October 20, 19 10] 



NATURE 



515 



soldered to the wire. The vacuum-tight seal is produced 

 between the inner surface of the elastic steel tube, which 

 on cooling is put under tension, and the glass, which 

 come> under compression. .Seals with wires of i mm. 

 diameter have been produced in this way. 



Dr. T. H. Havelock followed with deductions from the 

 relations between densities and refractive indices. Dr. 

 \. Li. Webster (of Worcester, Massachusetts) gave a 

 detailed account of a complete apparatus for the measure- 

 ment of sound. The producer of the sound was a steel 

 diaphragm rigidly driven by an electrically maintained 

 runing-fork, and made the back of a resonator of the 

 form of a small hollow chamber or of a tube of variable 

 length. The reaction of the sound upon the amplitude of 

 the fork enables the constants of the resonator to be 

 accurately determined, so that the rate of emission may 

 be measured in watts. The phonometer Cor measurer of 

 the sound) is a glass diaphragm, made the base of a 

 resonator, and bearing a light mirror, which constitutes 

 one mirror of a Michelson interferometer. The displace- 

 ment is measured stroboscopically by a telescope, and the 

 amplitude of the pressure change is read off on a scale in 

 dynes/cm.'. The instrument is as sensitive as the ear 

 for a pitch of 256 vibrations per second, and an accuracy 

 within 10 per cent, is claimed. 



In the short discussion aroused by this paper some 

 doubt seemed to be felt of the agreement of results given 

 with those obtained by Lord Rayleigh, but in the absence 

 of trustworthy memories the point was left unsettled. 

 Questioned as to the power exerted while he himself had 

 been speaking, Prof. Webster left it to be inferred from 

 the statement that ten million cornets (each of which 

 could be heard half a mile away) emitted at the rate of 

 a horse-power. 



Prof. W. M. Hicks followed with a paper on the rela- 

 tion of spectra to the periodic series of the elements. In 

 this he de.scribed some results recently obtained bv him 

 in a critical study of the spectral series of the second and 

 third groups of the periodic table of elements, more 

 especially their dependence upon the atomic volume of the 

 element. \'alues for atomic volume or of density very 

 close to observational values can be deduced in the case 

 of the first three groups of the periodic series. .Applying 

 the method to the spectrum of europium as given by 

 E.xner and Hashek, a density of 13.1 was predicted for 

 that element. Sir Norman Lockyer, discussing the paper, 

 emphasised the importance of a study of series, especially 

 in regard to the stars. In a paper on the series spectrum 

 of the mercury arc. Dr. S. R. Milner gave the results of 

 a photographic study of the mercury arc m vacuo, thus 

 avoiding tlie faint continuous background which fogs the 

 plate when the arc is observed in air ; very much longer 

 exposures can for this reason be given, and many new 

 lines are then observed, among which the lines forming the 

 continuation of the various series of mercury were 

 strikingly developed. Measurements have been made up 

 to the fifteenth line in the diffuse series and the thirteenth 

 of the sharp series. Rvdberg's difference law is exactly 

 satisfied. 



Mr. .\. E. Oxley described an apparatus for a pro- 

 duction of circularly polarised light obviating the lateral 

 displacement of the beam produced by Fresnel's rhomb, 

 but possessing the advantages of this rhomb in other 

 resoects. Two similar rhombs of glass are placed end to 

 end in contact so as to form a heni double rhomb. By 

 suitably choosing the dimensions the required object is 

 attained. The angle of the rhomb chosen is 74° 38-2' for 

 glass of index 1-5035. The length (15 cm.) is, however, 

 inconveniently long. .\ much shorter form, in which three 

 reflections of the light occur, is obtained by putting in 

 close contact two trapezium-soctioned orisms, the faces 

 in contact being those containing the shorter of the two 

 parallel edges of the trapezium. The relative retardation 

 can be made in+vh. and the emergent light will then 

 he circularly polarised. The apparatus can advantageously 

 r-nlacp a quarter-wave plate in the quantitative studv of 

 elliptic vibrations. Mr.- Oxley also described a new half- 

 shade analyser, consisting of a double rhomb of glass of 

 suitable angle to produce a retardation of 37r, which is 

 equivalent to —tt. 



