

 Epicurus, the subtle matter of De« Cartes, the effluvium 

 of bodies, the emanations of stars, and such-like impossi- 

 bilities." 



One doctor declared that he had seen the eflluvium of 

 magnets as a mist Probably he mistook a misty view 

 for a view of a mist. As the instrument improved, and 

 more knowledge was gained, the sham wonders ceased to 

 appear, and although little was known of the molecular 

 construction of matter, it was no longer imagined that its 

 minutest or ultimate particles would be seen with the 

 powers employed. We are not yet in a position to say 

 e.xactly where the limits of \nsion must necessarily end. 

 Dr. Pigott reduces the image of a spider's web to 

 less than one-millionth of an in.:h in diameter, and 

 then shows it by remagnificatiou. His process is 

 like diminishing an object by looking at it through an 

 inverted telescope, and magnifying the small image thus 

 obtained. After proving that " spider lines, miniatured 

 down to the fourteenth part of the hundred-thousandth of 

 an inch, could be made visible to ordinarily good eyesight, 

 under proper* microscopical manipulation," he sought for 

 actual objects comparable in minuteness with these optical 

 images, and succeeded in showing them. One plan he 

 adopted was to smash, with a watch-spring, very small 

 globules of mercury in a minute drop of petroleum, inserted 

 under a thin cover on a slide. Many of these mercurial 

 particles thus obtained were exceedingly minute, some 

 round, and others iri'egular. Upon some of the irregu- 

 lars he found minute Ijlack points, visible with a power of 

 1,000 diameter, and comparing them with the thinnest 

 spider line, he found one, in particidar, less than one 

 millionth of an inch in diameter. 



The visibility of extremely minute objects depends much 

 upon tlieir optical properties, and how they ai-e situated 

 in reference to neighbouring bodies. In his remarkable 

 investigation into the life-history of certain small objects, 

 called monads, Mr. Dallinger employed a magnitication of 

 live thousand linear, and could not with this great power, 

 see the minutest germs capaV>le of development into active 

 organisms. In tlie course of the spontaneous germination 

 controversy, the extreme minuteness of these germs was 

 not dreamt of by advocates of that theory, and Pouchet 

 thought the " pansperniists," as he called those who adhered 

 to the doctrine of omiie invrim ex ovo in the sense of regard- | 



• " Proc. Roy. Soc.,'" p. 208. 1S80. 



