March 9, 1876] 



NATURE 



379 



SOCIETIES AND ACADEMIES 



London 



Geological Society, Feb. 23.— Prof. P. Martin Duncan, 

 r.R.S., president, in the chair.— The Rev. David Charles, 

 D.D., Thomas Musgrave Heaphy, C.E., William Smethurst, 

 Edward Horatio W. Swete, M.D., and John Thomas Young 

 were elecfed Fellows, and Prof. Joseph Gosselet, of Lille, a 

 Foreign Correspondent of the Society. — The following com- 

 munications were read : — On the greenstones of Western Corn- 

 wall, by Mr. John Arthur Phillips, F.C.S. In this paper the 

 author brought forward evidence to show that the so-called 

 " greenstones " of Penzance really belong chiefly to the following 

 three classes -.—a. Gabbros or Dolerites, in which the origi- 

 nally constituent minerals are either to a great extent un- 

 changed, or, sometimes, almost entirely represented by pseudo- 

 morphic forms, b. Killas, or ordinary clay-slates, c. Highly 

 basic hornblendic rocks, exhibiting a tendency to break into 

 thin plates ; these under the microscope present the appear- 

 ance of metamorphosed slates. Slaty rocks of a character 

 intermediate between b and c also occur. In the Cape 

 Cornwall district the "greenstones" are chiefly hornblendic 

 slates, sometimes with veins or bands of garnet, magnetite, or 

 axinite. The rocks near the Gurnard's H ad are almost identical 

 with those of Mount's Bay. The crystalline pyroxenic rocks 

 and metamorphic slates of the St. Ives district exactly resemble 

 those of Penzance. The greenstones between St. Erth and St 

 Stephen's are probably altered ash-beds or hardened hornblendic 

 slates ; unlike the hornblendic and augitic rocks of the other 

 districts, they do not occur in the immediate vicinity of granite, 

 but elvan courses are always found near them. The percentage 

 of silica in the two series of rocks is nearly constant ; the horn- 

 blende slates contain about 10 per cent, less silica than 

 the crystalline pjToxenic rocks, and there is an excess of 

 iron oxides to nearly the same extent, their composition 

 in other respects being very similar. The Killas is an 

 acidic rock of essentially different chemical composition. —On 

 columnar, fissile, and spheroidal structure, by the Rev. T. 

 G. Bonney. Some of the above structures have compa- 

 ratively recently been discussed by Mr, Mallet and Prof. J. 

 Thomson. Both these authors agree in attributing columnar ' 

 structure to contraction due to loss of heat while cooling, but '\ 

 differ in their explanation of cross jointing and spheroidal struc- 

 ture. In this paper it is sought to show that the principle 

 proved by Mr. Mallet to be the explanation of the columnar 

 structure is capable of a wider application. After a brief notice 

 of some instances of coliminar structure, the author described 

 cases of a fissile structure seen in certain igneous rocks (especially 

 in the Auvergne phonolites), closely resembling true cleavage, 

 and often mistaken for it ; also the tabular jointing of rocks ; a 

 peculiar form of this, where most of the segtBents are of a flat- 

 tened convexo-concave form ; spheroidal structure and cup-and- 

 ball structure. He showed by examples that Prof. Thomson's 

 explanation of spheroidal structure was inadequate, and gave 

 reasons for considering all these structm-es to be due to con- 

 traction. He aUo discussed more paiticularly the cup-and-ball 

 structure, giving reasons for thinking that the spheroidal and the 

 horizontal fissures were often to some extent independent of each 

 other. 



Physical Society, Feb. 26. — The president. Prof. G. C. 

 Foster, F.R.S., in the chair.— The following candidates were 

 elected members of the Society : — The Rev. R. Abbay, M.A., 

 and Mr. W. Bottomley, sen. — Mr. A. Haddon exhibited and 

 described a form of tangent galvanometer, so arranged that by 

 the aid of an electric lamp an image of the needle can be pro- 

 jected on the screen, and its deflections thus made evident to 

 large audiences. A horizontal beam of light falling on a mirror 

 inclined at 45° is thrown vertically upwards. In its path it 

 meets with a glass box containing a lozenge-shaped magnet 

 about three-quarters of an inch long ; above this needle is a 

 graduated semicircle. The pivot supporting the needle is fixed 

 in the centre of the glass plate which forms the bottom of the 

 box. Above this box is a lens, and on the top of the whole is 

 a second reflector parallel to the first. On either side of the 

 needle is a hoop of stout brass wire, fourteen inches in diameter, 

 one end of each hoop being insulated by a piece of ebonite, 

 while the other end is in metallic connection with a brass ring 

 which slides easily over the circular base of the instrument. The 

 hoops are separated firom each other by a distance equal to half 



the diameter of either hoop, i.e., 7 inches. The instru- 

 ment having been placed at a distance from the screen equal to 

 the focal length of the lens, and the needle brought to zero 

 by rotating the graduated scale, the hoops are placed 

 parallel to the magnetic meridian, and the instrument is 

 ready for action. As an illustration of the manner in 

 which the galvanometer is employed. Ohm's Law was proved 

 in the cases of large and small external resistance. — Mr. 

