I go 



NA TURE 



\yunc 20, 1889 



points out that if the equation of an anemometer, whose constants 

 Tiave been determined by a whirling machine, be used to reduce 

 •observation^ made in the open air, the computed wind velocities 

 will be too high, by an amount which will depend upon the 

 •moment of inertia of the cups and revolving parts. Some of 

 the experiments described were of a delicate nature, the cups 

 being made of paper fastened to pieces of fine knitting-needles, 

 which served as arms.— The concluding article is by Prof. R. 

 Owen, on magnetic phenomena in the southern hemisphere, the 

 object being to give some particulars regarding the experiments 

 made in that hemisphere, as compared with results obtained in 

 the northern half of the globe. 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society, June 6. — " Report on the Efifects of 

 Contact Metamorphism exhibited by the Silurian Rocks near 

 the Town of New Galloway, in the Southern Uplands of Scot- 

 land." By S. Allport and Prof. Bonney, F.R.S. 



In the neighbourhood of New Galloway a mass of granite cuts 

 across and sends veins into a series of Silurian rocks, which 

 are considerably altered near the junction. These, originally, 

 were a variable series of more or less sandy rocks, such as older 

 authors would call greywacke. The minerals resulting from the 

 *' contact-metamorphism " were enumerated : these were quartz 

 brown mica (which, as was pointed out, must be much richer in 

 iron than in magnesia), white mica, and iron oxides, with some 

 hornblende, augite, garnet, and perhaps epidote. Chiastolite is 

 absent ; andalusite very inconspicuous ; tourmaline very rare 

 The point to which special attention was directed was that in 

 several of the slides, the larger fragments which had been pre- 

 sent in the original greywacke could still be recognized, such as 

 clastic quartz, feldspar (often more or less converted into white 

 mica and quartz), bits of argillite or earthy sandstone (repre- 

 sented by mixtures of brown mica and quartz in varying propor- 

 tions). The bearing of the results of the investigation on general 

 questions of metamorphism was indicated: (i) that heat, in 

 presence of water, and probably under considerable pressure, 

 had produced rocks which bore some resemblance to, but 

 could be distinguished from, crystalline schists, such as those 

 of known Archaean age ; (2) that, while these agents of meta- 

 morphism have produced a crystalline rock from a clastic 

 one, they have not obliterated the original structure, when this 

 was somewhat coarse. Hence that it is safe to conclude that, 

 at any rate in the less highly crystalline schists, the alternation 

 of mineral constituents which so closely simulates bedding is 

 due to an original stratification of clastic constituents. 



Physical Society, June 8. — Prof. Ayrton, F.R.S., Vice- 

 President, in the chair. — The following communications were 

 made : — A photograph of lightning flashes was exhibited and 

 described by Dr. Hoffert. The photograph was obtained 

 <luring the storm on the 6th inst. , whilst the camera was being 

 waved about in the hand, and shows three similar and parallel 

 flashes, thus proving that successive sparks in multiple flashes 

 ■may traverse the same path and maybe separated by appreciable 

 intervals of time. The supposed primary spark is intersected by 

 •numerous tributary ones spread out on both sides, the second 

 ■spark shows one tributary, and the third none. Faint bands of 

 light pass across the plate parallel to the direction of motion, 

 and these prove that some residual illumination exists during the 

 intervals between the successive flashes. Evidence of a dark 

 flash is also presented by the plate. Prof. Herschel, who had 

 "taken photographs during the same storm, referred to the 

 fluttering appearance of the flashes, and on their long dura- 

 tion ; in many cases the time was sufficient to allow him to 

 ■direct the camera towards the flash, and make a successful 

 ■exposure. He had also observed multiple flashes with the 

 unaided eye, and on waving his hand about he had sometimes 

 noticed about a dozen distinct images of it during one discharge. 

 Mr. Gregory said that he watched the storm along with two 

 others, and they could seldom agree as to the shape of the 

 flashes, or on their simple or multiple character. The want of 

 agi'eement as to multiplicity he thought might be caused by 

 their eyes being directed towards differetit parts of the sky when 

 a multiple flash occurred ; the one who happened to be looking 

 towards the flash might be conscious of only one impression, 

 whereas the others in directing their eyes would receive the 

 flashes on different parts of the retina. In some cases as many 

 as three distinct flashes (occurring at intervals of about ten 



seconds) traversed the same path, and a number of the discharges 

 presented a beaded or striated appearance. The beads seemed 

 to remain after the main flash had faded, and this might account 

 for the bands shown in Dr. Hoffert's photograph. Mr. C. V. 

