238 



NATURE 



[Jan. 9, 1890 



eventually, in iron wires, cyclic changes of the residual mag- 

 netization. In these, the maximum magnetization answered, as 

 in the induced magnetization in fields below the Villari point, to 

 tension " on." 



In the present investigation, the existence of a cyclic change 

 in the residual magnetization of cobalt accompanying cyclic 

 changes of pressure has been established, and the magnitude of 

 the eiTect examined in a large number of fields, extending from 

 o to 400 C.G.S. units. It was found that not only the mag- 

 nitude, but the sign even, of the effect depended largely on the 

 condition of the rod during the break of the current. When 

 the rod was under pressure during the break, the residual mag- 

 netization in the cyclic state showed a maximum underpressure, 

 whatever was the strength of the pre-existing field. When, 

 however, the rod was free from pressure during the break of the 

 current, it was only in the residual magnetization left after the 

 weakest fields that the maximum answered to pressure "on." 

 When the strength of the pre-existing field was raised, the effect 

 passed through the value zero and changed sign. 



" On the Extension and Flexure of Cylindrical and Spherical 

 Thin Elastic Shells." By A. B. Basset, M.A., F.R.S. 



The method which I have employed in dealing with problems 

 relating to the equilibrium and motion of thin cylindrical and 

 spherical elastic shells, is as follows : — 



Taking the case of a cylindrical shell, let OADB be a small 

 curvilinear rectangle described on the middle surface, of which 

 the sides OA, BD are generators, and the sides AD, OB are 

 circular sections. The resultant stresses per unit of length 

 across the section AD are completely specified by the following 

 five quantities, viz. (i) a tension, T^ ; (2) a tangential shearing 

 stress, M, ; (3) a normal shearing stress, N, ; (4) a flexural couple, 

 Go ; (5) a torsional couple, H^ ; and the stresses across BD may 

 be derived by interchanging the suffixes i and 2. If, therefore, 

 we resolve all the forces which act upon the element along OA, 

 OB and the normal, and take moments about these lines, we 

 shall obtain the six equations of motion in terms of these stresses. 



The expression for the potential energy is next found, which 

 ■differs from that obtained by Mr. Love (Phil. Trans., 1888), 

 owing to the fact that he has omitted to take into account 

 several terms involving the product of the extensions and the 

 cube of the thickness. 



The variational equation can now be written down, and if it 

 be applied to a curvilinear rectangle bounded by two lines of 

 curvature and worked out in the usual way, the line integral part 

 will determine the values of the edge stresses Tj, Tg, . . . in terms 

 of the displacements ; and the surface integral part will deter- 

 mine the three equations of motion in terms of the displacements. 

 These results furnish a test of the accuracy of the work, and 

 :also of the fundamental hypothesis upon which the theory is 

 'based (viz. that if the surfaces of the shell are not subjected to any 

 surface pressures or tangential stresses, the three stresses, R, S, 

 T, are of the order of the square of the thickness) ; for if we sub- 

 stitute the values of the edge stresses in the last three of our 

 original equations, they ought to reduce to identities ; whilst if 

 we substitute these values in the first three, we ought to reproduce 

 the equations of motion which we have obtained by means of 

 the variational equation ; and this is found to be the case. 



The boundary conditions are obtained by Stokes's theorem, 

 which enables us to prove that it is possible to apply a certain 

 •distribution of stress to the edge of a thin shell, without pro- 

 ducing any alteration in the potential energy due to strain. 



Geological Society, December 18, 1889.— W. T. Blanford, 

 F. R. S., President, in the chair. — The following communications 

 were read : — On the occurrence of the genus Girvanella, and 

 remarks on Oolitic structure, by E. Wethered. The author 

 referred to his previous work, wherein he had shown that 

 Girvanella is not confined to Silurian rocks, and that as a rock- 

 forming organism it is more important than was supposed, 

 occurring in the Gloucestershire Pea-grit, and also in the 

 Coralline Oolite of Weymouth. He now dealt more in detail 

 with its occurrence (l) in the Carboniferous Oolitic Limestone; 

 smdi {2) \n\h^ Jurassic Oolites. In the Carboniferous limestone 

 of the Avon valley. Oolitic limestone occurs on four horizons, in 

 .three of which the Oolites rest on dolomite. In none of these 

 .three cases are there signs of Girvanella. From beds partly 

 Oolitic, and not resting on dolomite, he has been able to de- 

 termine two new species. The Oolite not associated with dolomite 

 is less crystalline, and the original structure is better preserved. 

