Showing papers by "Imperial College London published in 1976"

Topology of cosmic domains and strings

Abstract: The possible domain structures which can arise in the universe in a spontaneously broken gauge theory are studied. It is shown that the formation of domain wall, strings or monopoles depends on the homotopy groups of the manifold of degenerate vacua. The subsequent evolution of these structures is investigated. It is argued that while theories generating domain walls can probably be eliminated (because of their unacceptable gravitational effects), a cosmic network of strings may well have been formed and may have had important cosmological effects.

The Semantics of Predicate Logic as a Programming Language

Abstract: Sentences in first-order predicate logic can be usefully interpreted as programs. In this paper the operational and fixpoint semantics of predicate logic programs are defined, and the connections with the proof theory and model theory of logic are investigated. It is concluded that operational semantics is a part of proof theory and that fixpoint semantics is a special case of model-theoretic semantics.

Balance between swelling pressure and collagen tension in normal and degenerate cartilage

Abstract: ARTICULAR cartilage contains a high concentration of acid glycosaminoglycans (GAG), reaching 6% by wet weight and associated with fixed charge densities up to 0.2 mEq g−1. This leads to considerable swelling pressure within cartilage, due to, first, the strongly non-ideal osmotic pressure, characteristic of polymer solutions, which increases sharply with concentration and, second, the ionic contribution, in accordance with the Gibbs-Donnan equilibrium. Ogston and Wells1–3 have estimated the values of these two components and I have calculated them from my experimental data on cartilage4,5. The two components of swelling pressure are approximately of the same order of magnitude and can reach values as high as 1.7kgcm−2 (refs 4 and 5). Since normal cartilage does not swell in solution, even when it is removed from the joint and cut into thin (250 µm) slices (lowest curve, Fig. 1), this implies that its high swelling pressure must be counteracted by considerable elastic forces within the collagen fibre network. It has been known for some time that the concentration of GAG gradually increases from the articular surface to the deep zone (for example, refs 6 and 7). A typical variation of total GAG content with depth, measured as fixed charge density, is shown in Table 1. I now suggest that this particular profile is adapted to the physiological function and mechanical properties of cartilage.

The kinetics of rock deformation by pressure solution

Abstract: A simple model for rock deformation by pressure solution, assuming grain boundary diffusive mass transfer to be deformation rate controlling, is presented. The model leads to a constitutive flow law which is of the same form as that for Coble creep. It is argued that the presence of a fluid film in stressed grain boundaries leads to enhanced diffusivity of solute particles in the grain boundary. Some simple experiments are described, which demonstrate rapid diffusion in solutions in pores, much slower diffusion in stressed interfaces and deformation by pressure solution. By using the theoretical model, and by assuming that the pressure of the interfacial solution is equal to the applied normal stress, so that available experimental data on the effect of pressure on mineral solubility could be used, rates of deformation by pressure solution have been calculated. These are compared with rates of deformation by crystal plastic and high temperature diffusive flow processes, by using deformation mechanism maps. Predicted transition conditions between various deformation mechanisms are found to be consistent with those inferred from the study of textures of naturally deformed rocks.

A model of large program development

Abstract: Discussed are observations made on the development of OS/360 and its subsequent enhancements and releases. Some modeling approaches to organizing these observations are also presented.

The dynamics and simulation of tropical cumulonimbus and squall lines

Abstract: Techniques of theoretical analysis and numerical simulation are combined to produce a dynamical model of tropical cumulonimbus convection which features a close cooperation between the updraught and downdraught circulations. The cloud-scale dynamics determine the structure and transfer properties. Sub-cloud-scale transfer is unimportant. A steady-state dynamical model shows that the upshear or down-shear propagation speed, c, of a cumulonimbus cell relative to the mid-level flow is determined as a function of the convective available potential energy, CAPE, and weakly influenced by the windshear through a non-dimensional number, R, of the large-scale flow. This propagation speed is almost constant for a wide range of R, with c≃ 0.3CAPE, but only possible if R ≳ 2.8 (small shear). This contrasts with a previous result for another regime of convection, obtained by Moncrieff and Green (1972), if R ⩽ 1 (high shear). The transfer of momentum is distinctive and of large magnitude. The initiation and growth of the convective circulation represent an essentially nonlinear, finite-amplitude process, whose properties are closely related to the wind profile in the tropical atmosphere. The numerical simulations attain a quasi-steady state of a complex, three dimensional nature, basic features of which can be represented in terms of the steady-state analysis. Moreover, the outflow of the downdraught air is closely related to the attainment of a steady circulation and suggests a mechanism both for the maintenance and the eventual breakdown of the convective regime.

