that they force their way into the continent in high latitudes and actually prevent the formation of anticyclones and their concomitant low temperature. These conditions lead to mild winters in Canada.

The Meteorological Service is investigating as to whether there is any connection between the temperature and position of the Japan current and the behaviour of these cyclonic areas.

14. Prof. A. H. COMPTON.-The Quantum Theory of the Scattering of X-rays.

15. Prof. J. A. GRAY.-Scattering of X- and Gamma-rays and the Production of Tertiary X-rays.

1. Experiments with X-rays show that the proportion of scattered rays of longer wave-length than the primary is independent of the crystalline structure and thickness of the radiator.

2. The rather peculiar intensity distribution curves of unmodified and modified scattered radiations suggest that they are not really independent types. 3. Results on the scattering of y-rays do not altogether agree with the quantum theory of scattering.

4. The tertiary' X-rays discovered by Clark and Duane must be formed in the atoms in which the photoelectrons are produced. They may even be a special type of scattered rays.

5. If X-rays consist of quanta, they should have a range. [If this is the in the writer's opinion the wave theory should be abandoned.]

case,

6. The writer, while aware of the difficulty of explaining certain results by the wave theory, believes that other results cannot be explained without its The quantum theory cannot explain interference.

aid.

16.

Prof. W. DUANE.-On Secondary and Tertiary Radiation.

This paper gives the results of recent experiments on secondary and tertiary radiation carried on in our laboratory by Doctors Allison, Clark, and Stifler. The experiments with an X-ray tube in one large room radiating through a hole in the wall into a second room that contains the X-ray spectrometer indicate the presence of scattered radiation, fluorescent radiation, and tertiary radiation only, in the beam of X-rays coming from a secondary radiator. The tertiary radiation has the short wave-length limit that would be expected if it were due to the bombardment of photo-electrons. No other radiation comparable in intensity with these three types appears in the spectra. In case the X-ray tube lies in a small box, the influence of the walls of the box on the spectra is discussed.

Experiments also are described illustrating the reflection by a crystal of X-rays characteristic of the chemical elements in the crystal itself, and the precautions necessary to procure photographic evidence of this characteristic reflection are mentioned.

By placing sheets of copper in the path of the X-rays incident on the crystal sufficient in thickness to cut off practically all the radiation having wavelengths of the characteristic line spectra of the crystal, it has been shown that this phenomenon cannot be regarded as an increase in the reflecting power of the crystal for certain rays already existing in the primary beam, but must be due to the fluorescent radiation generated in the crystal itself by X-rays of much shorter wave-lengths.

17. Prof. J. C. MCLENNAN, F.R.S.-Recent Developments in Low Temperature Research.

(A visit to the Cryogenic Laboratory, by kind invitation of Prof. McLennan, was paid during the meeting. Liquid helium and the luminescence of solid nitrogen, among other things, were exhibited.)

18. Joint Discussion with Section G on Optical Determination of Stress.

COSMICAL PHYSICS SUB-SECTION.

19. Mr. F. J. W. WHIPPLE.-The Diurnal Variation of Pressure: Facts and Theories.

The regular oscillation of pressure shows remarkable regularities all over the globe, and it is, therefore, probable that it is connected in a simple way with its cause. The object of this paper is to emphasise the fact that there is an opening here for speculation as well as for more analysis of the records. The preparation of critical tables of pressure at places where barographs have been maintained for long periods requires international co-operation. Observational material is exceptionally rich in the British Isles, where a number of photographic barographs properly compensated for temperature changes and with open time-scales have been in operation for more than fifty years. The British records indicate that the average diurnal variation of pressure for a given time of year can be regarded as due to the combination of a local wave (a pure sine-curve) and a planetary wave. The planetary wave is not a pure sine curve; the changes in its form conform closely to changes in the sun's declination. It is pointed out that these facts are difficult to reconcile with Lord Kelvin's resonance hypothesis, and in conclusion other objections to that hypothesis are also mentioned.

20. Prof. W. J. HUMPHREYS.-The Relation of Wind to Height.

:

On the average, perhaps, and especially on the equatorial side of cyclones, the wind varies as follows with height Increases rapidly, but decreasingly so, with height up to 400 to 500 metres above the surface; then decreases slightly through, say, 300 metres; after this increases a little, and then remains, roughly, constant up to round 2,000 to 3,000 metres above the surface; here again often slightly decreases; and then through the next several kilometres increases in proportion to decrease of density. Directions of the wind and its temperature also are interestingly related to height above the surface.

All these observed facts are plausibly explained as effects of mechanical and thermal turbulence.

21. Mr. J. BJERKNES.-The Importance of Atmospheric Discontinuities for Practical and Theoretical Weather Forecasting.

Empiric investigations show that new-formed depressions usually consist of two oppositely directed air currents, the one warm and the other cold. Initially each current occupies about one-half of the region covered by the depression. The area of the cold air is, however, always increasing, and finally it embraces the whole of the depression in the lower layers. The warm air covers at the ground a correspondingly decreasing space (the warm sector). During the development of the depression, air from the warm sector will escape upwards and spread in higher layers. This motion involves a transformation from potential into kinetic energy (strengthening of the wind and deepening of the depression). The kinetic energy of the depression decreases again as soon as there is merely cold air supply available for the ascending motion. The temperature distribution in the depression thus gives useful indications concerning the expected development.

