562 



NA TURE 



[October 4, 1900 



PHYSICS AT THE BRITISH ASSOCIATION. 



THE interesting way in which Dr. Larmor, in his Presidential 

 Address to Section A, touched on some of the problems of 

 theoretical physics appears to have had a considerable influence 

 on the subsequent proceedings of that section during the meeting 

 at Bradford. At few recent meetings has the number of im- 

 promptu discussions of theoretical questions been so great, and 

 even although these discussions may not always have been ended 

 in the settlement of some question previously in dispute, they 

 have provided in a way that only the Association meetings can, 

 opportunities for exchanges of opinions so necessary in these 

 days of specialisation, and so valued by those who have the 

 advance of their subject at heart. 



The large section room was well filled for the President's 

 address. After a vote of thanks moved by Prof. FitzGerald 

 and seconded by Principal Oliver Lodge had been carried, 

 a large proportion of the audience left to hear the address of 

 Prof. Perkin, the President of Section B, and the reading of 

 papers commenced. In what follows they aie given in order of 

 subject and not of reading. 



Dr. Trouton gave a short account of his experiments on the 

 creeping of liquids, and on the surface tensions of mixtures. 

 He has found that the tendency of certain liquids to creep up 

 the sides of their containing vessels is due to such liquids being 

 mixtures. The more volatile constituent creeps in advance of 

 the other, and the action is stopped if evaporation is prevented. 

 Zinc surfaces seem more favourable to the process than surfaces 

 of other metals or of glass. The surface tensions of mixtures 

 of liquids are as a rule less than the values calculated from the 

 surface tensions and proportions of their constituents, while 

 those of salt solutions increase with the number of gram 

 equivalents of the salt present at a rate nearly independent of the 

 nature of llie salt, a fact to which Quincke was the first to draw 

 attention. 



Prof. G. PI. Bryan, in a note on the partition of molecular 

 energy, explained how, in his endeavour to build up irreversible 

 thermal phenomena from reversible dynamics, he had been led 

 to a novel method of investigating the mean distribution of 

 energy amongst a number of particles moving in an external field 

 having a potential. He found that two such particles do not 

 follow Maxwell's law of partition of energy, and concluded 

 that the law would not be followed in a general assemblage of 

 particles. Prof. FitzGerald considered that two particles in an 

 external field did not sufficiently represent the molecules of a 

 gas, and suggested that if the case of three particles had been 

 worked out, they would have been found to follow Maxwell's 

 law. He hoped that physicists would accept that law as valid 

 for gases till a system had been constructed for which it could 

 be proved conclusively not to hold. Prof. Bryan, on the other 

 hand, challenged physicists to construct a simple system of 

 particles which would tend towards Maxwell's distribution of 

 energy. 



Dr. Larmor gave some results of his application of the 

 principle of least action to the statistical dynamics of gas 

 theory, as illustrated by meteor swarms and optical ray 

 systems. He finds that if a swarm of meteors is moving 

 under its own mutual attractions and conservative outside forces, 

 and if from some point vectors be drawn equal and parallel to 

 the velocities of the meteors, the product of the volume marked 

 out by the ends of these vectors into the volume occupied by 

 the meteors themselves will remain constant throughout the 

 motion. If the mutual attractions are insensible, the product 

 of the solid angle bounded by the velocity vectors into the 

 square of the mean velocity of the swarm will remain constant. 

 In optics this corresponds to the concentration in cross section of 

 a beam being proportional to the solid angular divergence of 

 the beam, into the square of the refractive index of the medium 

 in which it is travelling. In the case of a gas where encounters 

 Ijetween the particles may take place, the above distribution of 

 particles and velocities is found to be a possible steady state. 



Tne report of the Seismological Committee was presented by 

 the secretary, Mr. Milne. During the past year he has 

 analysed the records of the earthquakes which occurred during 

 1899, and has found that the earthquake wave takes about 

 no minutes to travel from its origin to the opposite end of the 

 earth's diimeter, but whether it is propagated through the 

 centre of the earth or as a surface wave cannot at present be 

 decided. He suggests that earthquakes may be connected with 

 the small changes of latitude knowm to occur, and that earth- 



NO. 1614, VOL. bl\ 



quake waves may have a disturbing effect on the timepieces of 

 observatories. Messrs. Clement Reid and Horace Darwin are 

 engaged in an attempt to detect movements at a geological fault 

 owing to earthquakes. 



