We have measured the absorption spectra of the two principal lines of 018H at frequencies 1637 and 1639 MHz approximately in the directions of the sources Sgr A and Sgr B2. The use of 64 x 100 kHz filters enabled us to observe both lines simultaneously. The equipment and methods of observation and reduction are discussed by Gardner, McGee, and Sinclair.

There is a large body of evidence to suggest that the anomalous OH emission sources—particularly those which are classified as class I sources—are associated with protostars. It is also known, from the theories brought forward over the past four years, that maser action can be caused by quite a variety of non-equilibrium situations. It is argued here that one particular kind of non-equilibrium situation which can plausibly be expected to exist in protostars is very similar to that proposed previously in the electron-pumping model, and that, in this new context, it can mimic most of the observed features of class I sources.

Most extra-galactic radio sources show angular structure over a wide range of size. Long-baseline interferometers and scintillation observations have shown that resolved sources extending over 10′ arc can have significant contributions from components <1″ arc. Observations at low frequency with the Molonglo cross telescope show also that a large fraction of the emission can come from halo, bridge or extended regions which may form a diffuse ridge or arc structure extending well beyond the small diameter components. However the majority of sources exhibit a basic double structure, and this has led to the adoption of two extended (Gaussian) components as an initial model for the interpretation of brightness distribution in most papers on the structure of extra-galactic sources. Parameters based on such a model will usually match the observed distribution quite well, but because of confusion, there is a danger in using intensity parameters for a model component to derive the spectral index for part of the source.

One of the basic tasks of the one-mile cross at the Molonglo Radio Observatory is the compilation of a catalogue of radio sources complete to a low flux density level. This level has been tentatively chosen as 0.2 f.u., which is well above the ultimate detection level which can be reached in selected areas of sky. Such a limitation is necessary if the catalogue is to be restricted to a manageable number of sources; a few tens of thousands, which will be achieved with the above limit, seems appropriate and useful at the present time.

Recent accurate measurements at 408 MHz and 2700 MHz of the positions of 4C catalogue sources have allowed a comparison of the positions of radio sources measured at the two frequencies and an investigation of the frequency dependence of these positions.

We present observations made at 80 MHz with the Culgoora radioheliograph of the compact central components of the radio source Centaurus A, which is associated with the peculiar galaxy NGC 5128.

Conner et al. have reported the sudden emergence of a strong X-ray source at 3-12 keV in a position well removed from any previously known X-ray source. This source has been confirmed by Kitamura et al. We wish to report the results of an attempt to detect the source during a balloon flight made from Mildura, Australia, on 1969 August 25.

The origin of the high degree of linear or circular polarization frequently observed in the 18 cm radiation of hydroxyl molecules has proved to be a formidable theoretical problem for which no adequate solution appears to exist at present. It is usually supposed that the radiation arises from some form of maser action and so it is possible that the polarization may arise from an interaction of radiation with the various frequencies and polarizations, through the coupling agency of the populations of the energy levels of the molecule.

The exact formula for the intensity of synchrotron radiation emitted by a single charged particle in vacuo was given by Schott, for the case of circular orbits, and Takakura for the case of helical orbits. In the general case the radiated power is expressed in terms of four variables which appear in (among other places) the arguments or orders of a Bessel function and its first derivative; hence the general formula gives little insight into the interpretation of synchrotron radiation and allows evaluation only in particular cases. There is a particular need for approximate formulae that yield the spectrum of the radiation in explicit form. Such approximate formulae were found by Vladimirskii and Schwinger for the case of highly relativistic electrons. In the present paper we outline the derivation of approximate formulae applicable to mildly relativistic electrons, especially those with velocity βc such that 0 ≪ β ≲ O.9. These approximations are also relevant to the case of highly relativistic electrons in a plasma with refractive index appreciably less than unity.

The object of this paper is to show that librations of the Earth follow conditions that are prescribed by nutations. The poles of rotation, excitation and inertia are treated as moving within a system that has approximately constant speed and direction relative to the Conventional International Origin. Applied excitations are presumed to cause other excitations, part forming at once and part forming progressively.

The problem of synchronizing clocks at a distance has been solved both by the use of travelling clocks (e.g. caesium beam devices) and by radio transmission of time signals, for which active satellites are used to overcome inherent errors. A group of Czech workers recently developed a method of intra-continental clock comparison which makes use of the properties of domestic television signals. This has been verified under Australian conditions in experiments carried out between Mount Stromlo Observatory and the Division of Applied Physics of the National Standards Laboratory, CSIRO, Sydney, using signals from the Australian Broadcasting Commission transmitter ABWN 5A Wollongong.