the sectors B1, &c., and will therefore produce a current equal and opposite to that produced by the negative ends of the long lines between the two sets of sectors. As the sectors C1, &c. move from left to right, the lines of force will tend to get sheared over towards the right, so that a line going for example from C1 to B1 will ultimately strike B, and form two lines, one from B2 to B1, and one from C1 to B2. Thus, at every gap between the sectors there will be a continuous production of lines going from one sector to the next across the gap. One end of each sector will therefore continually tend to get positively charged, and the other end negatively charged. These opposite charges will of course continually neutralize one another, which will constitute a current equal and opposite to that produced by the motion of the negative ends of the lines of force along the sectors B1, &c. This process of neutralization is shown more in detail in fig. 4, where only these "short" lines and their charges are represented. Fig. 4. + It is evident that what really happens is that the moving. sectors carry round their charges while a circulation of electricity is produced in the other sectors. Such a circulation. of course cannot produce a magnetic field, because as much electricity must pass across each sector in one direction as in the opposite direction. Thus, in this case, the motion of the disk should produce a magnetic field. Cremieu, however, only obtained a small field, which he supposed was a spurious effect. It is assumed in the above that the insulation between the sectors is perfect. If this is not the case, then the lines of force between the two sets of sectors will slip across the gaps instead of forming the short lines which produce the compensating currents. Since the actual currents in these experiments are in any case very small, whereas the electric intensity between the two sets of sectors is very large, it is easy to see that unless the insulation of the sectors is very good, the lines of force will nearly all slip across the gaps where the insulation is bad, so that there will merely be twonearly equal and opposite parallel currents, and consequently very little magnetic field. It seems therefore probable that in Cremieu's experiments without the drum-ends, the insulation of the sectors was not good enough, and consequently he failed to obtain the calculated magnetic field. Rowland & Hutchinson, in their experiments*, used an arrangement which is equivalent to Cremieu's without the drum-ends. It is shown diagrammatically in fig. 5. Two Fig. 5. E The charged ebonite disks, carrying metal sectors, were rotated on the same axis GH. Two glass plates, C and D, were fixed between them, and carried the opposite charges to those on the disks. These plates also carried metal sectors. magnetic field was measured by means of an astatic needle system E, placed between the plates so that the lower needle was on the axis of rotation of the disks. This needle system was enclosed in a metal tube. The observed field agreed well with that theoretically predicted. It is evident that this arrangement is practically the same as Cremieu's without the drum-ends, so that we may conclude that the insulation between the sectors in Rowland & apparatus was good enough to prevent the lines Hutchinson's of force slipping across the gaps, which was probably not the case in Cremieu's apparatus. It may be worth while to point out that the drum-ends in Cremieu's apparatus ought not to stop the magnetic effect except when they are close up to the gaps between the sectors on the mica. For if they are some distance away, then the *Phil. Mag. xxvii. p. 445 (1889). lines of force will get across the gaps without striking the drum-ends, so that the magnetic field should be produced just as if the drum-ends were not there at all. Thus the metal tube round Rowland's astatic needle system was too far away from the gaps between his sectors on the glass plates to catch the lines of force, so that it could not prevent the field being produced. The conclusions arrived at in this paper are: (1) That in Cremieu's attempt to detect the electrostatic effect of a varying magnetic field, the effect of the steady magnetic field on the charging currents was left out of account, and that this latter effect is equal and opposite to the former. (2) That, consequently, Cremieu's negative result constitutes an indirect proof of the existence of an electrostatic effect of a varying magnetic field, of the amount usually predicted theoretically. (3) That in Cremieu's attempt to repeat Rowland's experiment, his addition of a metallic screen placed close up to the fixed sectors should cause a current to be produced which should almost entirely compensate the desired effect. (4) That Cremieu's partial failure without the additional screen, is quite possibly due to defective insulation of his sectors. In conclusion I wish to say that my best thanks are due to Prof. J. J. Thomson and to Dr. Larmor for advice and encouragement during the preparation of this paper. IX. Notices respecting New Books. Magnetic Induction in Iron and other Metals. By J. A. Ewing, M.A., F.R.S., M.Inst.C.E. Third Edition, revised. London: 'The Electrician' Printing and Publishing Company Limited, 1900. L1 ITTLE need be said in praise of a book which has on all hands been acknowledged to be the treatise in the English language on the subject of magnetic induction, and the comparatively rapid exhaustion of the first two editions of which is sufficient evidence of the thoroughness with which its distinguished author has accomplished his task. The additions to the original work which appear