of shape, indicating that the spark-length varies with the current in different ways for different materials so that the material which gives the longest spark with a particular current does not necessarily give the longest spark with another current. Thus with a current of 15 amperes the spark is longest with carbon poles, shortest with platinum ones; but, on reducing the current to 3 amperes, the easily fusible metals bismuth and lead give the longest sparks. Perhaps the most interesting point brought out, however, is in connexion with the behaviour of the different materials on reversal of the current. With the metals iron, copper, tin, nickel, zinc, and with carbon, reversing the current produces no perceptible difference in the spark-length; but with lead, platinum, and aluminium poles the spark-length for the larger currents is notably greater when the fixed pole is negative than when it is positive. Owens College, Manchester. LXIII. Notices respecting New Books. Production et emploie des Courants alternatifs. Par L. BARBILLION. Paris: G. Carré et C. Naud, 1901. ("Scientia " Series, No. 11.) Pp. 103. WE E have already had occasion to review briefly some of the volumes belonging to the "Scientia" series, which is being brought out by the well-known French publishing firm, Messrs. G. Carré et C. Naud. The present volume of 103 pages contains a digest of alternating-current theory. The title of the book is misleading, and it seems to us that "the theory of alternating currents" would have been a much truer indication of the contents of the book than the title selected by the author. The brief discussion of continuous-current machines given on pp. 8-14 seems entirely out of place, and is, of course, very incomplete. On p. 8 we have a diagram (fig. 2) of the magnetic field in a two-pole dynamo which would be excusable in a pre-historic treatise on the subject, but which it is surprising to find in an up-to-date book; no attempt is made to show the true course of the lines and their refraction as they pass from iron to air or vice versa. Taken as a whole, however, the book is a useful and concise exposition of the principles underlying the applications of alternating currents, and contains a good account of several interesting developments of recent date. LXIV. Proceedings of Learned Societies. GEOLOGICAL SOCIETY. [Continued from p. 580.] June 19th, 1901.-J. J. H. Teall, Esq., M.A., V.P.R.S., HE following communications were read : THE : 1. On the Use of a Geological Datum.' By Beeby Thompson, Esq., F.G.S., F.C.S. A proper interpretation of geological phenomena frequently requires that allowance shall be made for differential earthmovements that have taken place since the period under consideration. Present differences of level in rocks of the same age may be due to actual differences in depth of the sea-floor on which they were deposited; but they may also be the result of subsequent differential earth-movements. The rock selected as a datum should combine as far as possible the following characteristics :-It should be thin, of considerable horizontal extension, having similarity in physical characters and paleontological contents over a large area, and situated as near as possible, in vertical sequence, to the reference deposit. In Northamptonshire three formations meet these requirements-the Rhætic Beds, the Marlstone Rock-bed, and the Cornbrash. The author applies the Marlstone Rock-bed as a datum to the study of the five chief deep explorations in Northamptonshire, with the following results :-While the old land-surface (below the Trias) now varies in height by more than 250 feet, the variation in thickness of the rocks between it and the Middle Lias only reaches 56 feet; and although the old land-surface is actually lowest where the Rhætic rocks have not been detected, when compared with the position of the Marlstone it is found to be the highest. The further application of the same method enables the author to recognize Rhætic rocks at Northampton, to correct the record of the Kingsthorpe shaft, and to explain the presence of Triassic saline water in the Marlstone. A revised section of the Kingsthorpe shaft is given. Another point proved is that a general levelling-up process was going on just before the beginning of the Lower Liassic Period, and another at the close of the Middle Liassic Period. 2. 'On Intrusive, Tuff-like, Igneous Rocks and Breccias in Ireland.' By James R. Kilroe, Esq., & Alexander McHenry, Esq., M.R.I.A. Many fragmental igneous rocks, although resembling tuffs, cannot be regarded as ejectamenta on account of their character and mode of occurrence in the field. Rocks of this type occur to the east of Lough Eake in Donegal, in the district of Forkhill in Armagh, at Blackball Head in Cork, in Waterford, near Arklow, in Wexford, and elsewhere. Sometimes they consist of partly fused and broken-up felspathic mica-schist merging into felsitedykes, at other times of brecciated slate, granite, and felsites embedded in a scanty andesitic matrix. At Blackball Head, the rocks cross the bedding of the associated sedimentary rocks of the region. The authors agree with Prof. Lapworth in considering it possible that igneous matter making its way between the moving masses may consolidate as sills when the pressure is great..... As movement progressed intermittently, we should have the formation of subterranean agglomerates, tuffs, and breccias, which would be forced sometimes between bedding-planes, sometimes into dykelike fissures.' A series of sections is exhibited to illustrate how tuff-like masses invade black slate of Llandeilo age in the South-east of Ireland, generally adhering to the direction of bedding, but frequently cutting across it and detaching numerous pieces from the slate, which are more abundant near the margins of the intrusion than elsewhere. The masses frequently assume a tuff-like appearance. At Arklow Rock tongues of tuff-like rock penetrating black slate of Llandeilo age contain pieces of Limestone of Bala age, as well as pieces of the slate. The development of vesicular texture in lapilli-like, contained, fragments may be due to the simple release of pressure. INDEX TO VOL. II. ACOUSTICAL notes, 280. Adams (E. P.) on the electromag- Ether and gravitational matter Amagat (E. H.) on the isothermals Appleyard (R.) on a direct-reading Ashton (A. W.) on a model which Atom, on the electrical structure of Balmer's formula, kinematical analy- Baly (E. C. C.) on the spectrum of Barrow (G.) on the occurrence of Barus (Prof. C.) on the absorption Phil. May. S. 6. Vol. 2. No. 12. Beats, notes on, 283. Beattie (R.) on the length of the Bismuth, on the electrical resistance tial of a symmetrical system, 237. Buchanan (Dr. J.) on magnetic Bullen (Rev. R. A.) on two well- 2 X Burbury (S. H.) on Boltzmann's law Chree (Dr. C.), applications of elastic Convection, on the magnetic effect Coomara-Swámy (A. K.) on the Crémieu (Dr. V.) on the magnetic effect of electric convection, 235. 65. Crystals, on rotatory polarization in 488. Current, on the dependence of the, 210. Cyanin, on the anomalous dispersion Cyanogen, on the spectrum of, 386. Double-refraction in moving viscous Elastic solids, applications of, to Eletric convection, on the magnetic field, on the dependence of the 300. resistance of bismuth, on the, Electrodes, on the drop of potential 616. Electromagnetic effects of moving Emission-function, on the complete, Energy, on the partition of, 416. Fit, on lines and planes of closest, 559. Fleming (Dr. J. A.) on a model Fluids, on the isothermals of, 651. on the 659. Geological Society, proceedings of Groom (Prof. T. T.) on the igneous Gunn (W.) on the geology of the Harmer (F. W.) on the influence of winds upon climate during the Heat, on the dynamical theory of, 1. Hill (J. B.) on the crush-conglome- Hind (Dr. W.) on the beds below Howe (J. A.) on the beds below the some of its compounds, 370; on |