August 4» 1892] 



NATURE 



315 



OUR BOOK SHELF. 



In Starry Realms. By Sir Robert S. Ball, D.Sc, 



LL.D., F.R.S. (London: Isbister and Co., 1892.) 

 This is another striking example of Sir Robert Ball's 

 skill in popularizing the most fascinating of the sciences. 

 Though the same story has been to a large extent told by 

 him before, there are several new features which prevent 

 the least suspicion of staleness. The author is perhaps 

 most interesting in his homely illustrations of astrono- 

 mical dimensions. Among these are the disc of the 

 moon projected on the map of Europe, and three lunar 

 craters similarly compared with the map of England. 

 The history of a falling star, as told by a particularly 

 intelligent meteorite, is also worth special notice. 



The two final chapters consist of '* An Astronomer's 

 Thoughts about Krakatoa," and " Darwinism in its Re- 

 lation to other Branches of Science." The former is a 

 popular account of the Report of the Krakatoa Com- 

 mittee of the Royal Society. The moral of the last 

 chapter is that the scientific method of Darwin is closely 

 related to that employed in astronomy. "Astronomers 

 were the first evolutionists : they had sketched out a 

 majestic scheme of evolution for the whole solar system, 

 and now they are rejoiced to find that the great doctrine 

 of Evolution has received an extension to the whole 

 domain of organic life by the splendid genius of Darwin " 

 (p. 349). We can confidently recommend the book to all 

 classes of readers. Those who are already familiar with 

 the subject will find much to delight them. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature, 

 No notice is taken of anonymous communications. ^ 



Basset's Physical Optics. 



I DESIRE to make a few remarks on Prof. Schuster's review 

 of my treatise on physical optics. 



The sentence in the preface to which he refers is not perhaps 

 very happily arranged, and might be amended as follows : — 



" I have a profound distrust of arguments based upon vague 

 and obscure general reasoning instead of upon rigorous mathe- 

 matical analysis." This, however, is a small matter; what I 

 wished to protest against was, the practice which has crept into 

 more than one recent work, of slurring over an investigation by 

 means of a page or two of general talk, instead of writing out a 

 careful mathematical demonstration ; or at any rate making a 

 serious attempt to grapple with mathematical difficulties, and 

 trying to arrive at a definite result. 



I fully admit, that when a subject is in a state of growth it 

 is often impossible to dispense with hypothesis. But whenever 

 this is necessary, the hypothesis should be expressed in clear and 

 definite language ; the evidence and arguments for and against 

 ■be hypothesis should be properly marshalled and discussed; 

 ^e reader should be plainly informed that the proposition 

 which forms the basis of the investigation is a hypothesis and 

 not an established fact, and that consequently further research 

 may show that the hypothesis must either be abandoned or 

 modified. When an investigation is conducted on these lines. 

 all obscurity and vagueness will be avoided ; for the reader will 

 t e thereby enabled to clearly understand the exact nature of the 

 assumptions which are made, and will be able to discriminate 

 between those portions of the investigation which consist of 

 hypothesis, and those which constitute results deduced from 

 hypothesis by the aid of mathematical analysis. 



The dangerous character of arguments based upon general 

 reasoning is well illustrated by the theory of the deformation of 

 thin elastic plates and shells. When a thin shell is deformed by 

 means of bodily forces, and stresses applied to its edges, the 

 effect produced is extension, change of curvature, and torsion ; 

 and it might be argued from this, that ihel potential energy due 

 to deformation is a homogeneous quadratic function of the 



NO. I 188, VOL. 46] 



quantities by which extension, change of curvature, and torsion 

 are specified. But if the expressions for the potential energy of 

 a cylindrical or of a spherical shell be examined (Phil. Trans. 

 1890, pp. 443, 467), it will be found that they contain certain 

 terms which involve the differential coefficients of quantities by 

 which extension is specified. 



With regard to the concluding portion of the review, I must 

 point out that one of the difficulties with which the author of an 

 advanced treatise is confronted is where to draw the line. Upon 

 this subject there is necessarily 100m for a wide difference of 

 opinion. As my object was to write a book on physical optics, 

 I considered that the reader might properly be expected to 

 obtain his information respecting the various theories of the 

 electromagnetic field, from the treatises and original memoirs on 

 that subject ; and for that reason I abstained from discussing 

 purely electromagnetic theories, further than was necessary for 

 the explanation of optical phenomena. A, B, Basset, 



July 25. 



Causes of the Deformation of the Earth's Crust. 



The communication from E. Reyer in Nature of July 7 

 (p. 224) "On the Causes of the Deformation of the Earth's 

 Crust" is interesting from several points of view. It is an 

 indication that the theory which looks upon mountain ranges as 

 the effects of the earth's contraction does not satisfy the condi- 

 tions of the geologist. 



It is welcome to me individually as in the main accepting the 

 principles of which I happen to be the exponent, and have sys- 

 tematized in the " Origin of Mountain Ranges," published in 

 1886, It is, however, the addition to this theory explaining 

 the folding of strata by what Mr. Reyer aptly calls " gliding" 

 that calls for examination. It is shown very clearly by experi- 

 ment and otherwise that under certain conditions strata, when 

 they reach a certain degree of inclination on the flanks of a 

 mountain chain during elevation, must glide downwards by 

 gravitation and produce folds and disturbances towards the low- 

 lands. We have only to consider the effects of land-slides 

 such as occur in the chalk districts in the south of England, and 

 their effects on the shore deposits, to admit the truth of this. 

 This aspect of the problem, though always present, has grown 

 on me since my work was published, and I have little doubt 

 that the "foot-hills" usually formed of the newer strata which 

 flank most great mountain ranges are to a considerable extent 

 due to gravitation and "gliding." I may point to the foot-hills 

 of the Canadian Rockies and of the Himalayas as illustrations. 

 The cases of folded lying upon undisturbed strata mentioned by 

 Reyer are, as he clearly shows, explanatory on this view, but 

 not by general contraction. 



There are no doubt other effects traceable to the gravitation of 

 masses of the earth's crust during elevation such as the lateral 

 spreading of the plastic cores of mountain ranges in fan-like 

 form, and the consequent shouldering of the strata on either 

 side intensifying the effects of the folding of the strata by thermal 

 expansion, as explained in the "Origin of Mountain Ranges." 



I cannot, however, follow Mr. Reyer if he considers "gliding " 

 an explanation of ^11 folding. I am not sure that this is his 

 meaning, though the last paragraph would seem to bear such an 

 interpretation. It seems obvious to me, to mention only one of 

 numerous examples, viz., the folds of Jurassic strata caught up 

 in the gneiss of the Central Alps, as shown in Heim's section, 

 reproduced in " Preslwich's Geology," and in plate xiv., 

 " Origin of Mountain Ranges." 



While looking upon " gliding" as only a partial explanation 

 of folding, I welcome Mr. Reyer's fresh and vigorous treatment 

 of the important problem of the causes of the deformation of the 

 earth's crust. It is evidence that geologists and physicists are now 

 allowing their minds to play freely round the subject of the orogenic 

 changes of the earth's crust, and of the growth of philosophical 

 conceptions on the geological evolution of our planet. 



Park Corner, Blundellsands, T. Mellard Reade. 



July II, 



An Obvious Demonstration of the 47th Proposition 

 of Euclid. 

 Some years ago in trying for a simpler demonstration of 

 this theorem I worked out the following. Its extreme sim- 

 plicity suggested that it could scarcely be original ; but as some 

 years have elapsed, and as none of my friends have seen it else- 



