Showing papers by "University of Cambridge published in 1979"

Economic theory and exhaustible resources

Abstract: 1. A preview 2. Resource allocation in a timeless world 3. Externalities 4. Intertemporal equilibrium 5. Renewable resources: some ecological and environmental models 6. Exhaustible resources: an introduction 7. Production with exhaustible resources 8. Resource depletion and capital accumulation in a competitive economy accumulation in a competitive economy 9. Measureability, comparability and the aggregation of intergenerational welfares 10. The optimal depletion of exhaustible 11. Imperfect competition and exhaustible resources 12. Taxation of exhaustible resources 13. Uncertainty, information and the allocation of risk 14. Uncertainty and the allocation of resources 15. price movements in resource markets 16. Conclusions Bibliography Index.

Activity types and language

Abstract: Repercussion des contraintes sur l'emploi de la langue dans des types specifiques d'activite sociale sur les concepts d'actes de langage, d'implication conversationnelle et de jeux de langage.

0957 + 561 A, B: twin quasistellar objects or gravitational lens?

Abstract: 0957 + 561 A, B are two QSOs of mag 17 with 5.7 arc s separation at redshift 1.405. Their spectra leave little doubt that they are associated. Difficulties arise in describing them as two distinct objects and the possibility that they are two images of the same object formed by a gravitational lens is discussed.

The roaring of red deer and the evolution of honest advertisement

Abstract: In many large animals, changes in fighting ability within breeding seasons or across the lifetime of individuals are related to changes in body condition but not to obvious changes in size. In situations where a conflict of interests is likely to lead to a fight, we might consequently expect opponents to assess each other on traits which are related to variation in body condition. This appears to be the case among red deer stags. Competing

Theoretical plasticity of textured aggregates

Abstract: The plasticity of metal polycrystals with preferred orientations is considered from a phenomenological standpoint. Some new general theorems are proved, in particular the existence of a work-equivalent function of the tensor strain-rate over any yield surface. The status of the classical theory of plastic anisotropy is re-appraised in the light of recent experiments, which are themselves critically reviewed. A new type of yield function is proposed to account for the so-called anomalous behaviour of some materials.

Seismic waves in a stratified half space.

Abstract: Summary. The response of a stratified elastic half space to a general source may be represented in terms of the reflection and transmission properties of the regions above and below the source. For P-SV and SH waves and both buried sources and receivers, convenient forms of the response may be found in which no loss of precision problems arise from growing exponential terms in the evanescent regime. These expressions have a ready physical interpretation and enable useful approximations to the response to be developed. The reflection representation leads to efficient computational procedures for models composed of uniform layers, which may be extended in an asymptotic development to piecewise smooth models.

Classification of gravitational instanton symmetries

Abstract: We classify the action of one parameter isometry groups of Gravitational Instantons, complete non singular positive definite solutions of the Einstein equations with or without Λ term. The fixed points of the action are of 2-types, isolated points which we call “nuts” and 2-surfaces which we call “bolts”. We describe all known gravitational instantons and relate the numbers and types of the nuts and bolts occurring in them to their topological invariants. We perform a 3+1 decomposition of the field equations with respect to orbits of the isometry group and exhibit a certain duality between “electric” and “magnetic” aspects of gravity. We also obtain a formula for the gravitational action of the instantons in terms of the areas of the bolts and certain nut charges and potentials that we define. This formula can be interpreted thermodynamically in several ways.

Aspects of Invariance in Solid Mechanics

Abstract: Publisher Summary This chapter is conceived primarily as an account of certain interrelated ideas that have contributed over a long period to the basic mechanics of rate-independent solids and which retrospectively appear to have proved influential. Central to this method is the representation of tensors on trace axes of deformation that sweep through a Lagrangian reference configuration. A Lagrangian viewpoint is also well suited to handling pure mechanics, as it is advocated in relation to incremental boundary-value problems. With “nominal” stress as the primary variable, allied to the notion of convexity with respect to functional, one has an infallible structural guide when investigating uniqueness of solutions and their external characters. The chapter also highlights that the failure of incremental uniqueness, or bifurcation, is arguably the phenomenon that is most typical of continuum response at unrestricted levels of deformation. It is in constitutive analyses of material behaviors that the mathematical apparatus assembled at the outset finds its justification.