in the present edition consist mainly of a chapter on "Practical Magnetic Testing," and of references to the more recent investigations, such as those of Dr. Morris on the effects of temperature, and those of Mr. S. R. Roget on the "ageing of iron by prolonged exposure to various temperatures. The only criticism which we venture to offer is a suggestion that in future editions the new matter should be worked up in such a manner as to become capable of being incorporated with the older portion of the book, to maintain homogeneity of treatment. One cannot help feeling, for instance, in reading the practical hints on the ballistic method of measurement contained in the last chapter, that the most logical arrangement would have been to append these hints to the account of the ballistic method given in Chapter III. As it is, a reader desirous of information regarding this method of measurement naturally turns to Chapter III., and in the account there given he fails to find what are-from the experimenter's point of view-exceedingly useful practical hints, such, e. g., as the one regarding the extreme convenience of using a ballistic galvanometer of the D'Arsonval type. We trust that in future editions this suggestion may be adopted. La Rivista Tecnica delle Scienze, delle Arti applicate all' Industria e dell' Insegnamento Industriale. Anno I. Fascicolo 3-4. Editori Roux e Viarengo: Torino. THIS very readable periodical contains a number of interesting articles dealing with various technical and scientific subjects. The first article is devoted to a discussion of alternating currents; this is followed by another in which the scientific foundations of electro-chemistry are dealt with. Next comes one on the subject of toothed gearing, and the remaining articles deal with a large variety of topics interesting from an industrial standpoint. Most of the articles are well illustrated, and are not too technical to be beyond the understanding of a reader with a moderate knowledge of the practical applications of science. The new periodical should appeal to a wide circle of readers. X. Proceedings of Learned Societies. GEOLOGICAL SOCIETY. [Continued from vol. 1. p. 670.] December 19th, 1900.-J. J. H. Teall, Esq., M.A., F.R.S., HE following communications were read : THE 1. On the Igneous Rocks associated with the Cambrian Beds of the Malvern Hills.' By Prof. T. T. Groom, M.A., D.Sc., F.G.S. The Cambrian beds of the Southern Malverns are associated with a series of igneous rocks which have commonly been regarded as volcanic, but are probably all intrusive. They consist of a series of bosses, dykes, sills, and small laccolites intruded into the Upper Cambrian Shales and into the Hollybush Sandstone. The dykes appear to be confined to the sandstones, the sills and laccolites chiefly to the shales, while the bosses are found in both. The rocks consist of a series of ophitic olivine-diabases, a related series of porphyritic olivine-basalts, and a series of porphyritic amphibolebearing rocks of andesitic habit, but probably to be classed with the camptonites. The three types have a different distribution, and do not appear to be connected together by intermediate gradations; the amphibole-bearing and the olivine-bearing rocks differ in their mode of occurrence. According to existing analyses, the former range in chemical composition from sub-basic to basic, and the latter from thoroughly basic to ultrabasic. All the rocks have a local stamp, but are probably most nearly related to the camptonitic rocks of the Central English Midlands. Intrusion took place at a period not earlier than the Tremadoc, and probably not later than that of the May Hill Sandstone. 2. On the Upper Greensand and Chloritic Marl of Mere and Maiden Bradley in Wiltshire.' By A. J. Jukes-Browne, Esq., B.A., F.G.S., and John Scanes, Esq. The district dealt with is on the borders of Wiltshire and Somerset. The general succession is as follows: Lower Chalk, with Chloritic Marl at the base Do. with siliceous concretions (cherts) Fine grey and buff sands Sandy marlstone Grey marl and clay (Gault) ... Feet. 3 to 8 20 to 24 15 about 120 15 90 The chert-concretions and the sands in which they occur consist very largely of spicules of lithistid sponges. One of the sandstonebeds has yielded several species of Necrocarcinus, and may be the chief source of the crustacea which have been quoted from the Warminster Greensand. Above the chert-beds, and below the horizon at which Stauronema Carteri comes in, is a variable set of beds which include a layer of concretions known as cornstones or popplestones. These beds are very rich in fossils, and include at Maiden Bradley a layer of phosphatic nodules. They contain the Rye Hill fauna of the Warminster Greensand, and it is proposed to call them the zone of Catopygus columbarius. In Southern Wiltshire there is usually a complete passage from this zone into the Chloritic Marl; and as the cephalopoda of this zone are all Chalk-Marl species, the natural inference from the local evidence would be to place the plane of separation between the Selbornian and Cenomanian stages at the base of the C.-columbarius beds. In Dorset, however, the break above these beds is so very marked and strong that the authors think that the beds with the Rye Hill fauna must be retained in the Selbornian. It is one of those cases in which the paleontological and the stratigraphical breaks do not coincide. January 9th, 1901.-J. J. H. Teall, Esq., M.A., F.R.S., President, in the Chair. · The following communications were read:1. The Geology of South-Central Ceylon. By John Parkinson, Esq., F.G.S. In this communication the author endeavours to give some |
