In the "Handbook of Physical Constants," Special Paper Number 36, published by The Geological Society of America in 1942, the editors sought to compile for the first time a wide variety of physical constants needed for geological and geophysical calculations. The choices of data were necessarily arbitrary; the compilations were contributed by the labors of volunteers with little bibliographical assistance. Nevertheless, the usefulness of the Handbook has been demonstrated, both by the exhaustion of several editions and by many citations in subsequent literature. For at least ten years, the desirability of a revised edition has been recognized: many sections are now badly out of date, and new kinds of measurements have appeared. As the amount of material has increased, however, so has the labor of dealing with it. A preliminary selection of chapters and compilers was drawn up in 1955, and a few manuscripts eventually appeared. It was apparent, however, that a vigorous effort was needed to push the work to completion, and Prof. Sydney P. Clark, Jr., agreed to assume the editorship. The present volume is the result. A primary consideration has been brevity, on the assumption that a volume of moderate size would be most generally useful. The introductory sections of the chapters have consequently been kept to the minimum length consistent with clarity and usefulness. A critical attitude toward measurements has been encouraged by the inclusion, in many cases, of measurements of ostensibly the same physical property by different observers. Without this confrontation, the real uncertainties of

Density and specific volume depend on stress as well as temperature. A stress consisting of uniform pressure in all directions is known as “hydrostatic” pressure, and the relationship between change of volume or density and hydrostatic pressure may be expressed in terms of a single coefficient, the compressibility β, defined by β = − 1 V ( ∂ V ∂ P ) T = 1 ρ ( ∂ ρ ∂ P ) T , where V is the specific volume, and π the density, at pressure P. Since ∂V/∂P is intrinsically negative, β is a positive number having the dimensions of the reciprocal of a pressure or stress. Its reciprocal K is known as the bulk modulus; K and β depend in general upon pressure and temperature. For small compressions, the volume change is often related to the specific volume V 0 or density π 0 at P = 0 (or 1 atmosphere) instead of to the volume or density at pressure P; the difference is proportional to the total change of volume between the pressures 0 and P. Compressibility may be determined directly as a volume change under pressure, or volume changes may be computed from changes of linear dimensions under pressure. If the material is not isotropic, measurements of linear changes in as many as three mutually perpendicular directions may be required to determine the volume change. Unless specified as linear, the tabulated data refer to volume changes. The directions of linear measurements are specified as “parallel” or “perpendicular” with reference to the axis of highest symmetry for crystals of the hexagonal, tetragonal, and trigonal classes; for the others, the directions are the

Several aspects of the physics of the crust may be distinguished for convenience, though they are necessarily interrelated: (1) the physical methods employed in the field to discover structure ( i.e. , gravitational, seismic); (2) the physical measurements of properties of geological materials required for the interpretation of field observations in terms of composition; (3) the physical conditions, such as pressure and temperature, which influence these properties; (4) the physical processes, particularly the transformations of energy, which give rise to geological dynamics. A brief review is given of subdivisions (2) and (3).