The problem of the estimation of parameters determined statistically from physical measurements is discussed. Emphasis is placed on the fundamental role played by the prior probability distribution for the parameter. The validity of "maximum likelihood" estimation is examined with particular reference to the case of the estimation of a parameter which actually has an unique but (originally) unknown magnitude. Situations in which the prior probability distribution for the parameter is completely unknown are treated and a method is described for the calculation of this distribution from appropriate experimental data. Many examples are given throughout from the field of cosmic radiation.

A review of the present state of knowledge of the variation with temperature of the thermal expansion of anisotropic metals. The theoretical significance of the experimental data is discussed.

Both electromagnetic and acoustic waves exert forces of radiation upon an obstacle placed in the path of the wave, the forces being proportional to the mean energy density of the wave motion. In electromagnetics the action of these forces is relatively easily understood through the concept of Maxwell's electromagnetic stress tensor.

The physical processes leading to these forces in a sound wave have been found to be considerably more complex; the difficulties belong to the fact that the acoustic wave equation is not linear and that a beam of finite cross section is subject to effects caused by the surrounding medium.

Though many papers have been devoted to the subject and though various theoretical approaches have been made, some difficulties still seem to stand in the way of a clear understanding of the physics of the problem.

The purpose of the present study is especially to throw light on the physical aspects. The approach adopted, which uses the momentum theorem, is believed to serve this purpose especially well. The expression for the radiation pressure is given in both Eulerian and Lagrangian coordinate systems.

Special consideration is given to liquids of constant compressibility, since in such media the processes involved can be dealt with mathematically in a simple manner. The general case of a plane reflector with arbitrary reflection coefficient is treated; the modus operandi of the forces at the interface between liquid and obstacle is explained for some special cases, including the radiation forces on the interface between two nonmiscible liquids.

Finally, a general relation is established between the energy density and the pressure caused by radiation falling normally upon a plane reflector, which, under certain assumptions, is valid in any fluid and at any amplitude.

A brief discussion is given of the meaning of initial permeability, the principles of its measurement, and the nature of "dimensional resonance." In considering magnetic spectra containing only a single resonance, typical data on sintered ferrites and on finely divided ferrite powders are reviewed, and some of the theoretical interpretations previously advanced are criticized. The investigations involving the two natural resonances found at this laboratory in a commercial Mg-ferrite are then summarized, including resonance in single-domain particles as a method for identifying dispersion mechanisms, experimental evidence for the inertia of domain walls, the nature of internal fields, and the temperature dependence of the natural resonances and of the spectroscopic splitting factor. Using the information obtained from these special studies, the following general hypothesis is advanced: In principle, two natural resonances are present in the magnetic spectrum of any sintered ferrite, and the single resonance sometimes observed represents the coalescence of two resonances. Reasons are given for expecting this coalescence to occur in some materials only, as in certain sintered mixed ferrites containing Zn-ferrite. The new hypothesis is consistent with all presently available data, including new spectra, some containing two resonances, that were recently obtained on pure ferrites at this laboratory. Additional confirmatory experiments are described which provide methods for separating or coalescing two resonances, and permit distinguishing between internal fields due to anisotropy and magnetic interactions. The limitations and possibilities of ferrites for obtaining useful microwave permeabilities are discussed from the new viewpoint, the criteria involved in this problem being shown to differ from those used at radiofrequencies.