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Showing papers by "Imperial College London published in 1977"


Journal ArticleDOI
01 Oct 1977-Nature
TL;DR: Simple mathematical models show that adaptations for achieving dispersal retain great importance even in uniform and predictable environments and provide a case where the evolutionarily stable dispersal strategy for individuals is suboptimal for the population as a whole.
Abstract: Simple mathematical models show that adaptations for achieving dispersal retain great importance even in uniform and predictable environments. A parent organism is expected to try to enter a high fraction of its propagules into competition for sites away from its own immediate locality even when mortality to such dispersing propagules is extremely high. The models incidentally provide a case where the evolutionarily stable dispersal strategy for individuals is suboptimal for the population as a whole.

1,271 citations


Journal ArticleDOI
A. Mahgoub1, L G Dring1, Jeffrey R. Idle1, R. Lancaster1, Richard Smith1 
TL;DR: Family studies supported the view that alicyclic 4-hydroxylation of debrisoquine is controlled by a single autosomal gene and that a defect in this metabolic step is caused by a recessive allele.

1,192 citations


Journal ArticleDOI
TL;DR: In this article, the magnetic field for an exact gauge group H (assumed compact and connected) exhibits an inverse square law behaviour at large distances, and the generalized magnetic charge, appearing as the coefficient, completely determines the topological quantum number of the solution.

752 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the nucleation of recrystallization at large second phase particles in deformed aluminium alloy single crystals and found that particle stimulated nucleation occurs above a critical particle size which increases with decreasing strain.

679 citations


Journal ArticleDOI
TL;DR: A tree-search algorithm for two-dimensional cutting problems in which there is a constraint on the maximum number of each type of piece that is to be produced is presented.
Abstract: We present a tree-search algorithm for two-dimensional cutting problems in which there is a constraint on the maximum number of each type of piece that is to be produced The algorithm limits the size of the tree search by deriving and imposing necessary conditions for the cutting pattern to be optimal A dynamic programming procedure for the solution of the unconstrained problem and a node evaluation method based on a transportation routine are used to produce upper bounds during the search The computational performance of the algorithm is illustrated by tests performed on a large number of randomly generated problems with constraints of varying "tightness" The results indicate that the algorithm is an effective procedure for solving cutting problems of medium size

499 citations


Journal ArticleDOI
03 Feb 1977-Nature
TL;DR: A concept is developed for the regulation of populations by density-dependent movement, rather than by overt competition alone, which is seen as maximising the reproductive advantage of a balance between migratory and congregatory behaviours.
Abstract: A concept is developed for the regulation of populations by density-dependent movement, rather than by overt competition alone. Fitness is seen as maximising the reproductive advantage of a balance between migratory and congregatory behaviours. Population density is shown to be spatially, as well as temporally dynamic and a mechanism is proposed that accounts for observed spatial behaviour.

430 citations


Journal ArticleDOI
TL;DR: The body size is a parameter which can influence tissue metal concentrations, and if significant variations due to size do occur, then clearly it is a factor which should be taken into consideration within these contexts as mentioned in this paper.
Abstract: Studies of tissue metal concentrations in shellfish have progressed along two major lines: first, to record the range of concentrations within commercial species, a response to the need to identify potentially hazardous levels from a human nutritional standpoint; and second, to compare concentrations within a single species on a geographical basis as a framework for an environmental monitoring programme. Body size is a parameter which can influence tissue metal concentrations, and if significant variations due to size do occur, then clearly it is a factor which should be taken into consideration within these contexts.

