June io, 1920] 



NATURE 



473 



president of . the Royal Horticultural Society ; Prof, 

 j. Bretland Farmer; Prof. Keeble ; Lady Northcliffe; 

 and \'iscountess Falmouth, chairman of the governin{4 

 l)(»dy, Swanley College. Donations may be sent to 

 Messrs. Child and Co., i Fleet Street, London, E.C.4. 

 Ten thousand pounds is needed at once, and 50,000/. 

 for the complete installation of the science depart- 

 ment* and for the reconstruction of the college and of 

 the intensive training grounds. 



Societies and Academies. 



London. 

 Physical Society, May 14.— Sir W. H. Bragg, presi- 

 dent, in the chair. — Dr. F. Lloyd Hopwood : Experi- 

 ments on the thermionic properties of hot filaments. 

 The experiments shown were some of those described 

 by Dr. Hopwood in the Philosophical Magazine for 

 -March, 1915, p. 362, in which the glowing filament of 

 a carbon lamp and glowing filaments of nichrome 

 and platinum in air are made to move under the 

 influence of positively and negatively charged rods 

 brought into or withdrawn from their vicinity, the 

 character of the effects observed being such as to give 

 a qualitative indication of the thermionic emission 

 from the filaments. In addition, he showed a type of 

 tilted electroscope in which the gold-leaf was re- 

 placed by a narrow loop of WoUaston wire. When a 

 current is passed through the wire so as to make it 

 glow, it forms an electroscope of different sensitivitv 

 for +ve and — ve charges. — G. D. West : A modified 

 theory of the Crookes radiometer. The paper gives a 

 short account of a theory of the Crookes radiometer 

 w orked out by Sutherland in 1896, but, unfortunately, 

 much neglected. The theory as it stands will not 

 explain many radiometric phenomena, but it is shown 

 that when modifications depending on the modern 

 knowledge of thermal surface conditions are made, 

 such explanations become jxissible. Radiometer 

 action, especially at the higher gas pressures, would 

 appear to depend essentially on the formation of gas 

 currents near the radiometer vane. These currents 

 are distinct from convection currents, but are closely 

 associated with the phenomena of thermal transpira- 

 tion. — .\. Campbell : The magnetic properties of silicon 

 iron (stalloy) in alternatini' mairnetic fields of low 

 value. Measurements are described of the hvsteresis 

 losses in silicon iron sheet and wires in verv low 

 alternating mag"netic fields at low and telephonic 

 frequencies, using' an alternating-current method 

 described in a former paper. The equations giving 

 the hysteresis losses as a function of B„,„, are deduced 

 in the case of the sheet material at low frequencies for 

 ranges of H„„^. from 00002 to 002. Comparisons are 

 made between sheet material and wires of different 

 diameters, and curves are given showing the great 

 improvement in the permeability of wires when thev 

 are annealed. The behaviour of the material is studied, 

 both bv ballistic tests and at telephonic frequencies, 

 as regards the alternating field when direct-current 

 fi.elds of various values are applied at the same time. — 

 T. Smith : Tracing? ravs through an optical svstem. 

 F.quations for tracing rays in an axial plane through 

 an optical svstem have the normal refraction terms 

 separated from those representing aberrations. By 

 expressing the latter as a fraction with the first-order 

 aberration as the numerator and a correcting factor, 

 which mav take various forms, as the denominator, 

 rays may be traced exactlv through the system, using 

 a short table of cosines in terms of sines in place of 

 the extensive tables, p^iving sines in terms of angles 

 generally employed. .\ considerable saving of time is 

 effected in the ^calculations, and the estimation, without 

 calculation, of the aberrations of other rays is 

 facilitated. 



NO. 2641, VOL. 105] 



Geological Society, May 19.— Mr. R. D. Oldham, 

 president, in the chair. — Dr. H. H. Thomas, with 

 chemical analyses by E. G. Radley : Certain 

 xenolithic Tertiary minor intrusions in the Island 

 of Mull (.\rgyllshire). The paper deals with a 

 series of minor intrusions, generally tholciitic but 

 occasionally composite in character, which are well 

 represented in the western peninsula of Mull, lying 

 between Loch Scridain and Loch Buie, and are 

 remarkable for the number and mineralogical pecu- 

 liarities of the xenoliths that they contain. Xenoliths 

 of a highly siliceous nature (quartzites, sandstones, 

 etc.) are met with, but more commonly the inclusions 

 are of a type rich in alumina (shales and clays). 

 Cognate xenoliths of noritic and gabbroic affinities 

 occur in several of the intrusions, and these, together 

 with the accidental siliceous xenoliths, are briefly 

 described ; but the communication deals more par- 

 ticularly with the aluminous inclusions which are 

 crowded together in most of the intrusions, range up 

 to several feet in diameter, and are characterised by 

 well-crystallised minerals such as sapphire, spinel, 

 sillimanite, cordierite, and anorthite. These xenoliths 

 offer the clearest evidence of the modification of a 

 more or less pure aluminous sediment by permeation 

 of magmatic matter, more particularly by the diffusion 

 of lime, ferrous iron, and magnesia. It is held from 

 the evidence afforded by the xenoliths that the meta- 

 morphism is of a deep-seated character, and has been 

 effected bv a tholeiitic magma on the walls of its 

 basin, which were composed mainly of aluminous 

 sedimentary rocks. 



Cambridge. 



Philosophical Society, May 3. — Sir Ernest Ruther- 

 ford, vice-president, in the chair. — W. J. Harrison : 

 Notes on the theory of vibrations, (i) Vibrations of 

 finite amplitude. (2) A theorem due to Routh. Ray- 

 leigh determined, in trigonometric form, the approxi- 

 mate effect of small terms varying as the square and 

 cube of the displacement in the equation of simple 

 harmonic motion. In the former of these notes exact 

 Fourier series are determined by the theory of ellip- 

 tic functions, and tables are computed. > The latter 

 note relates to the theorem that an increase of inertia 

 of any part of a vibrating system increases all the 

 periods in such a way that the new periods are 

 separated bv the original periods. If the effect of 

 the increased inertia be represented by an addition to 

 the kinetic energy of the square of a linear function 

 of the velocities, it is pointed out that the theorem does 

 not hold unless this" linear function involves all the 

 velocities. — W. Burnside : On cyclical octosection. 

 The complete solution of the problem of cyclical 

 quartisection was first given by V. .'\. Le Besgue in 

 Comptes rendus. vol. li., i860, without proof; he 

 forms the quartic eauation satisfied bv the sum of 

 iip-i) distinct primitive *th roots of unity, p being a 

 prime number of the form 4^+1. If p = l.' + iW, 

 where L=i (mod. 4), the equation involves p and L, 

 being 



where y is one more than four times the sum in ques- 

 tion. The onlv proof as yet published appears un- 

 necessarily long. The present paper deals with the 

 case when * is a prime' of the form 8n+i; it forms 

 and solves the equation satisfied by the sum of a(/>-i) 

 distinct primitive' * roots of unity, which is capable 

 of eif^ht values, bv a method capable of extended ao- 

 plication. Expressing p in both the forms n' + b', 

 a'^ + 2h", this equation involves *. a-, and a'. — Dr. 

 G. F. C. Searle : (i) A bifilar method of measuring the 

 rigiditv of wires. The uoper ends A, C of two equal 



