August 15, 1907J 



NA TURE 



OUR BOOK SHELF. 



Chrisliiian Hiivgciis, Traiii: De iis quae liquido 

 siipcnialaiil. Kfdii^'c par D. J. Kortf wfi,'. Pp. 

 lxxxiii + 210. (Extrait des Oeuvres completes de 

 Christiaan Huygens. Tome xi., n.d.) 

 This treatise of ;ibout a hundred uuarlo pages, printed 

 from a hitherto unpublished MS. of Huygens, forms 

 part of the eleventh volume of the collected edition of 

 his works which is now in progress. The accom- 

 plished editor, Prof. Korteweg, of .Amsterdam, who 

 has himself made valuable contributions to hydro- 

 dynamics and to mathematical history, has judged 

 wisely in issuing it separately, and so rendering it 

 accessible to a wider public. 



The contents are certainly remarkable. Book i. 

 begins by deriving the .-Xrchimedean conditions of 

 equilibrium of Heating bodies from the principle that 

 the altitude of the centre of gravity of the whole 

 svstem composed of solid and fluid must be a »i/i!!- 

 nuim, and the same principle is then applied to the 

 question of stability for angular displacements. Owing 

 to the manner in which the problem is stated, stable 

 positions are alone the subject of investigation ; the 

 recognition of the fact that the conditions of equi- 

 librium arc equally satisfied by a stalionayy altitude ol 

 the centre of gravity, belongs to a later period. The 

 special cases of the paraboloid of revolution, and the 

 cone, floating with the axis vertical, are treated in 

 some detail. 



In Book ii. we find a somewhat elaborate, but not 

 quite complete, treatment of the problem of the 

 stability in difi'erent positions of a floating log of 

 rectangular section ; but the method followed is 

 different, and is more closely related to .\rchimedean 

 principles. Book iii. deals, on similar lines, with the 

 problem of the floating cylinder. 



This brief recital will show how great is the histor- 

 ical interest attaching to this tract. The interest is 

 increased when we are told that it was a youthful 

 es.say, which the author himself condemned ;is frag- 

 mentary and incomplete. .\ note appended to the 

 MS. reads : " Haec de corporibus solidis in liquido 

 supernatantibus in prima adolcscentia scripsi, cum 

 nullum adhuc maioris moment! argumentum sese 

 obtulisset . . . E primis Theorematis quaedam retineri 

 possont, item de Cvlindris. Refiqua vulcano tradenda." 

 The student of mathematical history will be glad that 

 this elegant, and in some respects significant, essay 

 should have escaped the fate here threatened, and 

 will be grateful to the editor for having now placed 

 it bevond the reach of accident. 



The volume, which is (by the way) most beautifully 

 jjrintod, contains a valuable introduction and com- 

 mentary bv Prof. Korteweg, who points out in detail 

 the relation of Huvgens's work to later developments 

 of the theory. It includes also reprints of other MS. 

 notes by Huygens on the same subject, with interest- 

 ing facsimile reproductions of the original diagrams. 



H. L. 

 .1 Health Reader. By Dr. C. E. Shelly and E. 

 Stenhouse. Books i. and ii. Pp. vi -1-160 and 

 viii+iq6. (London : Macmillan and Co., Ltd., 1907.) 

 Price i.s'. and i.s". 4<f. 

 The great importance of imparting to children, while 

 at school, a knowledge of the broad principles of 

 healthy living is generally recognised ; and the past 

 few years have witnessed the outcome, in response to 

 a growing demand, of many handbooks upon ele- 

 mentary hygiene for school purposes. Some of these 

 have erred on the side of attempting to explain too 

 much, and the book, whether designed to meet the 

 needs of the school-teacher or of the scholar, has 

 dealt with non-essentials in an unnecessarily technical 

 manner. 



NO. 1972, VOL. 76] 



These two small works, both of which are fully 

 illustrated, have been compiled with great discretion, 

 for the sutjject-matter is that which every child should 

 know, and it is treated in such a manner that the 

 child, while acquiring useful information, will be inter- 

 ested. 



The earlier book is designed for use among chil- 

 dren of nine to ten years of age, and the later volume 

 for pupils of from ten to twelve years of age. A 

 noteworthy feature of both books is the omission of 

 all technical terms. For instance, the cranium and 

 sternum are called the brain-case and the breastbone, 

 and the red blood corpuscles of the blood are described 

 as " grains of red jelly " — " the oxygen-boats." An- 

 other noteworthy feature is the due appreciation shown 

 of the importance of presenting the subject-matter in 

 the simple manner which appeals to the child-reader. 

 Those who are unfamiliar with the difl'iculty of 

 appealing to the child of from ten to twelve yeai-s 

 with a subject such as hygiene may judge Book ii. 

 to be too elementary ; but others will not share that 

 view. 



It is most desirable that a health reader, the infor- 

 mation of which can (where necessary) be supple- 

 mented or explained by occasional conversational 

 digressions, should be introduced into every school; 

 for it is doubtless the easiest and the most effective 

 way of teachin.g the subject. The works under review- 

 arc exceedingly well designed to serve this purpose. 

 I'ti'Ieiudns oder Kopernikiis? Eine .S7iirf/c iiber die 

 Beiueguiig der Erde utid iiber den Begrifj dcr 

 Bewegung. By Dr. Karl Neisser. Pp. vi+154. 

 (Leipzig: J. A.' Barth, 1907.) Price 3 marks. 

 The author of this little book aims at showing that 

 all motion is relative, and that the people who three 

 hundred years ago disputed as to whether the earth 

 moved round the sun or the sun round the earth were 

 all equally right, and were only fighting about words. 

 In the first chapter it is set forth with needless pro- 

 lixity how all apparent phenomena produced by the 

 motion of an observer will be the same if he is at 

 rest, and all the surrounding objects are in motion 

 with the same velocity in the opposite direction. The 

 author next deals with the system of Copernicus, and 

 points out how it could only claim to represent the 

 planetary motions in a somewhat simpler manner, 

 but that the relative positions of the earth and a 

 planet were exactly the same according to the old 

 and the new system. But everything was changed by 

 the discovery of the proper motions of the fixed stars, 

 which made it theoretically impossible to refer the 

 celestial motions to a fixed origin of coordinates, while 

 the sun's own motion through space shows that the 

 earth cannot describe a closed curve round it. As 

 regards the rotation of the earth, the author maintains 

 that the deviation of a falling body towards the east 

 and Foucault's pendulum do not absolutely prove 

 that it is the earth and not the whole rest of the 

 universe which is turning. Whether we say that the 

 earth or the heavens, or both, move is thus a mere 

 matter of taste depending on the point of view ; and 

 the two first laws of motion are incapable of proof, 

 since there is no such thing anywhere as motion in a 

 straight line. 



The most unsatisfactory part of the author's reason- 

 ing is the way in which he lightly skims over the 

 aberration of light in a footnote only. One might 

 grant him that everything else he brings forward is 

 conceivable, but the human mind is hardly capable 

 of imagining that all stars might move in the course 

 of a year in circles, the planes of which are parallel 

 to each other. If aberration is not an absolute proof 

 of the earth's annual motion, we may give up the 

 hope of proving anything. 