375 citations


Journal ArticleDOI
TL;DR: In this article, Monte Carlo and molecular dynamic simulations of the surface of a system of Lennard-Jones (12, 6) molecules have been carried out at temperatures which span most of the liquid range, showing that the density profile, as a function of height, falls monotonically from the density of the bulk liquid to that of gas.
Abstract: The gas–liquid surface of a system of Lennard-Jones (12, 6) molecules has been simulated by Monte Carlo and by Molecular Dynamic methods at temperatures which span most of the liquid range. For systems of 255 molecules the two methods lead to similar results and this agreement confirms that the density profile, as a function of height, falls monotonically from the density of the bulk liquid to that of the gas. The thickness of the surface layer is sensitive to the surface area, and appears to approach its thermodynamic limit for surface areas of 400σ2 for a system of 4080 molecules. The density profile can be represented by a hyperbolic tangent of an appropriately scaled height. The thickness of the surface is of the order of two molecular diameters at temperatures near the triple point and increases rapidly as the critical point is approached. The computed surfacetens ions agree well with those calculated by statistical perturbation theory.Monte Carlo and Molecular Dynamic simulation of a binary mixture shows clearly the adsorption of the component of higher vapour pressure; the amount absorbed agrees with that calculated from Gibbs's isotherm.

330 citations


Journal ArticleDOI
TL;DR: Co-ignimbrite ashes as discussed by the authors are a type of fine-grained air-fall ash associated with many ignimbrites, and they have crystal/glass ratios systematically lower than artificially crushed pumice from the same ignimrites.

301 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured the dimensionless threshold stress and its dependence on grain protrusion and found that the threshold stress for grains resting on the top of an otherwise flat bed in a turbulent stream was measured and found to be 0.01 -considerably less than previously reported values of 0.03-0.06 for beds where all grains were at the same level.
Abstract: Shields (1936) found that the dimensionless shear stress necessary to move a cohesionless grain on a stream bed depended only on the grain Reynolds number. He ignored the degree of exposure of individual grains as a separate parameter. This report describes experiments to measure the dimensionless threshold stress and its dependence on grain protrusion, which was found to be very marked. The threshold stress for grains resting on the top of an otherwise flat bed in a turbulent stream was measured and found to be 0.01 –considerably less than previously-reported values of 0.03–0.06 for beds where all grains were at the same level. It is suggested that the new lower value be used in all turbulent flow situations where the bed is of natural sediments or unlevelled material. An hypothesis is proposed that the conventional Shields diagram implicitly contains variation with protrusion between the two extremes of (i) large grains and large Reynolds numbers, with small relative protrusion, and (ii) small grains, low Reynolds numbers, and protrusion of almost a complete grain diameter. In view of this, the extent of the dip in the Shields plot is explicable in that it represents a transition between two different standards of levelling as well as the transition between laminar and turbulent flow past the grains, the range of which it overlaps considerably.

286 citations


Journal ArticleDOI
TL;DR: In this paper, the role of climb as a tectonic deformation mechanism in quartzites is discussed, and it is argued that unbound dislocations within the interiors of sub-grains are not reliable indications of past conditions.

Journal ArticleDOI
TL;DR: It is shown that the use of low cation media enhances the separation of intact and broken chloroplasts in a new method of obtaining chloroplast retaining their outer membranes.

Journal ArticleDOI
TL;DR: There is a critical migration rate above which resistance is greatly retarded and the model suggests that density dependence is important in determining the optimal insecticide kill rate and that a larger kill rate will probably delay the onset of resistance.


Journal ArticleDOI
TL;DR: In this article, a model of turbulent shear layers on rotating or curved surfaces is presented, which is characterized by strong effects of streamline curvature on the turbulence structure, and the authors deal with the calculation of these effects with a model which solves transport equations for the turbulence kinetic energy and its local rate of dissipation.
Abstract: The development of turbulent shear layers on rotating or curved surfaces is usually characterized by strong effects of streamline curvature on the turbulence structure. The present contribution deals with the calculation of these effects with a model of turbulence which solves transport equations for the turbulence kinetic energy and its local rate of dissipation. The direct effect of curvature in the model is limited to a single empirical coefficient whose magnitude is directly proportional to a Richardson number based on a time scale of the energy-containing eddies. (In the absence of significant streamline curvature the model reduces to a form that has earlier been extensively tested in various thin shear flows.) Finite difference computations are reported of the following turbulent flows: the boundary layer on concave and convex surfaces; fully developed flow in a curved channel; axisymmetric flow over a spinning cylinder; and heat and mass transfer due to spinning cones of various vertex angles. Agreement with experiment is satisfactorily close in all these cases.