Structure of locust adipokinetic hormone, a neurohormone that regulates lipid utilisation during flight

Abstract: Adipokinetic hormone, isolated from locust corpora cardiaca, has been identified as a blocked peptide: PCA-Leu-Asn-Phe-Thr-Pro-Asn-Trp-Gly-Thr-NH2. The detailed structure is based on mass spectrometric data, substantiated in part by dansyl-Edman and carboxypeptidase data on thermolytic fragments. This is the first peptide hormone from an insect neuroendocrine organ to be fully characterised.

Contribution towards a Reynolds-stress closure for low-Reynolds-number turbulence

Abstract: The problem of closing the Reynolds-stress and dissipation-rate equations at low Reynolds numbers is considered, specific forms being suggested for the direct effects of viscosity on the various transport processes. By noting that the correlation coefficient is nearly constant over a considerable portion of the low-Reynolds-number region adjacent to a wall the closure is simplified to one requiring the solution of approximated transport equations for only the turbulent shear stress, the turbulent kinetic energy and the energy dissipation rate. Numerical solutions are presented for turbulent channel flow and sink flows at low Reynolds number as well as a case of a severely accelerated boundary layer in which the turbulent shear stress becomes negligible compared with the viscous stresses. Agreement with experiment is generally encouraging.

Risk and prognostic factors in trophoblastic neoplasia.

Abstract: Three hundred and seventeen patients with gestational trophoblastic tumors were investigated and treated between 1957-1973. The risk of trophoblastic tumor was influenced by the outcome of the antecedent pregnancy (hydatidiform mole, non-mole abortion, term delivery) and the ABO blood groups of the mating couple; it was also influenced by the patient's age. The response to treatment with chemotherapy and , where appropriate, with surgery and radiotherapy, was influenced prfoundly by several factors. These included 1) the outcome of the antecedent pregnancy, 2) the total body burden of tumor at the time treatment stated as reflected by the urinary output of human chorionic gonadotrophin (CG), 3) the interval between the antecedent pregnancy and the start of chemotherapy, 4) the ABO groups of the mating couple, 5) the extent of mononuclear cell infiltration in the tumor, 6) the immunological status of the patient at the start of treatment, 7) the size of tumor masses, 8) the site of metastases and particularly the presence of intracranial metastases, and possibly by 9) the age and 10) the parity of the patient. A detailed study of the HLA antigens of the patient, her husband, and antecedent child has shown no positive effect on risk or prognosis. These data provide a basis for a scoring system that allows the prognosis to be defined at the time of diagnosis and facilitates tisk of drug resistance. Applied retrospectively to the cases from which the scoring system was generated, prognostic groups with survival rates ranging from 0-100% can be defined. Unfavorable prognostic factors combine so as to increase the probability of drug resistance.

Genetics of Resistance to Infection with Salmonella typhimurium in Mice

Abstract: Eight strains of inbred mice fell into two sharply defined groups. Four strains (CBA, A/JAX, C3H/He, and DBA/2) were resistant (LD50, greater than 10(5)) to Salmonella typhimurium C5 given subcutaneously. The other four strains (Balb/c, C57BL, B10.D2 [new line], and DBA/1) were susceptible (LD50, less than 10). No intermediate resistance was seen. Examination of the F1, F2, and parental backcross generations bred from matings of CBA and Balb/c mice showed that resistance behaved as a simple Mendelian dominant. Resistance was not linked to H-2 genes, and no useful marker has yet been found. However, as previously demonstrated in the parent strains, resistance in the hybrids was related to the ability to produce a good delayed-type hypersensitivity reaction to an extract of S. typhimurium.

The mechanism of adsorption of poly(ethylene oxide) flocculant on silica

Abstract: Adsorption of poly(ethylene oxide) (PEO) of high molecular weight from water on to various forms of silica has been investigated and correlated with the varying degrees of flocculation produced. Both effects are most marked with silicas which carry “isolated” silanol groups and hydrophobic groups (either methyl or siloxane). A combination of hydrogen bonding and hydrophobic association may be responsible. Silicas which have been fully dehydroxylated by heating above 1000°C do not adsorb PEO and are extremely slow to rehydrate in water. The marked influence of pH on the adsorption of this nonionic polymer (and of others such as polyvinyl alcohol and polyacrylamide) needs explanation. It is suggested that ionization of silica reduces its affinity for nonionic compounds by a general “salting-out” effect, since any factor which favors the presence of water on the surface (in preference to organic compounds) tends to weaken the, already fragile, attachment of the solute. The concept of competition between water and ether units for surface sites provides a unified qualitative explanation of all the phenomena reported.

Turbulent flow in a rectangular duct

Abstract: A detailed experimental study of developing turbulent flow in a rectangular duct was made using a laser-Doppler anemometer. The purposes of the work were to obtain data of value to fluid mechanicists, particularly those interested in the development and testing of mathematical turbulence models, and to evaluate the performance of the anemometer. For the first purpose, contours of axial mean velocity and turbulence intensity were measured in the developing flow, and all three mean velocity components and five of the six Reynolds stresses were obtained in the nearly fully developed flow.The symmetry of the present flow appears to be better than that of previous measurements and the range of measurements is more extensive. In addition, the laser-Doppler anemometer has the potential advantage, particularly in the measurement of secondary velocities, of avoiding probe interference.

