Table of contents

The equations of the shock wave in air at n.t.p. are solved numerically for pressures on the high pressure side up to 1000 atmospheres. The solution enables all the physical entities, such as temperature, wave velocity, particle velocity, air density, etc., on the shock wave front to be expressed numerically as a function of the wave pressure. To solve these equations it is necessary to know the internal energy (E) and volume (ν) of one gramme of air over certain regions of the two-dimensional range 1<p<1000 atmospheres and 273<T<16000° K. Calculations are carried out to assess the numbers of the various types of molecules, atoms and ions present at any p and T. The only E values needed at high p are those for which T is also high, and the simple gas laws may therefore be assumed for the volume determinations. The resultant energy is obtained by summing the internal energies of the different groups of atoms and molecules present. Statistical mechanics furnishes equations whose solution fixes the composition. This set of equations is difficult to solve to any degree of accuracy if oxides of nitrogen are taken into account; of these only nitric oxide, NO, ever exceeds a concentration of 1% by weight, and its maximum concentration is less than 5% As a first approximation, the shock wave equations are worked out on the assumption that the species present are N₂, O₂, N and O. These calculations make full allowance for all quantum states apart from ionic states. They are refined later by taking into consideration the presence of NO and argon (1.3% by weight) and ionization possibilities.

A plane shock wave falls normally on the interface between two ideal gases of molecular weights M, m and constant ratios Γ, γ of specific heats. As a rule, the character of the reflected wave then depends only on the ratio ΓM/γm. In exceptional cases, however, the type of reflection may change at a critical value of the incident intensity. An example of this is given.

The rate of occurrence of transient bursts of ionization within the lower E region of the ionosphere has been systematically measured during the period January 1945 to July 1946.

It has been found that the activity varies both diurnally and seasonally and in such a manner as to lend support to the meteor theory of burst formation. Some measure of solar control of the rate of burst occurrence has also been detected; this effect may be explained in terms of the normal diurnal E region changes in density of ionization. From experiments performed during the solar eclipse of July 1945, it is concluded that the sun does not emit a burst-producing radiation.

Analysis of the observations suggests that the greater proportion of the bursts are created within a thin layer located at a height of 86 km.; the distribution of the bursts within this layer has proved to be uniform over wide areas and no latitude effect has been detected. It is established that the rate of incidence of bursts which present echoing areas between A and A + dA m² to a radio wave of frequency ν may be expressed in the form CdA/ν³A^3/2, with C constant.

This paper describes measurements of radio wave interaction, in special test transmissions, between the B.B.C. transmitters at Droitwich and Lisnagarvey (N. Ireland). It is shown that the theoretical formula M=M₀/[1+(n/Gy)²]^½ accurately describes the dependence of the interaction modulation on the modulation frequency (n/2π). The electronic collisional frequency ν at the seat of interaction is deduced from Gv and the laboratory value of G. Investigations of the phase of the modulation are also described.

The paper contains a sketch of the theory of wave interaction. There is also an addendum dealing with the possible use of wave interaction as a tool in ionospheric research.

The integrations reported in this paper domonstrate conclusively that the elementary coma image is of the size and shape determined by ordinary geometrical optics, and that the only effect of diffraction is to break up the image into an elaborate fine structure of dots and lines of light. Practically all the light in the image is confined within the triangular space between the principal ray at the tip of the figure and the sagittal focus. Between the sagittal and tangential foci there is a small amount of light broken up into a series of approximately concentric curved bands of darkness and light, centred about the brightest part of the image. Even when the amount of coma is very small, comparable to the Rayleigh Limit, the characteristic shape of a typical coma-image is already making its appearance. The accuracy of the theoretical predictions is fully and completely borne out by the actual photographs of a comatic image.

The structure of PbS deposits condensed from the vapour in vacuo on to {001}, {110}, {111} and {443} rocksalt faces has been investigated by electron diffraction. The results suggest that the deposit atoms take up positions of least potential energy relative to the substrate, as far as is permitted by the disturbing effects of collisions of incident atoms with the initial deposit crystal nuclei, and by the limited surface mobility of the deposited atoms over the substrate. This view is also supported by the nature of the changes in crystal orientation which occur when initially random deposits are heated in vacuo.

A study has been made of scattering of 40 protons and 160 mesons in photographic emulsions in order to determine the mass of the individual particles. The spread in the values so obtained is large, but the evidence suggests that the majority of mesons recorded by the emulsion can be identified both with the mesons, of mass ≃ 200 m_e, commonly observed in cloud-chamber experiments and with counters, and with the μ-mesons observed in the photographic plates. The slow particles producing nuclear disintegrations, σ-mesons, appear to contain a large proportion of particles with a mass equal to that of the π-mesons.

A process is outlined by which the computations arising in the application of Benoît's Method of Exact Fractions to interferometric measurements can be shortened and simplified. A graphical illustration of the method is given, and it is compared with other modifications of Benoît's original technique.