Book
01 Jan 1977
TL;DR: In this article, the trajectories of solid grains are photographed by a multi-exposure technique as they are propelled by water streams along the bed of a laboratory channel, and trajectories are analyzed for the coefficient of friction tan cc and for the height of the effective thrust.
Abstract: This paper continues the investigation of the motion of solitary grains in a water stream, reported by Francis (1973)- The trajectories of solid grains are photographed by a multi-exposure technique as they are propelled by water streams along the bed of a laboratory channel. Many thousands of photographs were taken and analysed to determine the positions, velocities and accelerations of the grain. The technique does not take into account the possible effect, in multi-grain transport, of intergranular collisions. The three different modes of transport of grains were all observed — rolling, saltation and suspension, and the proportion of each found for a variety of transport stage w\*/w\*0. The development of suspension is much less rapid than the development of saltation from rolling, but even at the highest stage used, about 3.0, there is still a small amount of rolling. The trajectory dimensions and geometry are shown in relation to the stage which uniquely determines the geometry. Experiments where the grain is suddenly entrained from a stationary position show that several features of the subsequent trajectory are the same as those of a trajectory with a prior history of movement: thus it is inferred that the start of a trajectory is by way of hydrodynamic forces rather than by the conservation of momentum of previous trajectories. Impacts and trajectories were analysed for the coefficient of friction tan cc and for the height of the effective thrust. While tan a is shown to be rather larger than has been suspected in the past, the variation of yn throws light upon predominance of slow fluid near the bed rather than high speed inrushes of fast fluid. Better information is now available for finding the mean forward speed of grains compared to that presented in the earlier paper. There are grounds for believing the existence of a * shear-drift ’ force on grains when they are in a velocity gradient, giving a force opposing gravity: but there is no evidence of a proximity effect of the bed independent of the velocity gradient.

Journal ArticleDOI
TL;DR: In this paper, the authors used laser-Doppler anemometry to quantify the velocity field of water in a 90° bend of 40 x 40mm cross-section; the bend had a mean radius of 92mm and was located downstream of a 1[sdot ]8m and upstream of a 2m straight section.
Abstract: Calculated values of the three velocity components and measured values of the longitudinal component are reported for the flow of water in a 90° bend of 40 x 40mm cross-section; the bend had a mean radius of 92mm and was located downstream of a 1[sdot ]8m and upstream of a 1[sdot ]2m straight section. The experiments were carried out at a Reynolds number, based on the hydraulic diameter and bulk velocity, of 790 (corresponding to a Dean number of 368). Flow visualization was used to identify qualitatively the characteristics of the flow and laser-Doppler anemometry to quantify the velocity field. The results confirm and quantify that the location of maximum velocity moves from the centre of the duct towards the outer wall and, in the 90° plane, is located around 85% of the duct width from the inner wall. Secondary velocities up to 65% of the bulk longitudinal velocity were calculated and small regions of recirculation, close to the outer corners of the duct and in the upstream region, were also observed.The calculated results were obtained by solving the Navier–Stokes equations in cylindrical co-ordinates. They are shown to exhibit the same trends as the experiments and to be in reasonable quantitative agreement even though the number of node points used to discretize the flow for the finite-difference solution of the differential equations was limited by available computer time and storage. The region of recirculation observed experimentally is confirmed by the calculations. The magnitude of the various terms in the equations is examined to determine the extent to which the details of the flow can be represented by reduced forms of the Navier–Stokes equations. The implications of the use of so-called ‘partially parabolic’ equations and of potential- and rotational-flow analysis of an ideal fluid are quantified.