 O. J. Lodge, B. Sc, then described some investigarions on 

 which he has recently been engaged in reference to the flow of 

 electricity in plane bounded surfaces, in continuation of a papar 

 read before the Society in the early part of last year, by Prof. 

 G, C. Foster and himself. After some introductory considera- 

 tions, he pointed out that all the conditions of the flow of elec- 

 tricity are known for any number of poles in an tmlimited sheet 

 The problem then consists in reducing cases of poles in bounded 

 plates to corresponding cases in the unlimited plane, such that 

 the flow conditions on the bounding line may be the same in 

 both cases. The determination of these data, however, for 

 limited planes of certain forms presents considerable difl&culty. 

 In studying questions of this nature there are two kinds of lines 

 which must be considered. These are " equipotential lines," 

 along which no electricity passes, and "lines of flow," across 

 which no electricity passes. The boundary of any conducting 

 surface will of course always be a line of flow, and, in a bad 

 conductor, we can form an equipotential line by laying a band of 

 copper in the required direction. If, therefore, in stud)ing a 

 surface of limited extent in contact with an electrode, we can 

 find a point or points outside the surface such that, if they be 

 made electrodes, the boundary line of the stuface becomes a line 

 of flow, we are at liberty to treat the surface as part of an infinite 

 plane, and all the circumstances are therefore known. To take 

 the simplest case, a straight line in an infinite surface will be a 

 line of flow if equal sources be placed in pairs on opposite sides 

 of the line so that one is the virtual image of the other ; but, if 

 the components of each pair are of opposite sign, it becomes an 

 equipotential line. To make a circle of radius (r) an equipotential 

 circle, we require a source A, within, and a sink B, wiihout, such 

 thatC A.CB = r^ . To make it a line of flow we require 

 two sources, such that C A^. C A = r^ and an equal sink at C, the 

 centre of the circle. The cases of an infinitely long straight 

 strip and of a surface bounded by two straight lints inclmed at an 

 angle Q were then referred to, and Mr. Lodge showed that the 

 first requires an infinite number of external sources arranged 

 on a straight line, and the second an infinite number on a circle 

 except when is a submultiple of ir, the numt>er then becoming 

 finite. Diagrams of the images for certain cases of triangles and 

 squares were also shown. The dimensions of the electrodes in 

 contact with conducting surfaces are not matters of indifiierence. 

 In a plane bounded by straight lines the electrodes within and 

 without the boundary are of equal size, but when the boundary 

 is a circle the areas of electrodes vary as the squares of their dis- 

 tances from the centre. It was then pointed out that not only 

 the poles may be reflected in this way, but also every point in the 

 sheet ; and il the lines of flow or of potential are drawn inside a 

 given circle for any arrangement of poles, the lines outside can 

 be immediately obtained from them by inversion with regard to 

 the centre of the circle by means of a Peaucellier cell. The 

 author then described the maimer in which the principle of 

 Wheatstone's Bridge can be employed for tracing out lines of 

 equal potential If A and B be a source and sink on a con- 

 ducting ring, and P any point on the ring between A and 

 B and Q any point between B and A, then P and Q are of 



PA O A 

 equal potential whenever ^-^ = q^. If now the wire tmder 



the point P be flattened out into a surface, the above expression 

 holds good for a certain line on that surface, which is therefore 

 an equipotential line. Similarly by flattening out the wire under 

 the point A, the line for which the expression then holds good is 

 a line of flow for a certain distribution of poles. At this point 

 the reading of the paper was adjourned to the next meeting of 

 the Society. — Prof. McLeod exhibited a glass plate covered with 

 a film of silver which had in places been deflagrated by means of 

 Leyden jars, the poles being placed at varying distances apart 

 The form of the svurface acted upon tended towards the Lem- 

 niscate of Bemouilli. 



Paris 

 Academy of Sciences, Feb. 21, — Vice-Admiral Paris in the 

 chau:, — The death of M. Brongniart was annotmced. — The 

 I following papers were read : — Meridian observations of small 



