70 



NA TURE 



[May 17, 1894 



magnet on it»annati]re is the same, whether it be excited by a 

 conlinuoas or by an alleroating current of equal strength. 

 For higher inductions the continuous current gives 

 slightly greater force. Another experiment made with 

 solenoids and the U-shaped plunger of a Brush alternate cur- 

 rent arc lamp, gave similar lesults. In considering the question 

 of winding alternate current electromagnets so as to obtain a 

 given excitation when the current is supplied at constant 

 voltage, it is shown that the ampere-turns are inversely pro- 

 portional to the number of luins, for the impedance varies 

 nearly as (he square of the number of turns. One important 

 properly of such electromagnets when supplied at constant 

 voltage is that ihey give a fairly constant pull over a long range, 

 for as the arm.iture moves away from the magnet the current 

 increases, thus counteracting to some extent the eftect of dis- 

 tance. On the other hand, the allemating voltage required to 

 obtain a given force is much greater than that needed with con- 

 tinuous currents. With the armature in contact with the core 

 the ratio of the two voltages was found to be 170, whilst 

 separating them by 92 mm. reduced the ratio to 21 5. Prof. 

 Perry pointed out that the constant pull of the alternate current 

 magnet followed immediately from the fundamental equ,ation 

 e = TC ■'r «I when re is small, for if c be constant then I and 

 therefore 1 and I- are constant. He was interested to see that 

 the pulls for equal ampere-turns were the same, and indicated 

 how the problem could be worked out mathematically when 

 hysteresis was taken into account. Mr. Ulakesley thought it 

 belter to fill the space on a magnet full of copper, rather than 

 use wire only just large enough to carry the current, for the 

 loss of energy would be reduced. Some of the formulae given 

 might be put in simpler form. Mr. Swinburne said an alternate 

 current magnet would only give constant pull in special cases. 

 As another rule for winding alternate current magnets, he said, 

 wind the magnets as for continuous currents, and put a condenser 

 in to take the same current, thereby reducing the voltage re- 

 quired. Mr. Blakesley said it would require a capacity of 600 

 microfarads lo suit the magnet mentioned in the paper. Dr. 

 Thompson, in reply, agreed with Mr. lilakesley that the con- 

 denser was impiaclicable in many cases. — Major R. L. 

 Hippisley, K.E , read a paper on a graphical method of con- 

 structing the curves of current in electromagnets and trans- 

 formers, ar.d exhibited a machine for drawing these curves. 

 Taking the ordinary equation for a simple a'ternate current 



circuit E sin // - -^ = Ri, where B is the total magnetic flax, 

 and t the current ; the author writes it in the form 



E sin // ■ 





where L is the cotfTicient of self-induction of the circuit with the 

 iron withdrawn and «n >he tangent of the inclination of the BH 

 curve at the point corresponding to the instant considered. 

 This cqualicn is inlegraKd for a short part of the C}cle during 

 which m may be considered constant giving 



E - **' 



i = j^ cos »» sin (// - «») -f An '"*•'■ 



Melhcds of finding the ccnslanis in the last equation arc then 

 described, and the method of drawing the current curves step 

 by step opiaincd. The machine for [lerforming the process is 

 illuslialcd in the paper, and the curve for an electromagnet fed 

 with allemating current shown. The case of a transformer 

 wilhonl magnetic leakage is worked out at some length, and 

 the curves rjl primaiy and secondary current deleimlr.ol. Dr. 

 firmi ner said the author had used great care in working out a 

 dlfhculi prcblim more C( mpletely than usual. In case^ such as 

 aii.'e in practice the Ki itim is small, and for this case Evcrshed 

 thowed how to determine the current curves of a transformer 

 when the periodic state had been reached, some five or six years 

 ago. He (Dr. .Stmpncr) had also shown how to graphically 

 determine the current curves for circuits containing iron in 1S88. 

 The author's method was. however, of more general application. 

 Mr. Trotter inquired if whether the machine could be used in 

 practical alleinate current problems, say, for example, to pre- 

 deteiminc the E. M.F. curve of an alternator. Major Hippisley 

 replied to the point.'* raised. 