The result may be formulated mathematically as an equation giving the acceleration of the different air masses relatively to each other. One may thus, at least theoretically, arrive at a mathematical forecast, provided that sufficient observational data are at hand. This is exemplified in a depression passing Central Europe on February 1, 1923.

22. Mr. L. F. RICHARDSON.-Turbulence and Temperature-gradient among Trees.

The writer has previously derived from theory a criterion for the increase of turbulence, applicable at a height in the free air great compared with the irregularities of the ground. By contrast the present investigation relates to

observations made among trees. The temperature gradient was measured by a pair of thermo-junctions placed at different heights. This is compared with ⚫ the gustiness as shown by a Dines pressure-tube anemometer.

23. Dr. H. JEFFREYS.-Tidal Friction.

24. Discussion of observations, &c., made by Meteorological party during journey from England to Canada on R.M.S. Caronia.

(a) Mr. M. A. GIBLETT.-Remarks on the Daily Weather Charts constructed from data received by Wireless.

(b) Mr. L. F. RICHARDSON.-Measurements of Up-gradient of Temperature in the Air at Levels below the Mast-head of the Ship, using an Electrical Resistance Thermometer.

(c) Mr. F. J. W. WHIPPLE.-The Measurement of True Air Temperature and Humidity at Sea.

A comparison of readings of screened and unscreened thermometers with those of an Assmann aspiration psychrom ter in various positions on the ship.

(d) Dr. J. S. OWENS.-Haze Observations.

A white haze observed on the St. Lawrence in the neighbourhood of Anticosti Island shortly before sunset was found to consist of hygroscopic salt particles, although readings of an Assmann psychrometer showed the air to be relatively dry. [An apparently similar haze observed later over Lake Ontario was found to consist of solid particles, almost certainly soot.]

Monday, August 11.

25. Presidential Address by Prof. Sir WILLIAM BRAGG, F.R.S., on The Analysis of Crystal Structure by X-rays. (Page 34.)

26. Joint Discussion with Section B on Crystal Structure, including the following papers :

(a) Prof. W. L. BRAGG, F.R.S.-The Relation between Crystal Structure and Refractive Index.

When an electromagnetic wave passes through a medium the electric field polarises the atoms, their positive and negative components being displaced from the normal configuration. This creates a field around each atom equiva lent to that of an electric doublet whose moment is proportional to the electric vector at each instant. The velocity of the wave in the crystal depends on the ratio between the total polarisation per unit volume and the field.

Each atom is affected by the general field and by its polarised neighbours. In a crystal which has been analysed by X-rays the effect of neighbouring atoms on each other can be calculated. The double refraction of Calcite and Aragonite. for instance, can be satisfactorily explained quantitatively. This holds for other crystals, and similar calculations are of interest when applied to gases. It appears probable that a better measure of atomic refractivity will be arrived at when due allowance is made for atomic arrangement.

(b) Prof. C. H. DESCH, F.R.S.-The Crystal Surface.

The atoms in a crystal being arranged on a space lattice, all atoms in the interior of the crystal must be held in position by forces symmetrically disposed. At the surface of the crystal this symmetry disappears, so that the surface layer of a crystal must possess properties different from those of the mass. A part of

this difference manifests itself as surface tension. With increase of temperature, the cohesion and the surface tension in general diminish, but not necessarily at the same rate. At high temperatures, the surface tension may be sufficient to cause rounding of the sharp angles of a crystal. This is shown by experiments with gold. Beads of gold, slowly cooled from the liquid state, have a skin which has properties like those of a film of gelatin, the normal structure being exposed when the film is removed by etching. The sharp octahedral 'etch-figures' on large crystals of gold, or the angles of minute crystals prepared by precipitation, become rounded at temperatures several hundred degrees below melting-point. Other effects of surface tension in solid metals are given.

(c) Dr. G. SHEARER.-The Chemical and Physical Significance of X-ray Measurements of Compounds containing Long Chains of Carbon Atoms.

Such measurements appear to give reliable values for the lengths of these molecules; they show that the basis of the structure is actually a chain of carbon atoms which may have one or other of a limited number of forms. The observed rates of increase of the lengths of the molecules with the number of carbon atoms they contain are in accordance with the assumption that successive pairs of carbons are linked together at the tetrahedral angle; this would imply a marked directive property of the valency bond in carbon. The method of investigation would appear capable of throwing light on many problems of stereochemistry-geometrical isomerism, the nature of double and triple bonds, &c. The existence of spacings often as great as 60 A.U. makes it possible to observe a large number of successive orders of reflection, and a study of the intensity distribution among these different orders is of interest.

