Showing papers by "Imperial College London published in 1983"

Generalized Linear Models

Abstract: The technique of iterative weighted linear regression can be used to obtain maximum likelihood estimates of the parameters with observations distributed according to some exponential family and systematic effects that can be made linear by a suitable transformation. A generalization of the analysis of variance is given for these models using log- likelihoods. These generalized linear models are illustrated by examples relating to four distributions; the Normal, Binomial (probit analysis, etc.), Poisson (contingency tables) and gamma (variance components).

A binary plant vector strategy based on separation of vir- and T-region of the Agrobacterium tumefaciens Ti-plasmid

Abstract: The soil bacterium Agrobacterium tumefaciens is a plant pathogen that causes crown-gall tumours after infection of wounded dicotyledonous plants. Large plasmids (Ti-plasmids) are responsible for the oncogenicity of the bacterium1–3. Crown-gall tumours contain a DNA segment, called the T-DNA, which is homologous with a defined part of the Ti-plasmid present in the tumour-inducing bacterium, and is stably integrated into the plant genome4–7. Apart from the T-DNA another region of the Ti-plasmid-called the vir-region, is essential for tumour induction8–11. We report here the interaction of two compatible plasmids, one containing the vir-region, the other carrying the T-DNA on a wide host-range replicon. An A. tumefaciens strain harbouring both plasmids has a normal tumour-inducing capacity, although neither plasmid is functional alone. With this approach, the T-DNA on one plasmid can, because of its size, be easily genetically manipulated using Escherichia coli as a host. Transfer of this plasmid into an A. tumefaciens strain harbouring the plasmid with the vir-region allows introduction of the manipulated T-DNA into plant cells. In this way, sophisticated binary vector systems for plant genetic engineering can be developed.

Aspects of Computer Simulation of Liquid-Fueled Combustors

Abstract: An existing ''discrete droplet'' model of liquid sprays has been extended to include a stochastic representation of turbulent dispersion effects. Applications to simple test cases, including the dispersion of single particles, produce reasonable agreement. However, two further applications involving volatile and combusting sprays show that the turbulent dispersion effects are small in comparison to those due to uncertainties about the initial conditions of the spray.

The red edge of plant leaf reflectance

Abstract: A detailed study of the red edge spectral feature of green vegetation based on laboratory reflectance spectrophotometry is presented. A parameter lambda is defined as the wavelength is defined as the wavelength of maximum slope and found to be dependent on chlorophyll concentration. Species, development stage, leaf layering, and leaf water content of vegetation also influences lambda. The maximum slope parameter is found to be independent of simulated ground area coverage. The results are interpreted in terms of Beer's Law and Kubelka-Munk theory. The chlorophyll concentration dependence of lambda seems to be explained in terms of a pure absorption effect, and it is suggested that the existence of two lambda components arises from leaf scattering properties. The results indicate that red edge measurements will be valuable for assessment of vegetative chlorophyll status and leaf area index independently of ground cover variations, and will be particularly suitable for early stress detection.

Epidemiology and Genetics in the Coevolution of Parasites and Hosts

Abstract: Recent studies suggest that parasites (interpreted broadly to include viruses, bacteria, protozoans and helminths) may influence the numerical magnitude or geographical distribution of their host populations; most of such studies focus on the population biology and epidemiology of the host-parasite association, taking no explicit account of the genetics. Other researchers have explored the possibility that the coevolution of hosts and parasites may be responsible for much of the genetic diversity found in natural populations, and may even be the main reason for sexual reproduction; such genetic studies rarely take accurate account of the density- and frequency-dependent effects associated with the transmission and maintenance of parasitic infections. This paper aims to combine epidemiology and genetics, reviewing the way in which earlier studies fit into a wider scheme and offering some new ideas about host-parasite coevolution. One central conclusion is that 'successful' parasites need not necessarily evolve to be harmless: both theory and some empirical evidence (particularly from the myxoma-rabbit system) indicate that many coevolutionary paths are possible, depending on the relation between virulence and transmissibility of the parasite or pathogen.

Dynamics of film growth of GaAs by MBE from Rheed observations

Abstract: Detailed observations have been made of the intensity oscillations in the specularly reflected and various diffracted beams in the RHEED pattern during MBE growth of GaAs, Ga x Al1−x As and Ge. The results indicate that growth occurs predominantly in a two-dimensional layer-by-layer mode, but there is some roughening, which is enhanced by deviations from stoichiometry and the presence of impurities. In the case of the GaAs (001) −2×4 reconstructed surface a combination of dynamic and static RHEED measurements has provided firm evidence for the presence of one-dimensional disorder features as well as surface steps.