.'ipction G (Engineering) then joined to participate in 

 a discussion on the principles of mechanical flight, opened 

 NO. 2138. VOL. 84] 



by Prof. G. H. Bryan. This discussion wandered away 

 from the title, and developed into one on the relative 

 positions of the mathematician and the practical engineer 

 in the origination and development of new ideas on the 

 subject of aviation. The engineers were present in 

 strength, and the evidence brought forward from the 

 parallel case of motion on water seemed to be greatly in 

 their favour. Of the points more particularly bearing 

 upon the advertised title of the discussion we may mention 

 Mr. Dugald Clerk's advocacy of much lighter engines 

 and of a considerable modification of the usual thermo- 

 dynamic cycle. He recommended differences of pressure 

 of two to one instead of five or more to one as at present. 

 Mr. Scoble's account of his propeller tests was also of 

 much value. 



The proceedings of Tuesday, September 6, were opened 

 with a discussion on atmospheric electricity, initiated by 

 Dr. Charles Chree. He explained that atmospheric elec- 

 tricity includes a great variety of phenomena. Omitting 

 aurora as a subject so large as to require a separate dis- 

 cussion, there are the questions of the potential gradient 

 in the atmosphere, the influence of potential gradient and 

 electrical charges on the growth of vegetation, the pheno- 

 mena of thunderstorms, the loss the charge experienced 

 by insulated bodies, the number and nature of the positive 

 and negative ions in the air, the vertical earth-air currents, 

 and the phenomena of radio-activity. The potential 

 gradient and its diurnal and annual variation have been 

 the subject of study for a good many years, and we know 

 that the phenomena vary largely with the season of the 

 year at any given station, and that there are notable 

 differences between different stations at ground-level. As 

 yet but little is known of the diurnal and annual changes 

 at different heights in the free atmosphere. Observations 

 made near the top of the Eiffel Tower suggest that the 

 phenomena alter rapidly as the height above the ground 

 increases. Thus observations from balloons and kites, if 

 these could be maintained at a fixed level, would be of 

 great importance. The influence of electricity on the 

 growth of plants, first seriously studied by Prof. 

 Lemstrom, seems not unlikely to prove in the future to 

 be of economic importance. The phenomena of thunder- 

 storms have received considerable attention from meteor- 

 ologists, but many lines of investigation present them- 

 selves. The loss of charge of insulated bodies and the 

 ionic charges in the atmosphere have been studied most 

 in Austria and Germany — Elster and Geitel, in particular, 

 having done much pioneer work. In this country Mr. 

 C. T. R. Wilson has investigated the electric charge 

 brought down by rain, and has invented an instrument 

 for measuring the earth-air current. While a great 

 number of theories have been advanced to account for the 

 several phenomena, there are few, if any, of them which 

 command anything like universal acceptance. 



Sir Oliver Lodge referred to the existence of a positive 

 gradient of potential during fine weather and a negative 

 gradient during wet weather, and spoke of the possibility 

 of influencing the weather by varying the potential 

 gradient, and thought that rain might be produced in this 

 way, and that here was a field of experiment for the 

 enterprising capitalist. Referring to the effect of electricitv 

 on plant life, he explained that the experiments with 

 which he had been connected showed that in dull weather 

 the plants were stimulated by the electricity, and the effect 

 was good, but in bright, sunny weather they were over- 

 stimulated, with consequent bad results. He did not think 

 that the action on plants was due to nitrification, but to 

 some effect on the growing tips. 



Dr. .Shaw pointed out that in order to produce rain 

 in considerable quantities it would be necessary to find 

 a source for an enormous amount of energv, and he urged 

 that anv attempt to reverse the electric field should be first 

 of all thoroughly worked out in the laboratory. He asked 

 whether fine weather electricity had anything to do with 

 thunderstorms. The two phenomena seemed to be distinct 

 from one another. 



Sir J. J. Thomson thought the real difficulty about 

 producing rain by electrical methods would be political. 

 If one's efforts resulted in deluging neighbouring countries 

 with rain they did not want, or in depriving them of their 

 normal supplv, difiiculties would begin. .As he understood 

 Sir Oliver Lodge, it was not a question of supplying 