(d) Prof. W. L. BRAGG, F.R.S., and Prof. S. CHAPMAN, F.R.S.A Theoretical Calculation of the Rhombohedral Angle of Calcite. The paper describes an attempt to calculate theoretically the rhombohedral angle, or axial ratio, of crystals of the calcite type. A number of carbonates form rhombohedral crystals similar to calcite, which can be referred to three equal axes. The angle between the axes is in the neighbourhood of 102° for the whole series.

There must be some fundamental reason for the occurrence of this angle throughout the series. The crystals consist of ions R++ and CO,--, where R may be Mg, Ca, Mn, Fe, Co, Ni, Zn, Cd. One can picture the rhombohedral crystal symmetrically extended or compressed along a trigonal axis, keeping constant the distance between neighbouring oxygen and metal centres, and neighbouring carbon and oxygen centres. The electrostatic potential energy per gram-molecule has been calculated for a series of such positions, and it has been found that it has a minimum value in the neighbourhood of 106°. Taking into account the approximate nature of the assumptions on which the calculation is based, this may be regarded as in satisfactory agreement with the observed value of 102°. Further, it is possible to calculate the effect on the rhombohedral angle of substituting other metal ions for calcium, and it has been found that the slight variation in angle, which is 101° 55' in CaCO, and 103° 2' in MgCO,, is accurately explained.

(e) Mr. W. T. ASTBURY.-The Determination of Molecular Symmetry in Crystals and its Possibilities as a Method of Deriving Structural Formula.

Given the class of any crystal, we can, with two exceptions, decide definitely the space-group, and thence, from the number of molecules per cell, derive the only symmetry elements possible to the molecules as they are built into the crystal. Then, by a critical examination of the dimensions of the cell and the relative positions of the molecules within it as given by the space-group, it is often possible to make a clear choice between two or more suggested structural formulæ. even though the molecule be fairly complex. But, in general. it is essential to know from chemistry the various possibilities. The molecular symmetry in the crystal is in general less than what would be expected of a

molecule in the free state. A brief account will be given of the application of this method to the problem of the isotrimorphism of the tervalent metallic acetylacetones and the structure of the acetylacetone group.

(f) Mr. S. H. PIPER.-X-ray Crystallographic Methods as an Aid to Chemical Research.

(g) Prof. W. L. BRAGG, F.R.S.-Exhibit of Models illustrating Crystal Structure.

27. Prof. V. BJERKNES.-Lecture on The Forces which Lift Aeroplanes. (Illustrated by experiments.)

Now that aviation has become a reality much work is being expended upon the investigation of the hydrodynamic forces which make flying possible. The result has in one respect been surprising these forces are seen to belong to an extensive class which was investigated long before the period of aviation in connection with a totally different problem, that of action-at-a-distance.

:

After Newton's discovery of universal gravitation, action-at-a-distance was for two centuries the leading idea of natural philosophy. But a decided change of our ideas came about thirty years ago, after a development connected with the names of Faraday, Maxwell, and Hertz. In the period before this reversal had taken place C. A. Bjerknes, the father of the lecturer, found, first by mathematical investigations and later by experiments, that spherical and cylindrical bodies moving in a fluid exert apparent action-at-a-distance upon each other. These actions form part of a remarkable analogy which exists between hydrodynamic and electromagnetic phenomena. After the theory had been generalised by the lecturer, and made independent of every special sup position concerning the form of the bodies, this analogy may now be stated thus:

1. Regarding geometric structures full identity can be established between hydrodynamic fields of motion and static or stationary electromagnetic fields of force.

2. The mechanical forces which act in corresponding hydrodynamic and electromagnetic fields are oppositely equal to each other.

This direct geometric and inverse dynamic analogy can be illustrated by striking experiments. Regarding the apparent actions at a distance, the experiments will show the following phenomena, the opposite sign of the forces being tacitly understood: pulsating bodies act upon each other as if they were electrified, oscillating bodies as if they were magnets; neutral bodies take induced oscillations from the fluid and become subject to the same forces as iron and bismuth; rotating cylinders act upon each other like electric currents.

The forces which act upon the rotating cylinder are the same as those that carry the wing of an aeroplane. Theoretically the simplest aeroplane should have rotating cylinders instead of rigid wings. The lift depends upon the circulation of the air round the wing. We produce and regulate at will the required circulation by rotating the cylinders, while in the case of the common rigid wings we get it by a spontaneous process.

COSMICAL PHYSICS SUB-SECTION.

28. Dr. J. S. OWENS.-The Automatic Measurement of Atmospheric Pollution.

Refers especially to results of the automatic recorder designed by the author for the Advisory Committee on Atmospheric Pollution. The function of this is to measure the pollution of city air by smoke. A short description and references to fuller descriptions are given. The results obtained in London by this apparatus are compared with those of the author's dust counter (Proc. Roy. Soc. A., Vol. 101, 1922) and show a good correspondence. Curves obtained by both methods in investigating the effect of suspended matter on obstruction of light are given; the relation between obstruction and dust content is shown