The bainite transformation in a silicon steel

Abstract: An experimental silicon steel has been used in a detailed kinetic and structural study of the bainite transformation in an attempt to resolve some of the controversies concerning the reaction mechanism. Distinct reaction ‘C’ curves and transformation mechanisms were observed for the upper and lower bainite reactions. The observed set of three minima in transformation kinetics were found to be incompatible with the solute drag explanation of the kinetic Bs temperature. Transmission electron microscopy indicated the growth of both upper and lower bainite by the propagation of displacive subunits, with adjacent nucleation in the latter case. Definite evidence for carbon supersaturation was obtained for the lower bainitic ferrite. The results are best explained in terms of a shear mechanism for the ferritic component of bainite rather than a ledge mechanism (as is observed in Widmanstatten ferrite growth). Carbide precipitation events were also characterized and the evidence suggested that precipitation resulted from the aging of a supersaturated matrix in lower bainite. The evidence also suggests that carbide precipitation events are of secondary importance to the essence of bainite formation. It was further proven that the concept of a metastable equilibrium1 controlling the transition from upper to lower bainite was not applicable to the present steel and indeed, if any metastable equilibrium does exist in any other steel, it does not constitute a general phenomenon and hence is not essential to the bainite transformation mechanism.

The laminar horseshoe vortex

Abstract: The horseshoe vortex formed around the base of a cylinder by a separating laminar boundary layer has been investigated experimentally. Smoke flow visualization shows that both steady and unsteady vortex systems exist. Pressure distributions beneath both types of vortex system have been measured and the variation of the horseshoe vortex position on the plane of symmetry upstream of the cylinder has been determined. Unsteady horseshoe vortex systems are shown to have a complex oscillatory behaviour and the nature of this oscillatory behaviour is described. Using smoke flow visualization techniques some measurements have been made of the velocity distributions within horseshoe vortex systems.

The use of a priori data to resolve non‐uniqueness in linear inversion

Abstract: Summary. The recent, but by now classical method for dealing with non-uniqueness in geophysical inverse problems is to construct linear averages of the unknown function whose values are uniquely defined by empirical data (Backus & Gilbert). However, the usefulness of such linear averages for making geophysical inferences depends on the good behaviour of the unknown function in the region in which it is averaged. The assumption of good behaviour, which is implicit in the acceptance of a given average property, is equivalent to the use of a priori information about the unknown function. There are many cases in which such a priori information may be expressed quantitatively and incorporated in the analysis from the very beginning. In these cases, the classical least-squares method may be used both to estimate the unknown function and to provide meaningful error estimates. In this paper I develop methods for exploring the resolving power in such cases. For those problems in which a continuous unknown function is represented by a finite number of‘layer averages’, the ultimately achievable resolving width is simply the layer thickness, and perfectly rectangular resolving kernels of greater width are achievable. The method is applied to synthetic data for the inverse‘gravitational edge effect’problem where yi are data, f(z) is an unknown function, and ei are random errors. Results are compared with those of Parker, who studied the same problem using the Backus—Gilbert approach.

Using probabilistic models of document retrieval without relevance information

Abstract: Most probabilistic retrieval models incorporate information about the occurrence of index terms in relevant and non‐relevant documents. In this paper we consider the situation where no relevance information is available, that is, at the start of the search. Based on a probabilistic model, strategies are proposed for the initial search and an intermediate search. Retrieval experiments with the Cranfield collection of 1,400 documents show that this initial search strategy is better than conventional search strategies both in terms of retrieval effectiveness and in terms of the number of queries that retrieve relevant documents. The intermediate search is shown to be a useful substitute for a relevance feedback search. Experiments with queries that do not retrieve relevant documents at high rank positions indicate that a cluster search would be an effective alternative strategy.