Journal ArticleDOI
TL;DR: It was concluded that the gametocytes of P. falciparum are both long-lived and show persistent infectivity to mosquitoes, which can stimulate antibody production but the immune response appears to play no part in their elimination.
Abstract: The longevity and infectivity of isolated populations of Plasmodium falciparum gametocytes were studied. Following chloroquine treatment gametocyte numbers fell with a constant rate of loss over a period of 16–24days; the populations had a half-life of 2·4 days. The sex ratio stayed constant throughout at 4 female:1 male. The ability of the microgametocytes to exflagellate and the infectivity of the population to mosquitoes persisted for 3 weeks. Antibodies to the gametocytes were detected but not in every patient studied. It was concluded that the gametocytes of P. falciparum are both long-lived and show persistent infectivity to mosquitoes. They can stimulatae antibody production but the immune response appears to play no part in their elimination, which probably takes place in the spleen as a part of the normal process of removing old, damaged and malformed red cells.

Journal ArticleDOI
TL;DR: In this paper, the development of a stable c-axis fabric pattern in quartz-mylonites from the Cap de Creus shear zones is summarized and the subsequent effects of fold development on these fabrics is analyzed in more detail.

Journal ArticleDOI
TL;DR: A heuristic method is proposed for solving the problem where n is large; this method requires very little computing and was found to produce very good results for a sample of problems of varying size.
Abstract: The paper considers the problem of n given jobs to be processed on a single machine where it is desirable to minimise the variance of job waiting times. A theorem is presented to the effect that the optimal sequence must be V-shaped i.e., the jobs must be arranged in descending order of processing times if they are placed before the shortest job, but in ascending order of processing times if placed after it, and an algorithm for determining the optimal solution is given. A heuristic method is proposed for solving the problem where n is large; this method requires very little computing and was found to produce very good results for a sample of problems of varying size. The concept of the “efficient set” is examined and heuristic methods for generating this set are given.

Journal ArticleDOI
TL;DR: The cryo-sectioning technique has preserved for the first time the axial periodicities in the A-band and that this technique may help to bridge the gap between electron microscopy and X-ray diffraction.


Journal ArticleDOI
TL;DR: In this paper, three case studies of the nocturnal jet at dawn were made over southern England where the terrain is considerably less homogeneous than that over which previous published data concerning the jet have been obtained, for example the Great Plains and Wangara.
Abstract: Three case studies of the nocturnal jet at dawn are presented. Observations were made over southern England where the terrain is considerably less homogeneous than that over which previous published data concerning the jet have been obtained, for example the Great Plains and Wangara. It is shown that previously suggested layer models of the boundary layer can be usefully quantified to give the diurnal evolution of a layer-average wind. This model, which involves a decoupling of the flow in middle levels from surface constraints by thermal stratification at night, exhibits a nocturnal jet. Two surface-stress parameterizations, which make the stress proportional to the velocity and to the velocity squared, give a similar wind evolution. The observed abrupt transition in boundary layer structure soon after sunrise is studied by considering the growth and transfer characteristics of perturbations to a homogeneous Boussinesq fluid, which is stratified and possesses an idealized jet wind structure. Depending on the value of an average Richardson number, Ri, two regimes of dynamically unstable eddies are indicated. When +0.35 ≳ Ri −0.03 the preferred eddies are oriented transverse to the shear and when Ri ≲−0.03 they are oriented parallel to the shear. the latter regime, associated with longitudinal convective circulations, is efficient at smoothing the jet momentum distribution to give the characteristic well-mixed daytime wind profile.

Journal ArticleDOI
01 Dec 1977
TL;DR: In this article, a simple recursion technique is employed to generate the matrix representing the annular region, where all nodes are eliminated from the external element except those on its inner surface, so that the final matrix is no larger than that required to describe the region of interest.
Abstract: Electric- and magnetic-field problems with boundaries at infinity are treated in finite-element terms by constructing an element to model an extremely large annulus surrounding the region of interest. A simple recursion technique is employed to generate the matrix representing the annular region. All nodes are eliminated from the external element except those on its inner surface, so that the final matrix is no larger than that required to describe the region of interest only. The method is simpler to program and requires less computing effort than boundary-integral techniques. It has been tested by solving several 2-dimensional magnetostatic and electrostatic problems and comparing the results with analytic solutions. The method can be applied to any 2-dimensional field problem bounded by a large empty region in which the field satisfies Laplace's equation.

