# Societies and Academies

## Abstract

LONDON. Royal Society, November 27.—Sir Archibald Geikie, K.C.B., president, in the chair.—Prof. B. Hopkinson: A method of measuring the pressure produced in the detonation of high explosives or by the impact of bullets. A steel shaft about11/2 in. diameter and 4 ft. long is suspended horizontally from strings so that it can swing in a vertical plane as a ballistic pendulum. At one end it carries an end-piece of the same diameter and several inches long. The end-piece is held on by magnetic attraction; the surfaces of the joint are care-fully faced. If a bullet be fired at the other end a wave of pressure travels along the shaft, the length of which represents the duration of the blow on the scale 1 in. = 5 ×10—6 second approx. The wave passes the joint without change and is reflected as a tension-wave from the free end. If length of wave exceeds twice that of end-piece, the tail of pressure-wave will have passed the joint when the head of tension-wave reaches it and the piece will fly off, having trapped within it the whole momentum of the blow, leaving the shaft at rest. By experimenting with different lengths of end-piece and finding that which is just long enough to stop the shaft, the duration of blow can be determined. The end-piece is caught in a ballistic pendulum and its momentum measured; thus, knowing the time, the average pressure is determined. Applied to investigation of the blow given by a lead bullet, the method gave results in close accord with those expected on the assumption that the bullet be-haves as though it were liquid, the measured duration of blow being nearly that required by the bullet to travel its own length. Measurements by the same method of pressures produced by detonation of a i-oz. dry guncotton primer showed that, at a distance of I in. from surface of cotton, the pressure is practically all gone in 1/50,000 second, the average pressure during that period being about 25 tons per sq. in.—, and the maximum of the order of 45 tons per sq. in.- J. H. Jeans: Gravitational instability and the nebular hypothesis. The work of Maclaurin, Jacobi, Poincare, and Darwin on rotating fluids has applied only to the abstract case in which the mass is considered perfectly incompressible and homogeneous. To estimate the bearing of their results on astronomical problems, it is important to know to what extent these results remain valid for actual, compressible, heterogeneous masses. The result of the present investigation is summed up concisely by saying that the ideal mass of incompressible fluid has been found to supply a surprisingly good model by which to study the be-haviour of the more complicated natural systems con-sidered in astronomy.—B. A. Keen and A. W. Porter: The diffraction of light by particles comparable with the wave-length. A suspension of finely-divided sul-phur, obtained by precipitation from a solution of thiosulphate of soda by the addition of acid, ordinarily diffracts an excess of blue light, so that a white source of light seen through it looks red. One of the authors discovered that if the particles be allowed to grow the red image gradually changes over in colour, be-coming at one stage a deeo indigo blue, and after-wards passing through various shades of green to white. The present investigation was undertaken to obtain quantitative information in regard to this phenomenon.—Prof. R. J. Strutt: Note on the colour of zircons, and its radio-active origin.—Prof. W. H. Bragg: The influence of the constituents of the crystal on the form of the spectrum in the X-ray spectrometer. The energy of the pencil of X-rays which falls on the crystal of the X-ray spectrometer is in part spent within the crystal through absorption, which implies the production of kathode and characteristic X-rays, and in part is scattered, producing the reflected ray when circumstances are favourable. It is found that where there is much absorption there is little reflection. The best reflectors are therefore those crystals of which the absorption coefficients are smallest in comparison with their weights or their scattering powers. For this reason alone the diamond must be a very good reflector.—W. L. Bragg: The analysis of crystals by the X-ray spectrometer. By a quantitative comparison of the intensities of the successive orders of reflection by various crystal faces, it is shown that the X-ray spectrometer can be made to give a very complete analysis of the crystal structure. The structures par-ticularly investigated in the paper are those of the isomorphous sulphides, pyrites, and hauerite, and of the series of compounds which compose the calcite family of minerals. By a study of these last com-pounds, it is concluded that the diffracting power of an atom is proportional to its atomic weight.—Dr. Havelock: Ship resistance: the wave-making proper-ties of certain travelling pressure disturbances. The paper contains a theoretical comparison of the wave-making resistance associated with certain distributions of surface pressure. Various inferences are drawn in regard to variation of resistance with speed, and the speeds at which typical interference effects occur. In particular, types are examined which are similar in general form to those associated with the motion of ship models in recent work at the William Froude tank in the National Physical Laboratory.—Dr. R. A. Houstoun: The mathematical representation of a light pulse. The object of this paper is to direct attention to a new series of expressions representing the initial form and dispersion of a light pulse. TJiey have been suggested by one of Kelvin's hydrodynamical papers, and are derived from his instantaneous-plane-source solution in the conduction of heat.

### DOI

https://doi.org/10.1038/092416a0