 In referring to G. pisolitica, he discussed whether Girvanella is 



most allied to the Challenger Foraminifer, Hyperammina vagans, 

 or to Syringaniviina fragilissima. Traces of the organism occur 

 in the Clypeus-^xii, but none are quoted from beds of the Great 

 Oolite, nor from the Portland Oolite. The author had already 

 shown that the pisolites in the Coralline Oolite of Weymouth were 

 not concretions, but forms of Girvanella. Excluding these, he 

 showed that the spherules are of four types, of which one is the 

 ordinary Oolitic granule, while each of the others suggests the 

 presence of Girvanella. The characters of the genus, as seen 

 under the microscope were indicated, and four new species were 

 described. The President remarked on the importance of in- 

 vestigating the question whether these appearances are organic 

 or not. We should take warning from Eozoon as to possible 

 differences of opinion in the interpretation of tubular structure, 

 though these mystifying appearances seem more common in 

 serpentine and chalcedony than in calcite. In the bodies 

 depicted, the wall, the irregularity, and the manner in which 

 the tubes curve round each other are in favour of their being 

 organic. Prof. Rupert Jones thought that these forms were not 

 due to mineral but to organic laws. Dr. Evans, while dis- 

 claiming any special knowledge of the subject, suggested that 

 the appearances might be interpreted on the supposition of an 

 organism boring into a comparatively hard substance. Dr. 

 Hinde, who had seen most of the known species of Girvanella, 

 spoke of the wide distribution of these organisms. Remarks 

 were also offered by Dr. Hicks, Prof. Bonney, Prof. Judd, the 

 Rev. H. H. Winwood, and the author. — On the relation of the 

 Westleton Beds or ' ' Pebbly Sands " of Suffolk to those of Norfolk, 

 and on their extension inland, with some observations on the 

 period of the final elevation and denudation of the Weald and 

 of the Thames Valley, Part 2, by Prof. Joseph Prestwich, 

 F.R.S. — The author having, in the first part of this paper 

 (Proc. Geol. Soc, June 5, 1889), discussed the relationship of 

 the Westleton Beds to the Crag series and to the Glacial 

 deposits, proceeded in the present contribution to consider 

 the extension of the Westleton Beds beyond the area of the Crag, 

 and described their range inland through Suffolk, East, West, 

 and South Essex, Middlesex, North and South Hertfordshire, 

 South Buckinghamshire, and North and South Berkshire, 

 noticing their relationship to the overlying Glacial beds, where 

 these were developed, and the manner in which they reposed 

 upon older deposits. He gave an account of the heights of the 

 various exposures above Ordnance Datum, and mentioned 

 the relative proportion of the different constituents in various 

 sections, thus showing that in their southerly and westerly 

 extension they differed both in composition and in mode of 

 distribution from the Glacial deposits. Distinction was also 

 made between the Westleton Beds and the Brentwood Beds. 

 Attention was next directed to the occurrence of the Westleton 

 series, south of the Thames, in Kent, Surrey, and Hamp- 

 shire, and their possible extension into Somersetshire was 

 inferred from the character of the deposits on Kingsdown and near 

 Clevedon. In tracing the deposits from the east coast to che 

 Berkshire Downs, the author noticed that at the former place the 

 beds lay at sea-level, bat ranging inland, they gradually rose to 

 heights of from 500 to 600 feet ; that in the first instance they 

 underlay all the Glacial deposits, and in the second they rose 

 high above them, and their seeming subordination to the Glacial 

 series altogether disappeared ; thus at Braintree, where the 

 Westleton Beds were largely developed, they stood up through 

 the Boulder-clay and gravel which wrapped round their base, 

 whilst further west, where they became diminished to mere 

 shingle-beds, they attained heights of from 350 to 400 feet, capping 

 London-clay hills, where the Boulder-clay lay from 80 to 100 

 feet lower down the slopes, the difference of level between the 

 two deposits becoming still greater in a westerly direction, until 

 finally the Boulder-clay disappeared. The origin of the 

 component pebbles of the beds was discussed, and their de- 

 rivation traced (i) to the beds of Woolwich age in Kent, 

 North France and Belgium, and possibly to some Diestian beds, 

 (2) to the older rocks of the Ardennes, (3) to the Chalk and older 

 drifts, and (4) to the Lower Greensand of Kent and Surrey, or 

 in part to the Southern drift. The marine nature of the beds 

 was inferred from the included fossils of the type-area, and the 

 absence of these elsewhere accounted for by decalcification. 

 The southward extension of the beds was shown to be limited 

 by the anticlinal of the Ardennes and the Weald, and the scanty 

 palasontological evidence of the nature of that land was noted, 

 and the possible existence of the Scandinavian ice-sheet to the 

 north was referred to in connection with the disappearance of the 