The scaling theory of electrons in disordered solids: additional numerical results

Abstract: We present extensive numerical results applying the finite size scaling method to the theory of electrons in disordered systems. A method is developed for studying the localisation length in 1-dimensional systems of finite cross section. By studying these results as a function of cross-section and using scaling ideas, we derive the critical properties of 2-D and 3-D systems. We calculate transport properties as a function of temperature which can then be compared with experiment.

The propagation of eddies in diffluent jetstreams: Eddy vorticity forcing of ‘blocking’ flow fields

Abstract: Numerical experiments are performed with barotropic models to test a hypothesized mechanism by which barotropic eddies can, through Reynolds stresses, reinforce blocking flow patterns. Eddies propagating into a split Jetstream suffer an east-west compression and north-south extension of their vorticity fields and this enhanced, local enstrophy cascade is associated with energy transmission to the straining flow (i.e. the blocking flow field) and a characteristic pattern of vorticity forcing by transient motion. As a first step in demonstrating this mechanism, the barotropic vorticity equation is integrated in a form linearized about a state chosen to represent a split Jetstream with a prescribed eddy forcing function upstream. After thirty days of model integration, the time-mean eddy kinetic energy, enstrophy and vorticity flux divergence are calculated, together with mean eddy vorticity flux vectors. The resulting eddy vorticity forcing field can be used to determine the second-order induced flow. Experiments with the nonlinear barotropic vorticity equation show that dipole blocking patterns can be created simply by the introduction of an eddy generator into sufficiently weak, uniform westerly flow. Eddies alone are responsible for the block since the time-mean vorticity equation has no external forcing functions such as an orographic term. Monthly mean statistics are again calculated and terms in the time-mean vorticity equation are compared. Anticyclonic eddy vorticity forcing appears just upstream of the ‘blocking high’ in agreement with data analysed by Illari (1982) for the anomalous circulation over Europe in July 1976. Model flow fields fluctuate in a manner highly reminiscent of blocking episodes as observed in 500mb contour maps with strong bursts of southerly winds on the western flank of blocking highs during the approach and subsequent stagnation of each depression. The proposed straining mechanism and model simulations are in agreement with early observational studies of blocking.

Route first—Cluster second methods for vehicle routing

Abstract: In this paper we consider route first—cluster second methods for the vehicle routing problem. Extensions to the basic method both to improve its effectiveness and to enable it to cope with practical constraints are described. Computational results are given for the method applied to standard vehicle routing problems drawn from the literature.

Myositis autoantibody inhibits histidyl-tRNA synthetase: a model for autoimmunity

Abstract: In autoimmune disorders such as rheumatoid arthritis and systemic lupus erythematosus (SLE), autoantibodies are generated against a variety of macromolecules. Myositis is a human autoimmune disease characterized by weakness and wasting of muscle. In American studies, antibodies directed against soluble cellular constituents were detected by immunodiffusion in about 60% of cases; the commonest of these, found in 25% of patients, was antibody to the Jo-1 antigen. An antibody system referred to as PL-1 was recognized at a similar frequency in a series of patients studied at Hammersmith Hospital, London. We show here that this system is identical with the Jo-1 system and demonstrate that the antigen is a polypeptide of molecular weight (Mr) 50,000. The protein is immunoprecipitated with tRNA His and appears to be histidyl-tRNA synthetase. The identity of the Jo-1 antigen, the first of the RNA-associated antigens familiar in autoimmune disease to be characterized as a specific enzyme, suggests a model for virus involvement in autoantibody generation.

Linkage analysis of two cloned DNA sequences flanking the Duchenne muscular dystrophy locus on the short arm of the human X chromosome

Abstract: The inheritance of two restriction fragment length polymorphisms (RFLPs) on the short arm of the human X chromosome has been studied relative to Duchenne muscular dystrophy. This provides a partial genetic map of the short arm of the human X chromosome between Xp110 and Xp223. The data were derived from the segregation between a RFLP located at Xp21-Xp223, the DMD locus, and a RFLP located at Xp110-Xp113. The genetic distance from Xp110 to Xp223 was found to be approximately 40 centimorgans (cM). This provides experimental confirmation that 1cM corresponds to approximately 1,000 kilobase pairs of DNA for this region of the human X chromosome. Our data confirm that the DMD mutation lies between Xp223 and Xp110. The availability of flanking probes surrounding the DMD locus will assist in the ordering of further DNA sequences relative to the mutation.

