;cS 



NATURE 



[March 14, 19 18 



pletely polarised.— Dr. J. R. Airey : The Lommel- 

 Web^r i^ function and its application to the problem of 

 electric waves on a thin anchor ring. — W. Harrison : 

 Investigations on textile fibres. (i> Dry fibres, when 

 subjected to stress, exhibit a kind of plasticity in 

 which the strains produced remain when the stress is 

 removed, but are accompanied by corresponding in- 

 ternal stresses. (2) Fibres deformed in the above 

 manner return to their original shape when placed in 

 cold water. (3) Fibres in contact with cold water are 

 elastic; strains produced by the application of stress 

 disa|>pear when that stress is removed, more quickly 

 with some fibres than with others. (4) In boiling 

 water fibres are plastic, and the application of stress 

 produces permanent deformation with no corresponding 

 internal stresses in the case of wool and only slight 

 stresses with other fibres. (5) The double refraction 

 exhibited by the natural fibres is due to the presence 

 of internal stresses. (6) The swelling produced by 

 treatment of cotton fibres with solutions of sodium 

 hydrate and of wool fibres with sulphuric acid is due 

 to the internal stresses naturally present in those 

 fibres. (7) The . internal stresses present in natural 

 fibres appear to originate in the moulding of the fibres 

 during growth and in their subsequent drying, and can 

 be imitated experimentally with artificial fibres. — W. L. 

 Cowley and H. Levy : Critical loading of struts and 

 structures. This paper is concerned with the elastic 

 stability of structures composed of members under 

 compression, and treats problems relating to the 

 strength of such a construction as a beam under end 

 thrusts and supported at intermediate points. The in- 

 vesitigation shows that failure does not necessarily 

 occur when one of the bays is of Euler's lowest critical 

 length. In this instance, however, the two equations 

 of three moments inv'olving this bay take an indeter- 

 minate form and must be replaced by two other equa- 

 tions which can easily be derived. The structure, will 

 not fail, in general, through the bending moments be- 

 coming excessive, even if several of the bays are of 

 Euler's critical length, provided at least one bay is not 

 of that length. 



Geological Society, February i5.^Annual general 

 meeting.— Dr. k. Harker, president, in the chair. — The 

 President : Anniversary address. The present position 

 and outlook of the study of metamorphism were dis- 

 cussed. For the first time it seems possible to approach 

 the subject of metamorphism systematically from the 

 genetic point of view. For the geologist this implies 

 the critical study, not only of the great tracts of crystal- 

 line schists and gneisses, but equally of metamorphic 

 aureoles, of pneumatolysis and other contact-effects, 

 and of the phenomena, mechanical and mineralogical, 

 related to faults and overthrusts. It implies, rrioreover, 

 the recognition that these are all parts of one general 

 problem, that of the reconstruction of rocks under 

 varying conditions of temperature and stress. This 

 problem is complicated by the fact that perfect adjust- 

 ment of chemical equilibrium cannot be assumed, 

 either in the rocks prior to metamorphism, or during 

 the process of metamorphism itself. The most funda- 

 mental characteristic of metamorphism was consi- 

 sidered, namely, that recr3-stallisation takes place in a 

 solid environment, and so may be profoundlv affected 

 by the existence of shearing stress. Stress of this 

 type arises from the crystal growth itself, and is called 

 into play by external forces. The automatic adjust- 

 ment of the internally created stress to neutralise that 

 provoked from without affords the key to all struc- 

 tures of the nature of foliation. The mineralosfical 

 peculiarities characteristic of the crvstalline schists 

 must find their explanation in kindred considerations ; 

 for it can be shown that the chemistry of bodies under 



NO. 2524, VOL. lOl] 



shearing stress differs in important respects from the 



\ chemistry of unstressed bodies. The result is seen in 



the appearance of a certain class of "stress-minerals " 



I where the dynamic element has figured largely in meta- 



1 morphism, while in the same circumstances the forma- 



I tion of minerals of another class seems to have been 



inhibited. The conditions governing metamorphism 



are temperature and shearing stress, with uniform 



pressure as a factor of less general importance. If 



; the orogenic forces are sufficient to maintain shearing 



: stress everywhere at its maximum, the stress itself 



becomes a function of temperature, since this deter- 



■ mines the elastic limit, and the principal conditions 



of metamorphism come to depend upon a single vari- 



1 able. This degree of simplification, however, is not to 



I be expected universally. 



\ February 20. — G. W. Lamplugh, president, in the 

 chair. — Prof. W. M. Davis : The geological aspects of 

 the coral-reef problem. A voyage in the Pacific, in 

 1914, enabled the author to collect new evidence bear- 

 j ing upon this question, and to make observations that 

 j have influenced him in his support of Darwin's theory. 

 j All theories that postulate a fixed relation between reef- 

 I formation and ocean-level are disproved, and are in- 

 applicable to the case of atolls. Reef-upgrowth is 

 j intimately associated with submergence wherever the 

 I matter can be tested. The solution of the coral-reef 

 I problem turns, at present, upon some means of dis- 

 j criminating between a submergence caused by subsi- 

 1 dence, and a submergence caused by a general rise of 

 the ocean-level due either to the uplift of the ocean- 

 floor beyond the coral-reef region, or to the melting 

 of the Pleistocene ice-sheets. Reasons to regard 

 changes in ocean-level as of secondary importance are 

 presented, and the submergence demanded by self- 

 encircled islands is attributed to local subsidence, in 

 accordance with the views of Darwin and Dana. It is 

 concluded that fringing-reefs do not mark stationary or 

 rising islands so generally as Darwin supposed. With 

 regard to elevated reefs, the impossibility of explaining 

 their features by regarding them as having been sta- 

 tionary while the ocean-surface was lowered is demon- 

 strated, and it is held that they must be due to local 

 and diverse uplift affecting the islands themselves, 

 following on epochs of subsidence which were the 

 epochs of reef-formation. The theory that such reefs 

 were formed during pauses in the elevation and emerg- 

 ; ence is considered to be seriously defective, and is 

 ' contrary to Darwin's views. 



Zoological Society, February ig.^Dr. A. Smith Wood- 

 ward, vice-president, in the chair. — L. A. Lantz : A 

 collection of reptiles made in Transcasipia and now in 

 the Zoological Museum of Moscow University. — Prof. 

 E. W. MacBride : Recent investigations into the de- 

 velopment of the sea-urchin {Echinocardium cordatum). 

 Physical Society, February 22.— Prof. C. H. Lees, 

 president, in the chair.— T. Smith : A note on the use 

 j of approximate methods in obtaining constructional 

 data for telescope objectives. The paper discusses 

 I the reason why satisfactory telescope objectives are 

 j obtained by neglecting thicknesses and solving for 

 freedom from first-order aberrations. It is shown that 

 ! the introduction of thicknesses into such an objective 

 without any alteration in the curvatures of the surfaces 

 yields a lens corrected for aberration for a zone which 

 I is a constant fraction of the full aperture obtainable. 

 I For objectives of the usual type this zone is very 

 i approximately the one that would be selected for cor- 

 ' rection to obtain the most favourable balance between 

 first- and second-order aberrations. It follows that 

 i objectives calculated from first^rder formulae in which 

 i thicknesses are neglected do not require trigonometrical 

 verification or correction unless the conditions are very 



