Feb 17, 1882.] 



KNOWLEDGE ♦ 



331 



MAGi^ZlNE OF SCIENCE 



PlAINLY ^yfORDED -EXACTLY DESCRIBEg^ I 



LOXDOX: FRIDAY, FEBRUARY 17, 1882. 



Contexts of No. 16. 



PAOB. 



Birth ot Iho Moon l>v Tidal Evolu- 

 tion. Bv Pr. Ball, Aitronomor- 

 Rovnl for Ireland. Part 1 331 



Microscopic Vision and Minute Life. 

 Bt H. J. Slack, F.G.S., F.R.M.S. 332 



The Last Tratisit of Venus. By the 

 E.liior 333 



TheCrrstal Palace Electrical Eilii- 

 bition. Second Xoticc. (Illiul.) ... 335 



Popular Astronomy bv the Chief 

 of a Great National Ohssrratory . 336 



EsviEWS; Sir Edmund Be;'kett on 

 the Laws of Nature— The Food we 

 Eat— Land.auer's Blowpipe .\na- 

 iTsis— Science for All 339 



Night Slioima of Algol. 18Si 339 



FAGS. 



The Jfenaeing: Comet. Bv the 



Editor. {lUa'Iraleil) '. 3W 



The Effects of Tobacco.— Part II. 



Bv Dr. Muir Howie »J2 



How Spiders Fly 343 



Intelligence of Dogs 3i4 



CoBRESFONDBXCB: Optical Illusion. 

 -Cat's Eve Timepiece— 'Weather 



Forecast, 'ic 3M 



Queries 346 



Replicii to Queries 346 



Answers to Correspondents 847 



Notes on .Art and Science 34.9 



Our Mathematical Column 34S 



Our Whist Column 3J9 



Our Chess Column 350 



BIRTH OF THE MOON 



By Tidal Evolution. 



Bv Dii. Ball, Astronomer-Rotal for Ireland. 



PART I. 



THE daily rise and fall of the sea, which we call the 

 tide, has long been known to be connected with tlie 

 moon. The discovery of the law of gravitation enabled 

 Newton to explain how the tides were caused. Newton 

 showed that the tides were partly due to the attraction of 

 the sun, but chietly to the attraction of the moon. 



In the present paper we shall principally consider the 

 tides which are produced Ijy the moon, and the reader will 

 please understand that this is the tide to which we refer, 

 except otherwise stated. The tides are of the utmost im- 

 portance in our seaports. They are not, perhaps, very often 

 employed to do useful work, in the sense of driving ma- 

 chinery, but on work of one kind or another the tides are 

 unceasingly busy. No one who has watched the rise and 

 fall of the tide on the beach, or the currents of the tide in 

 a river, can doubt that the tides do work. We need not here 

 atten-.pt to enumerate all the varieties of tidal work. Let 

 '■* bo sufficient to mention one kind, as an ilhistration. 

 iho waters of a strong tidal river like the Avon, at Bristol, 

 ii- heavily charged with mud in suspension. The tides are 

 carrying that mud, and in doing so, they accomplish work, 

 at an expenditure of energy which could be expressed by an 

 equivalent amount of horse-power. 



The steam-engine will only yield an appropriate horse- 

 power when the boilers are heated by a proportionate 

 quantity of fuel. So also the tides can only accomplish 

 their gigantic work all over the world because they are 

 bountifully fed with energy. Whence do the tides obtain 

 their energy- ? They draw it from a certain store which is 

 being steadily squandered and never replaced. The supply 

 in the store may be great, but it is not inexhaustible. It 

 is easy to discover the store when we consider the circum- 

 stances of the case. Fixing our attention solely upon the 

 earth and the moon, we can enumerate the different forms 



of energy wliich may conceivably be available. The case 

 can be very simply stated ; there is a store of energy in the 

 earth due to the fact that the earth is rotating on its axis. 

 There is a similar store of energy due to the rotation of the 

 moon on its axis. The latter is, however, very small, and may 

 be left out of sight for the jjreseut. A third source of energy is 

 due to the fact that the moon is separated from the earth, 

 and that, as it would reijuire energy to force the earth and 

 the moon asunder, so, if the earth and the moon were 

 allowed to draw together, energy would be given out. To 

 this must be added the energy due to the motion of the 

 moon in its path around the earth. To put the matter 

 brieflv, we may say that the a^•ailable soiirces of energy 

 for the tidal work must be sought either in the rotation of 

 the earth on its axis, or in the distance of the moon, in- 

 cluding in the latter case the energy duo to the velocity of 

 the moon in its path, which is intimately connected with 

 the distance of the moon from the earth. As the tides are 

 incessantly drawing on this store, it is imperatively neces- 

 sary that one or both of these sources of energy be decreas- 

 ing ; we are therefore forced to admit tliat the velocity of 

 the earth's rotation on its axis must be diminishing, or that 

 the distance of the moon is decreasing, or that both 

 velocit}' and distance are decreasing. There can be no 

 doubt as to wliich is the true explanation, for the question 

 is determined by a well-known dynamical principle. This 

 principle assures us that the supply of energy required by 

 the tides must be drawn from the rotation of the earth. 

 Indeed, we may go further than this. It is most curious 

 to observe that a second draft is made upon the reserve 

 energy stored up in the earth's rotation ; this second draft 

 is actually expended in pushing the moon away from the 

 earth. 



We have, then, two very remarkable astronomical con- 

 sequences of the tides. These consequences are founded 

 on dynamic principles, but in a manner not very easy to 

 explain without going into technical matters. The first 

 consequence is that the velocity with which the earth 

 rotates must be abating— in other words, that the tides on 

 the earth are increasing the length of the day. The other 

 consequence is not a little remarkable. It states that the 

 moon must be describing an orbit around the earth, which, 

 in the course of ages, is gradually liecoming larger and 

 larger. It must be remembered that these two conse- 

 quences of the tides are not mere speculations. They are 

 as true as the laws of dynamics, which have been conllrmcd 

 by universal experience. The propositions just stated will 

 not be questioned for a moment by anyone who is acquainted 

 with mechanical principles. Let us take first the impoi-tant 

 fact that the length of the day is gradually increasing. It 

 must be admitted that the change in the length of the day 

 is excessively slow. Even in centuries, the change is but 

 a fraction of a second ; but the change is always in one 

 direction, and, consequently, ever since the earth and the 

 moon commenced to have a separate existence, the length 

 of the day has been getting steadily greater and gi'eater, 

 until it has at present attained the well-known 24 hours. 

 We are now to look back into the history of the earth and 

 the moon in verj' remote antiquity. Our ordinary chrono- 

 logies of thousands of years are here quite inadetiuate. The 

 unit of time adapted for the earth-moon hi.story is one 

 million of years. A million years ago the length of the 

 day was appreciably shorter than it is at present. 

 There was a time when the day, instead of being 

 24 hours, was only 23 hours ; earlier still, we find the 

 day still less and less, but we shall not halt at any 

 intermediate stage ; let us at once project our view back to 

 the earliest and the most interesting epoch in the liistory 

 of the earth-moon system. At the very remote epoch to 