On the solution of the Coqblin-Schreiffer Hamiltonian by the large-N expansion technique

Abstract: The authors employ a functional integral approach to the Coqblin-Schreiffer Hamiltonian, similar to that of Yoshimori and Sakurai (YS-1970) for the Kondo Hamiltonian, except that a field lambda enforcing the constraint nf=1 is introduced. By a gauge transformation they show that the phase of the complex sigma field introduced by YS may be absorbed into lambda , leading to a new two-field formulation in terms of lambda and s= mod sigma mod . The static approximation leads to a simple Friedel resonance on the impurity of width approximately TK/N, where N=2J+1 is the number of channels, and whose position is determined by the Friedel sum rule, as in the 'local Fermi liquid' theory. The authors show that the charge susceptibility is zero, is required. Gaussian fluctuation corrections to the static approximation are determined' taking account of fermion propagator renormalisation they then find that the Wilson chi / gamma ratio R=N/(N-1) is correct to order 1/N2. The value of TK is that of a lowest-order renormalisation group treatment, but may be corrected by fluctuation effects.

Vaccination against rubella and measles: quantitative investigations of different policies.

Abstract: This paper uses relatively simple and deterministic mathematical models to examine the impact that different immunization policies have on the age-specific incidence of rubella and measles. Following earlier work by Knox (1980) and others, we show that immunization programmes can, under some circumstances, increase the total number of cases among older age groups; the implications for the overall incidence of measles encephalitis and of congenital rubella syndrome are examined, paying attention both to the eventual equilibrium and to the short-term effect in the first few decades after immunization is initiated. Throughout, we use data (from the U.K., and U.S.A. and other countries) both in the estimation of the epidemiological parameters in our models, and in comparison between theoretical predictions and observed facts. The conclusions defy brief summary and are set out at the end of the paper.

Theory of hydromagnetic waves in the magnetosphere

Abstract: Many of the significant theoretical advances in understanding the origin and behaviour of low frequency hydromagnetic waves originating in the magnetosphere in the last decade are reviewed. Topics covered include wave generation mechanisms, wave damping, effects of inhomogeneity, signal behaviour in the ionosphere and atmosphere.

Zigzagging and casting as a programmed response to wind‐borne odour: a review

Abstract: . ‘Counterturning’ (meaning here the execution of a succession of alternating left and right turns) is the common feature in upwind zigzagging and cross-wind casting by flying insects manoeuvring towards a small source of wind-borne odour. Recent progress in understanding its control and function is discussed. Counterturning is internally controlled (‘self-steered’) in the limited sense that, once initiated by a chemical stimulus, it continues without further changes in the chemical input both in clean air and in a homogeneous cloud of odour. As a reaction it appears to be the kind of chemotaxis distinguished as longitudinal klinotaxis, for which the stimulus is a difference of chemical concentration detected over time along the insect's path, not across it. The new directions taken in response to the stimulus, being self-steered in the above sense, have no necessary relation to the direction of the chemical gradient that provided the stimulus but are influenced by the visual cues generated by wind drift. In wind, the counterturning programme is modulated by changes in the chemical input and simultaneously integrated with anemotaxis, but it can then continue in similar form after the wind has ceased. Unambiguous evidence for these conclusions is so far available only for certain flying male moths responding to sex pheromone. The primary function of counterturning, of all amplitudes and in both zigzagging and casting, appears to be the regaining of contact with an elusive scent.

Natural philosophy and public spectacle in the eighteenth century.

Abstract: Exploration des liens entre philosophie naturelle et spectacle au 18 siecle. L'A. exploite trois themes: la notion de production scientifique comme performance, les liens entre les styles de la philosophie et de l'histoire naturelle, et les liens entre philosophie naturelle et pouvoir politique.

The Effect of Free-Stream Turbulence on Turbulent Boundary Layers

Abstract: Mesures d'ecoulement moyen et de quantites turbulentes dans une couche limite turbulente bidimensionnelle incompressible a pression constante a proximite d'un courant turbulent quasi homogene et quasi isotrope. On confirme l'existence d'une relation nettement non lineaire entre le coefficient de frottement superficiel et autres parametres de couche limite et l'intensite quadratique moyenne de la turbulence

On Edge Detection of X-Ray Images Using Fuzzy Sets

Abstract: The effectiveness of the theory of fuzzy sets in detecting different regional boundaries of X-ray images is demonstrated. The algorithm includes a prior enhancement of the contrast among the regions (having small change in gray levels) using the contrast intensification (INT) operation along with smoothing in the fuzzy property plane before detecting its edges. The property plane is extracted from the spatial domain using S, ? and (1 ?) functions and the fuzzifiers. Final edge detection is achieved using max or min operator. The system performance for different parameter conditions is illustrated by application to an image of a radiograph of the wrist.