Elemental mobility during zeolite facies metamorphism of the Tertiary basalts of eastern Iceland

Abstract: Major and trace element analyses are reported for 70 lavas from the Tertiary succession of eastern Iceland The low grade regional metamorphism responsible for the zeolite zones appears to have caused significant mobilization of some elements, particularly Si, Mg, K, Rb, Sr and light rare earth elements (LREE) In contrast, values for Ti, P, Zr, Y, Nb, Ta, Hf and some of the rare earth elements show a high degree of correlation and this is taken to imply that these elements have been relatively unaffected by metasomatic transport The demonstrated mobility of Sr and LREE suggest that Sr isotopic and rare earth data obtained from the eastern Iceland lavas must be interpreted with caution

Due dates in job shop scheduling

Abstract: Much of job-shop scheduling research is based on the assumption that due dates are outside the control of the scheduler. The paper examines several procedures for specifying due dates, largely dependent on the expected processing time for jobs and on the level of congestion in the shop. The effect of these procedures is examined in the case of three loading rules (FIFO, St and SI∗) On waiting times, on missing due dates and on notional cost functions. The study is based on numerous simulations of a given shop operating at various levels of load ratio and the case of a fluctuating load is briefly examined.

Phenomenological Analysis of Plastic Spherical Indentation

Abstract: Functional relationships describing the “expected” deformation behavior of a spherically indented surface are obtained by statistical analysis of published data. The representative strain eR of the indentation, the ratio ψ of mean contact pressure to representative flow stress YR (eR ), the shape of the interfacial pressure distribution p(r), the plastic boundary dimensions, and the displacement at the contact circumference can each be expressed, with specified precision, as functions of the altitude to base radius ratio h/b of the displaced spherical segment, the flow stress to Young’s modulus ratio YR /E (a measure of elastic strain capacity), and the Meyer index m (a measure of strain-hardening rate). Only eR and ψ are independent of m. Whereas eR = 0.43h/b, all other variables are predicted more precisely by φ = (h/b)/(YR /E) than by h/b. After plastic flow commences (φ = 1.15), ψ = 1.1 + 0.53 lnφ until φ = 27, beyond which ψ = 2.87. The extent of plastic flow during unloading can be predicted from p(r), which becomes flatter as φ and/or m increase.

The turbulence structure of a highly curved mixing layer

Abstract: As part of a general investigation of complex turbulent flows, extensive one-point measurements have been made of the turbulence structure of the mixing layer bounding a normally impinging plane jet with an irrotational core. The ratio of shear-layer thickness to streamline radius of curvature reaches a maximum of about 0.2, the sense of the curvature being stabilizing. Downstream of the impingement region the shear layer returns asymptotically to being a classical plane mixing layer. The most striking feature of the results is that the return is not monotonic: after decreasing in the region of stabilizing curvature, the Reynolds stresses, triple products, energy dissipation rate and other turbulence quantities overshoot the plane-layer values before finally decreasing. Some conclusions are drawn about the nature of the turbulent transport of Reynolds stress, and about the representation of this and other processes in calculation methods for complex turbulent flows. An incidental result of the work is a comprehensive set of measurements in a plane mixing layer.

Gametogenesis and Fertilization in Plasmodium yoelii nigeriensis: A Transmission Electron Microscope Study