The Committee on the sizes of pages of periodicals reported 

 that it had succeeded in some cases in inducing societies pub- 

 lishing proceedings of exceptional sizes to conform to the rules 

 the committee laid down in its 1895 report. It did not seek 

 re-appointment. 



A paper on the relation of radiation to temperature was con- 

 tributed by Dr. Larmor. The late Prof. Balfour Stewart pointed 

 out at the meeting of the Association in 187 1 that if an enclosure 

 at constant temperature contained a moving body at the same 

 temperature, the radiation received from the body at a point in 

 advance would, by Doppler's principle, differ from that received 

 by a point behind the body. Dr. Larmor applies this principle 

 to the case of a spherical enclosure shrinking in size, in which, 

 therefore, the wave-lengths of all radiations will decrease as tlie 

 radius. Further, there will be a pressure of the radiation on the 

 inside surface of the sphere, which will require work to be per- 

 formed during the shrinkage. This work is converted into radi- 

 ation, and changes the temperature of the radiation inversely as 

 the radius, and the energy of the radiation inversely as the fourth 

 power of the radius. From this Stefan's law that radiation is 

 proportional to the fourth power of the temperature follows ; 

 and further, the energy of radiation between A. and K+d\ is of 

 the form A~^/(aT).SA. Prof. FitzGerald pointed out the great 

 simplification which Dr. Larmor had introduced into the treat- 

 ment of the problem by the consideration of the radiation in the 

 ether only, a method of which the legitimacy could not be 

 doubted. 



Dr. S. P. Langley sent over from America a chart of the 

 infra red spectrum from 7 to 5 3 fi, obtained by the bolometric 

 method described in his communication to the Association at 

 Oxford in 1894. His bolometer is now arranged so that a 

 difference of temperature of one-millionth of a degree centigrade 

 is detected ; and the whole operation of producing the charts is 

 automatic. They show distinctly the variation of atmospheric 

 absorption with the seasons, and may possibly, he thinks, lead 

 to a new method of weather forecasting. 



The Committee on Meteorological Photography reported that 

 as the result of about 400 photographic observations of clouds 

 made from two stations near Exeter, the following mean heights 

 have been found : — Cirrus, 10,200 ; cirro-cumulus, 8600 ; 

 cumulus top, 3000; base, 1300; strata-cumulus, 2200 metres. 

 During the early part of the day the clouds rise, attain their 

 maximum altitudes about 2 or 3 p.m., and fall during the after- 

 noon and evening. The greatest altitudes are associated with 

 thunderstorms and the lowest with cyclones. 



The Committee on Solar Radiation reported that experiments 

 had been made under the direction of Prof. Callendar, with 

 a view to testing the modified copper-cube actinometer and 

 reducing its records to an absolute scale. During the course of 

 these experiments it has been found necessary to introduce 

 further changes, and the instrument now used consists of 

 a blackened copper disc provided with thermojunctiohs, 

 suspended within a tubular water jacket around which a 

 stream of water at constant temperature is maintained. The 

 radiation to be measured passes down the tube and falls 

 normally on the copper disc. This instrument has been tested 

 by exposing it to the radiation from an electric lamp at a known 

 distance, and has been found capable of giving consistent results 

 for weak radiation, but the intensity of solar radiation is too 

 great to permit the elementary theory of the instrument to be 

 applied. It is, therefore, proposed at present to record only the 

 vertical component of the radiation from sun and sky by means 

 of the bolometric method described in the 189S report. Two 

 flat platinum thermometers, one bright and the other blackened, 

 are placed horizontally side by side and exposed to the radiation 

 from the sky. Their difference of temperature is automatically 

 recorded, and is taken to be proportional to the radiation to 

 which they are exposed. By means of an observation with an 

 electric lamp at a fixed distance, the indications of the instrument 

 can be converted into absolute measure. 



Mr. A. S. Davies described a novel form of mercurial baro- 

 meter, in which a fixed volume of the gas the pressure of 

 which is to be determined, is compressed isothermally by a 

 column of mercury of known length and the compression 

 measured. The instrument consists of a glass bulb, from which 

 a tube of small bore projects downwards, and ends in another 