Geological Society, April 25.— Dr. Henry Woodward, 

 F.K.S., I'residcni, in the chair. — Mr. A. R. Sawyer, referring 

 lo spccimcnt exhibited by him from the Transvaal, Orange Free 



NO. 1281, VOL. 50] 



I 



State, Cape Colony, Masbonaland, and Matabeleland (the last 

 mentioned collected during the recent war), remarked that 

 gneisses and gneissose granites cover a large portion of Mashona- 

 land, together with patches of schistose rocks and a few strati- 

 fied rocks. He drew attention to the faniastic shapes .assumed 

 on weathering by the granitic gneiss, which he considered solely 

 due lo atmospheric agencies, and not to ice-action or lo ihe eflects 

 of submersion. The schistose rocks are, for the most part, 

 sheared and altered igneous masses. There are numerous 

 examples of dolerites and epidiorites passing into hornblende- 

 schists, and of more acid igneous rocks. Masses of magnetite 

 occur in various parts of Mashonaland, and serpentinous rocks 

 (which probably owe their origin to the alteration of peridotites) 

 in the north-west corner of the Victoria gold-field. Extremely 



\ auriferous veins occur amongst the sheared acid igneous rocks 



. of the Umhungwe Valley in the Manica district, and gold occurs 

 in the kaolin produced by the disinlegr.ition of these rocks.— 

 The following communic.itions were read : — Eurlher notes on 

 some sections on the new railway from Romford to Upminster, 



I and on the relations of the Thames N'alley beds to the boulder 

 clay, by T. V. H jimcs. The author alluded to his discovery of 

 boulder clay on this new railway at Horiicharch {Quart. Journ. 

 Geol. Soc. August 1S92), and described the finding of more 

 boulder clay close to Romford during the deepening and widen- 

 ing of a cutting there. The boulder clay was on precisely the 

 same level as that at Hornchurch, a mile and a half to the 

 south-east, and, like il, was covered by gravel belonging to the 

 highest, and presumably oldest, terrace of the Thames Valley 

 system. A portion of the silted-up channel of an ancient 

 stream-course was also found in this Romford cutting. Its re- 

 lations to the boulder clay could not be seen, as they were not 

 in contact, but Ihey were alike covered by the oldest gravel be- 

 longing lo the Thames Valley system. The author discussed 

 the probable direction of the How of this stream-course, and the 

 way in which it was superseded by the ancient Thames. After 

 noticing certain points brought forward during the discussion on 

 his former paper, he concluded with a criticiim on the vic*vs to 

 which Dr. Hicks inclines in his paper on the sections in and 



; near Endsleigh Street (Quart. Journ. Geo!. Sac. vol. xIviiL 

 1892) as regards the age of those beds, asierting that ihey are, 

 in all probability, simply river drift of the Thames Valley system, 

 and consequently post-glacial, in the sense of being later in date 

 than the boulder clay of Essex .and Middlesex. ^On the geology 

 of the Pleistocene deposits in the v.xlley of the Thames at 



j Twickenham, with contributions to the flora and fauna of the 

 period, by Dr. J. R. Leeson and G. B. LafPan. The section 

 described in this paper w.ts exposed during the construction of 

 an effluent from the Twickenham sewage-works to the Thames. 

 Its length was about one mile. The beds exposed were (1) 

 coarse reddish-yellow gravels, coloured blue below, lying on an 

 eroJed surface of (2) dark blue loam, varying in thickness, the 

 greatest thickness seen being three feet, at a place wlicic the 

 bottom was not reached ; (3) dark sand ; (4) coarse ballast 

 gravel ; (5) London clay. The loam (which is quite a local 

 deposit) yielded eight species of niollusca and fourteen spci 1 

 of plants, all still living in the neighbourhood. A numbci 

 mammalian bones, referable to seven species, were lying just >in 

 the surface of the loam. Amongst the forms were bison ani 

 reindeer. The authors consider that the loam was deposited in 

 a small lake, and they alluded to similarities between it and a 

 deposit described by Dr. Hicks as occurring in the Endslcigli 

 Street excavations. In Ihe remarks on these two papers, the 

 President congratulated the authors of the second paper on| 

 having succeeded in rescuing so interesting a collection of re- 1 

 mains of Thames Valley mammalia. Sir John Evans expressed t 

 his pleasure at .Mr. Holmes's further discovery of evidence wto , 

 the superposition of the old Thames Valley gravels upon the 

 boulder clay, as these discoveries supported the view he bad 

 always held that these gravels, whether at a high or at a low 

 level, were " post-glacial " in Ihe sense indicated by the author, 

 lie also remarked that the finding of Ihe mainmaliaii remains 

 by Dr. Leeson in the low-level gravels at Twickenham was of 

 interest, as proving the existence of Ihe reindeer and bison in 

 the Thames at the lime of the deposition of these beds. As lo 

 some of the remains of olher animals, however, he enleilaincd 

 doubts whether, though found in the course of the excavation, 

 Ihey really belonged lo the gravels. Mr. E. T. Newton, Mr. 

 Lewis Abbott, Mr. G. B. Lallan, Prof. Hull, and Dr. Leeson 



also spoke, and Mr. T. V. Holmes briefly replied to the remarks 

 made on his paper, — On a new goniatite Irom the lower coal 