Abstract: The ultrastructure of the micro- and macro-gametocytes of Plasmodium yoelii nigeriensis and the nuclear and cytoplasmic changes during gametogenesis and fertilization were examined with the electron microscope. Osmiophilic bodies, dispersed in the cytoplasm, served to distinguish the gametocytes from other erythrocytic stages and were thought to play a part in the gametocytes' escape from the host cells by attachment to the parasite's plasmalemma causing dissolution of the overlying erythrocyte cytoplasm. Macrogametocytes were distinguished from the microgametocytes by their greater density of ribosomes, more elaborate endoplasmic reticulum, which contained electron dense material, more numerous mitochondria and smaller nucleus. In microgametogenesis nuclear division was endomitotic and the genome was segregated on three successive spindle formations. Microtubule organizing centres developed adjacent to the nuclear envelope and gave rise to orthogonal tetrads of kinetosomes which were found at opposite poles of the first nuclear spindle. Axoneme assembly from the kinetosome followed the usual pattern by the addition of sub-units terminally on to the A subfibres, and, with a slight lag, on to the B subfibres. The kinetosomes were closely linked to centriolar plaques in pores of the nuclear envelope, at the spindle poles. The attachment of the kinetosomes and their axonemes to the spindle poles provided the mechanism by which each haploid set of chromosomes was eventually endowed with a single axoneme. At the time of the final nuclear segregation the kinetosome and a newly formed juxta-kinetosomal sphere and granule became surrounded by a basket work of irregular tubules which lay close to a bud of the nucleus containing a spindle pole and the now highly condensed chromatin. During exflagellation the juxta-kinetosomal sphere and granule, together with the kinetosome and axoneme were forced through the perikinetosomal basket perpendicularly towards the surface and distended the plasmalemma. In the final stages of gamete formation, the gamete slid off tangentially to the surface and the nuclear bud also passed through the perikinetosomal basket, became separated from the main body of nucleoplasm and was incorporated as the nucleus of the gamete. The free microgamete contained a single axoneme with its kinetosome and distal juxta-kinetosomal sphere and granule. The condensed nucleus was intertwined with the axoneme. After emergence from the erythrocyte there were contrastingly few changes in the macrogamete. The absence of intranuclear spindles and maturation bodies provided evidence that meiosis did not occur at this stage. At fertilization one pole of the microgamete was closely applied to the surface of the macrogamete, and amorphous material on the outer surfaces became confluent. This allowed contact and fusion of the plasmalemmas. The naked axoneme and the nucleus of the microgamete passed into the cytoplasm of the macrogamete. Decondensation of the microgamete chromatin may have occurred before fusion of the two nuclei. A chromosome number of about ten was estimated for the microgametes from numbers of kinetochores and microtubules in the intranuclear spindles. This accords with the estimate for the sporogonic stages. The perikinetosomal basket and juxta-kinetosomal sphere and granule have not been described previously in malaria parasites. The former may have a skeletal function in directing the axoneme and nuclear bud towards the surface. The latter, in their position at the distal end of the microgamete may assist in the penetration of the macrogamete by mediating the fusion of the plasmalemmas of the two gametes, though we have not been able to determine that the kinetosomal end is that one which contacts the macrogamete.

The prevalence, relative abundance and site distribution of nematodes of the subfamily Cyathostominae in horses killed in Britain

Abstract: A total of 21 species of Cyathostominae was found in the lumen of the large intestine of 86 mature horses of various ages and breeds killed in south-west England during 1972–1974. Cylicostephanus longibursatus, C. goldi, C. calicatus, Cyathostomum catinatum, C. coronatum and Cylicocylus nassatus were found in over 80% of the horses, while 12 of the remaining species were detected in less than 30%. Quantitative studies on 55 horses showed the adult cyathostome burdens to rise to a sharp peak (average over 400,000/horse) in April-June and a lower one in November-December. Parallel fluctuations occurred in the lumen populations of 4thstage larvae. The most numerous cyathostomes were generally those species with the highest prevalence. The five most plentiful ( C. nassatus, Cyathostomum catinatum, Cylicostephanus longibursatus, C. minutus and C. goldi ) comprised over 90% of the total number of adult cyathostomes recovered. The three regions of the large intestine had adult cyathostome faunas of different composition. That of the dorsal colon was dominated by C. longibursatus and C. goldi , the ventral colon by Cylicocyxlus nassatus and Cyathostomum catinatum , while the sparsely inhabited caecum accommodated several equally numerous species. Each species showed a characteristic site distribution, 11 occurring predominantly in the ventral colon, eight in the dorsal colon and two in the caecum. Seasonal changes in the site distributions of C. catinatum and Cylicocyclus nassatus and in the size of the cyathostome burdens are discussed in relation to the epidemiology of cyathostome infections.

Influence of crop background on aphids and other phytophagous insects on Brussels sprouts

Abstract: SUMMARY More Brevicoryne brassicae and other alate aphids were caught in yellow water-traps in a weed-free crop of Brussels sprouts than in a crop with a weedy background. More B. brassicae colonized Brussels sprout plants in bare soil than in weeds; very few alatae were attracted to cruciferous weeds in the crop. Results in 1 yr suggest that initially larger populations on the weed-free sprouts became smaller than populations on the weedy sprouts because the larger aphid population attracted more natural enemies. Aleyrodes brassicae and certain Lepidoptera were also more abundant on sprout plants in bare soil than on sprouts surrounded by weeds; more adult A. brassicae were caught in water traps over the bare soil. More A. brassicae were present on sprout plants surrounded by a green than by a brown cloth background but the differences were not significant (P < 0–05). Numbers of B. brassicae on sprout plants with green and brown backgrounds varied greatly and did not differ significantly. In field cages, B. brassicae alatae were more attracted to potted sprout plants surrounded by bare soil than to ones surrounded by rings of living or cut grass or by artificial green rings. This effect was greater with small than with large sprout plants surrounded by grass rings. The maintenance of a limited weed cover is considered potentially useful in integrated control of some brassica pests.