Journal ArticleDOI
01 Nov 1977
TL;DR: In this paper, a range of choice of magnetic and electric vector potentials is examined, including the ℰ and H vectors as particular examples, and it is shown that the problem formulation is greatly affected by the choice of gauge, or vectorpotential divergence.
Abstract: The numerical calculation of 3-dimensional eddy currents is complicated by the need to use vector functions to describe the interlinked flux and current distributions. The three vector components are coupled, particularly by the conditions imposed by the copper and iron surfaces, and by other changes in material properties. The range of choice of magnetic and electric vector potentials is examined, including the ℰ and H vectors as particular examples, and it is shown that the problem formulation is greatly affected by the choice of gauge, or vector-potential divergence. This introduces scalar potentials which can be used in various ways. They control the coupling conditions between the vector components. One advantage of an electric vector potential, or generalised H, formulation is that the gauge can be chosen so that the vector is zero outside the conductors, and is replaced by a scalar. Magnetic vector-potential formulations are generally less attractive, both because the vector has to be computed throughout the region of interest and because of the greater complexity of the magnetic-and electric-interface conditions.

Journal ArticleDOI
TL;DR: The uptake and distribution of inorganic mercury (HgCl2) within higher plants (Pisum sativum and Mentha spicata) was examined using solution culture and radiotracer techniques as discussed by the authors.
Abstract: The uptake and distribution of inorganic mercury (HgCl2) within higher plants (Pisum sativum and Mentha spicata) was examined using solution culture and radiotracer techniques. Plants were found to tolerate an external level of 1 mgHg/kg of solution but both physiological and biochemical processes were affected at 5 mgHg/kg and 10 mgHg/kg. The uptake of Hg into plants grown in hydroponic solution was a function of external concentration. Over the concentration range considered the accumulation of Hg in the roots was linear on a log-log basis although the uptake of the element into the shoots appeared to be two-phased. The distribution of Hg in plants was asymmetrical with much greater amounts of the element in the roots than the shoots. Although the level of Hg increased generally in plant tissues with increasing external levels, the proportion retained in the roots, relative to the shoots, was constant (approximately 95%). Two binding characteristics of the Hg within plant tissue were detected. A major proportion of Hg was tightly bound, being unaffected by treatment with ethanol and hydrochloric acid. The remaining Hg in the tissue was removed by either water or hydrochloric acid treatment. Cell fractionation indicated that the major binding component of Hg in plant tissues was the cell wall.

Journal ArticleDOI
TL;DR: It is proposed that stimulation of the sympathetic nervous system by postural challenge provides a means for identifying a subgroup of younger patients with essential hypertension in whom sympathetic overactivity may play an important rule.


Journal ArticleDOI
01 Jan 1977
TL;DR: In this paper, a new expression for the time-average reaction rate in a turbulent flame, whether of uniform or non-uniform fuel-air ratio, is presented, based on the idea of coherent gas "parcels", which are subjected to a stretching process while reaction and small-scale mixing take place.
Abstract: A new expression is presented for the time-average reaction rate in a turbulent flame, whether of uniform or non-uniform fuel-air ratio. It is based on the idea of coherent gas “parcels,” which are subjected to a stretching process while reaction and small-scale mixing take place. The expression has been used for the prediction of flame spread behind a baffle, and for the turbulent diffusion flame. An outline is given of a more complete theory, still under development.

Journal ArticleDOI
TL;DR: The group receiving oral non-absorbed antibiotics had significantly few infections, fewer deaths from infection, fewer pyrexial episodes, and consequently received less systemic antibiotic therapy than the controls.