Finding of a sex pheromone source by gypsy moths released in the field

Abstract: “Moths display some of the most striking examples of long-distance chemical communication in the animal kingdom”1. Recent theories of how the male moths find a distant female have invoked steering by two different directional cues: (1) gradients of odour concentration in the air, used in ‘aerial odour-trail following’, and (2) the wind, used in ‘odour-modulated anemotaxis’2–7. Laboratory evidence has favoured the latter theory as well as contradicting the earlier radiation theories8–11, but the issue could not be settled without recording in the field the flight track of a male moth and, simultaneously, the track of the odour plume. This has now been done with gypsy moths and wind-borne bubbles, confirming theory (2) and showing in addition that cross-wind casting itself contributes to a moth's progressive approach to the odour source, an unexpected consequence of the way odours are dispersed by the wind12.

Sexual Development of Malarial Parasites

Abstract: Publisher Summary This chapter describes the sexual development of malarial parasites. In the genus Plasmodium , sexual development begins when a merozoite invades the host erythrocyte, where it differentiates into a mature gametocyte. The sexual phase is completed only on the formation of a uninucleate zygote within the bloodmeal of the insect vector. Sexual development in vivo has the following stages: temporal distribution of gametocytes within the infected host and population, spatial distribution of gametocytes within the infected host, and gametocyte infectivity. The chapter discusses the cytology and ultrastructure, along with the biochemical organization of gametocyte and the gamete development. The stages of the culture of gametocytes in vitro include gametocytogenesis, gametogenesis, gametocyte and gamete purification, and zygote production and artificial feeding of mosquitoes. The chapter also discusses the strategies for the control of the sexual development of Plasmodium. The action of the unique assemblage of organelles involved in exflagellation can be blocked with standard anticytoskeletal agents; it should prove possible to develop similar agents with selective antimalarial activity.

On reactions between silicon and nitrogen

Abstract: An attempt is made to rationalize various reaction mechanisms for the formation of Si3N4 from gaseous nitrogen and solid silicon It is suggested that a fundamental difference betweenα- andβ-Si3N4 is that the former results from silicon complexing with molecular nitrogen and the latter results from complexing with essentially atomic nitrogen Well established mechanisms (as well as controversial ones) are analysed in the light of this difference and are shown to be operative under various reaction conditions The influence of some common reaction variables are interpreted with respect to their influence on either silicon or nitrogen or both

THE RELATIONSHIP BETWEEN 17β-HYDROXYSTEROID DEHYDROGENASE ACTIVITY AND OESTROGEN CONCENTRATIONS IN HUMAN BREAST TUMOURS AND IN NORMAL BREAST TISSUE

Abstract: SUMMARY The activity of 17β-hydroxysteroid dehydrogenase (17βHSD) was measured in human breast tumours and in normal breast tissue from premenopausal, perimenopausal and postmenopausal women. Enzyme activity was higher in tumour tissue than in normal tissue from the same breast and under the conditions of the assay the oxidation of oestradiol was higher than the reduction of oestrone. The physiological status of the women in the study did not relate to the activity of the enzyme in either normal or tumour tissue although fibroadenomas had less activity than adenocarcinomas. In postmenopausal women tumour tissue oestrogens were 2–3 fold higher than in normal tissue from the same breast. Furthermore, tumour tissue concentrations of oestradiol tended to be higher than those of oestrone although in normal tissue the two oestrogens were present in similar concentrations. In plasma from the same women oestrone was the predominant oestrogen. There appears to be no direct relationship between 17βHSD activity and oestrogen concentrations but the enzyme may play a part in determining the balance between oestrone and oestradiol according to substrate and cofactor availability.

Microscopic Theory of the Temperature-Pressure Phase Diagram of Iron

Abstract: The free energy of bcc, fcc, and hcp iron has been calculated as a function of temperature and pressure within the single-site spin-fluctuation theory of band magnetism, which has been developed recently by Hubbard and Hasegawa The simple theory can account for the occurrence of the $\ensuremath{\alpha}$, $\ensuremath{\gamma}$, $\ensuremath{\delta}$, and $\ensuremath{\epsilon}$ phases in the $T\ensuremath{-}P$ phase diagram of iron, the phase transitions being driven by the magnetic contributions to the free energy

Paper 8 – the numerical computation of turbulent flows

Abstract: The paper reviews the problem of making numerical predictions of turbulent flow. It advocates that computational economy, range of applicability and physical realism are best served at present by turbulence models in which the magnitudes of two turbulence quantities, the turbulence kinetic energy k and its dissipation rate ɛ, are calculated from transport equations solved simultaneously with those governing the mean flow behaviour. The width of applicability of the model is demonstrated by reference to numerical computations of nine substantially different kinds of turbulent flow.