A novel coenzyme from bacterial primary alcohol dehydrogenases

Abstract: METHYLOTROPHIC bacteria are able to use methane derivatives as their sole source of carbon and metabolic energy and so can sustain growth on methane, methanol and other organic compounds which lack carbon–carbon bonds1. They are not autotrophic and, being unable to use carbon dioxide, rely ultimately on their ability to oxidise these substances. Although the primary alcohol dehydrogenases from a variety of methanol-grown bacteria have been purified and compared, and despite the increasing commerical importance of these organisms as sources of ‘single cell protein’, the essential features of these oxidative pathways remain unknown. The dehydrogenases exhibit a rather broad substrate specificity for primary alcohols and formaldehyde, and seem to contain a common and novel cofactor2–6, which may be more generally associated with the oxidation of single-carbon compounds. The structure of the cofactor is reported here.

On the duality of operating system structures

Abstract: Many operating system designs can be placed into one of two very rough categories, depending upon how they implement and use the notions of process and synchronization. One category, the "Message-oriented System," is characterized by a relatively small, static number of processes with an explicit message system for communicating among them. The other category, the "Procedure-oriented System," is characterized by a large, rapidly changing number of small processes and a process synchronization mechanism based on shared data.In this paper, it is demonstrated that these two categories are duals of each other and that a system which is constructed according to one model has a direct counterpart in the other. The principal conclusion is that neither model is inherently preferable, and the main consideration for choosing between them is the nature of the machine architecture upon which the system is being built, not the application which the system will ultimately support.

Finite deformation during fluid flow

Abstract: Summary. Typical upper mantle circulations obtained by solving Stokes' equation produce finite deformations which differ in important ways from those produced by pure or simple shear. Finite strain, defined by the ratio of the long to the short axis of the deformation ellipse, in most cases shows a steady increase with superimposed oscillations. Similarity solutions for the flow near plate boundaries demonstrate that the observed seismic anisotropy in the oceanic lithosphere can be produced by the finite deformation beneath the ridge axes. The same mechanism should give rise to strong anisotropy in the mantle above sinking slabs. Such anisotropy has not yet been detected, perhaps because the observed high velocities have been attributed to thermal effects. Convection in the mantle remote from plate boundaries produces complicated deformation which varies rapidly with position and will therefore be difficult to map seismically. The fabrics of nodules in lavas and kimberlites suggest that large strains can occur in the mantle under stresses which are too small to produce dislocation movement. The large and complicated finite deformation produced by the convective circulation in the mantle also affects closed geochemical systems, and leads to thorough mixing of any convecting region.

The anthropic principle and the structure of the physical world

Abstract: The basic features of galaxies, stars, planets and the everyday world are essentially determined by a few microphysical constants and by the effects of gravitation. Many interrelations between different scales that at first sight seem surprising are straightforward consequences of simple physical arguments. But several aspects of our Universe—some of which seem to be prerequisites for the evolution of any form of life—depend rather delicately on apparent ‘coincidences’ among the physical constants.

A New Approach to Animal Flight Mechanics

Abstract: The mechanics of lift and thrust generation by flying animals are studied by considering the distribution of vorticity in the wake As wake generation is not continuous, the momentum jet theory, which has previously been used, is not satisfactory, and the vortex theory is a more realistic model The vorticity shed by the wings in the course of each powered stroke deforms into a small-cored vortex ring; the wake is a chain of such rings The momentum of each ring sustains and propels the animal; induced power is calculated as the rate of increase of wake kinetic energy A further advantage of the vortex theory is that lift and induced drag coefficients are not required; estimated instantaneous values of these coefficients are generally too large for steady state aerodynamic theory to be appropriate to natural flapping flight The vortex theory is applied to hovering of insects and to avian forward flight A simple expression for induced power in hovering is found Induced power is always greater than simple momentum jet estimates, and the discrepancy becomes substantial as body mass increases In hovering the wake is composed of a stack of horizontal, coaxial, circular vortex rings In forward flight of birds the rings are elliptic; they are neither horizontal nor coaxial because the momentum of each ring balances the vector sum of parasite and profile drag and the bird9s weight Total power consumption as a function of flight velocity is calculated and compared for several species Power reduction is one of the major factors influencing the choice of flight style A large body of data is used to obtain an approximate scaling between stroke period and the body mass for birds Together with relations between other morphological parameters, this is used to estimate the variation of flight speed and power with body mass for birds, and on this basis deviations from allometric scaling can be related to flight proficiency and to the use of such strategies as the bounding flight of small passerines Note: Present address: Department of Zoology, University of Bristol, Woodland Road, Bristol BS8 IUG, UK