 Boys, in referring to multiple flashes, said that although his 

 statements made in the discussion of Mr. Whipple's paper on 

 April 13 were not readily accepted, yet no one who watched the 

 recent storm could doubt their existence. Prof. S. P. Thomp- 

 son thought the order of the flashes on the photograph may 

 have been the reverse of that supposed, for he observed that the 

 band of light extended on both sides of the (so called) primary 

 flash, whereas the outside of the third flash was quite dark. 

 Mr. E. W. Smith noticed many cases of "sympathetic dis- 

 charge," in which a flash in the north seemed to preciphate 

 another in the north-west within a few seconds, and in this he 

 was corroborated by Mr. Gregory, who viewed the storm from 

 a different locality. Mr. C. V. Burton thought the heating 

 of the air by the first spark might give rise to the tributary 

 . sparks in subsequent ones. Mr. A. W. Ward mentioned along 

 flash observed at Cambridge which passed from the zenith, and 

 struck some farm-buildings at a distance, and he was particularly 

 impressed by the considerable time occupied in its progress. — 

 On the methods of suppressing sparking in electro-magnets, by 

 Prof. S. P. Thompson. The object of this paper is to classify 

 the methods which have been suggested, and to draw attention 

 to a novel method of some importance. The classification is as 

 follows': (i) Mechanical devices : {a) simple snap switch; (/') 

 break in magnetic field ; {c) break under liquid ; {d) wiping 

 break (asbestos, &c., brushes) ; {e) blow out. Electrical deziccs : 

 (A) use of condensers, (a) placed across gap, and {b) across 

 terminals of magnet ; (B) mutual induction protectors, [a) copper 

 sheath around core, and {!)) layers of foil between windings ; 

 (C) short-circuit working; (D) differential winding; (E) high- 

 resistance shunt (non-inductive) ; (F) voltameter or liquid resist- 

 ance across gap ; (G) multiple wire arrangement of Mr. Langdon 

 Davies ; (H) electro-magnet with two bobbins in series or parallel. 

 The merits and demerits of the different methods are indicated. 

 The multiple-wire arrangement used by Mr. Langdon Davies in 

 his harmonic telegraph consists in winding each layer separately 

 and uniting all in parallel. The effect of this is to make the 

 time constants of the layers different, and on breaking the 

 circuit the energy is spent in mutual discharges. — A shunt trans- 

 former, by Mr. E. W, Smith. Two conductors, A and B, of 

 equal impedance, are placed in series between alternaie current 

 mains, and the same mains are connected through two incan- 

 descent lamps in series. The conductor A has great resistance, 

 and B has large self-induction, and when their junction is joined 

 to that of the two lamps, both lamps become brighter, and the 

 main current is reduced. These phenomena were shown before 

 the Society. Since A may consist of lamps and B may be a 

 choking coil, the arrangement will serve to increase the P.D. 

 between the terminals of the lamps without wasting much energy. 

 Experiments of a similar nature have been made on a Mordey 

 transformer wound with three equal coils. One coil was used 

 as primary, and the other two as separate secondaries, their 

 respective circuits consisting of lamps and an alternate current 

 motor. Under these conditions the arithmetical run of the mean 

 secondary currents exceeded the primary current by about 14 per 

 cent,; the secondary volts were 8 per cent, less than the primary. 

 All these experiments strikingly illustrate the effects of accelera- 

 tion and lag in alternate current circuits, and (as was pointed out 

 by the author) show that meters registering "ampere hours" 

 merely, may give readings differing greatly from the numbers 

 representing the energy used. — Notes on geometrical optics : (i) 

 on the deduction of the elementary theory of mirrors and lenses 

 from wave principles ; (2) on a diopteric spheromeler ; (3) on 

 the formula of the lenticular mirror, by Prof. S. P. Thompson. 

 Instead of deducing the formulte for lenses and mirrors by 

 means of "rays," and the relations between angles of incid- 

 ence, reflection, and refraction, the author considers it better 

 to derive them from the curvatures impressed on waves at 

 the bounding surfaces of the different media. Indices ofj 

 refraction are replaced by their reciprocals, which express 

 the relative velocities of light, and curvature is measured 

 by the camber at the middle of chords of equal, but small,! 

 lengths. An example will assist in making the method' 

 of treatment clear. Suppose «',«', a,a two successive position 

 of a plane wave-front in air, which impinges on a curved surfaceJ 

 EAF, at A, and let the curvature at A be R, and the velocitj 

 constant of the substance h. Then, whilst the part a travels 



