Showing papers by "University of Leeds published in 1975"

The mechanics of hopping by kangaroos (Macropodidae)

Abstract: Force-platform records and films of kangaroos and a wallaby hopping have been analysed to obtain data about the energetics of hopping and about the stresses which act in muscles, tendons and the tibia. The quantitative anatomical data required for the analysis have been obtained from X-radiographs and dissections. It is shown that fluctuations of potential energy and external kinetic energy account for most of the energy cost of hopping; fluctuations of internal kinetic energy are relatively unimportant. Evidence is presented that large savings of energy are effected by elastic storage of energy in the gastrocnemius and plantaris tendons. The elastic mechanism is particularly effective at high speeds and seems to account for the observation of Dawson & Taylor (1973) that oxygen consumption is more or less constant over the whole range of hopping speeds. A mathematical model of hopping is presented. The stresses which occur in hopping are discussed. Summary Force platform records and films have been made of kangaroos and a wallaby hopping. The maximum forces exerted on the ground were about six times body weight. The force exerted on the ground changes direction, throughout the period when the feet are on the ground, so that it is always more or less in line with the centre of mass. Consequently the animal decelerates a little and then accelerates again, during the contact phase. The fluctuations of potential energy which occur in each hop are slightly smaller at high speeds than at low ones. Fluctuations of external kinetic energy increase with speed and account for most of the energy cost of hopping at high speeds. Fluctuations of internal kinetic energy (due to acceleration and deceleration of the limbs) are relatively small. While the feet are on the ground the extensor muscles of the hip do positive work, those of the knee negative work and those of the ankle negative work followed by positive work. The energy cost of hopping is reduced substantially by elastic storage of energy in the Achilles tendon. In the case of a wallaby hopping at moderate speed the calculated saving was 40%. The maximum stresses developed in leg muscles, tendons and the tibia have been calculated and are discussed in relation to the known properties of muscle, tendon and bone. The trunk pitches as the animal hops because the two legs swing forwards and back simultaneously. Appropriate tail movements reduce, but do not eliminate, this effect. A mathematical theory of hopping is presented and used to investigate the merits of different hopping techniques. Dawson & Taylor's (1973) discovery that the rate of oxygen consumption of kangaroos decreases a little, as hopping speed increases, is probably to be explained by the increased role of elastic storage of energy at high speeds.

Pedogenic carbonates and flood sediment accretion rates; a quantitative model for alluvial arid-zone lithofacies

Abstract: A general model for pedogenic carbonate development in alluvial sediments is based upon of rates flood sediment accretion versus time taken for particular carbonate profiles to develop. Conclusions regarding rates of Holocene carbonate pedogenesis and the changing morphology and thicknesses of carbonate profiles with time are used to predict critical values of floodbasin accretion rates which would have enabled particular ancient profiles to develop. Local (intra-drainage basin) and regional profiles are separated and a number of models for calcrete genesis are outlined. The presence or absence of particular genetic stages of calcrete may be used as a rough guide to estimate ancient floodbasin accretion rates.

Crystal deformation in aromatic polyesters

Abstract: A study has been carried out of the changes in the x-ray diffraction patterns which occur when oriented fibers or tapes of poly(trimethylene terephthalate) (3GT) and poly-(tetramethylene terephthalate) (4GT) are subjected to mechanical tensile stress. Although the polymers show very different behavior in detail, in both cases comparatively large reversible lattice strains are observed (∼ several %). The diffraction pattern of 3GT changes monotonically with increasing macroscopic strain, suggesting that the lattice responds immediately to the applied stress, and deforms as though it were a coiled spring. In 4GT, on the other hand, there is no detectable change in the x-ray diffraction pattern at low macroscopic strains, i.e., low values of the applied stress. At higher stresses, changes in the pattern occur which suggest a definite change in the crystal structure. Finally at the highest values of applied stress, the lattice deformations cease to increase. A preliminary discussion is presented of the relationship of these x-ray diffraction results to the mechanical stress–strain behavior.

Flow separation in meander bends

Abstract: IT is generally assumed that downstream flow through meander bends is helicoidal and is accompanied by a transverse, bottom flow component directed towards the inner bank1–3. Thus particles of sediment on a point bar are transported at some angle inwards from the generalised local downstream flow vector4,5. As pointed out by Bagnold2, however, “… a stage must be reached at which the flow along the inner boundary becomes unstable and breaks away from the boundary, leaving an intervening space occupied by a zone of unstable and confused motion …”. The experimental results of Leopold et al.5 leave no doubt that this phenomenon of flow separation (Fig. 1) can be a highly important feature in river hydraulics. Since 1960, however, no workers have extended these initial experimental ideas into field situations. Existing models of sedimentation and erosion in sinuous channels4,5 ignore any possible effects of flow separation. Here we draw attention to examples of flow separation in natural meander bends and attempt to define an empirical criterion for predicting the onset of separation.

Variations in the composition of developing rat incisor enamel.

Abstract: The developing enamel of rat incisors was dissected into a series of samples extending from the newly-formed partially-mineralised matrix to the mature enamel. Chemical analysis showed that, on a dry weight basis, the tissue achieved the composition of mature enamel well before the completion of mineral uptake. The enamel at this stage was porous and relatively soft. As more mineral was acquired, its hardness increased. Throughout the developing region, the Ca:P ratio remained fairly constant, but the CO2:P and Mg:P ratios both decreased due, apparently, to dilution by an influx of relatively CO2- and Mg-free mineral.

A Statistical Analysis of Association Football Attendances

Abstract: SUMMARY Little quantitative work has so far appeared in the growing literature on professional sports. In this paper a model is constructed and estimated for the attendances of four English First Division clubs. The possibility of a common law-like relationship is examined and account is taken of common cross-sectional influences in order to raise the efficiency of the estimates.

Regularity and Related Problems for Deterministic Pushdown Automata

Abstract: It ~S shown that to decide whether the language accepted by an arbitrary deterministic pushdown automaton is LL (k), or whether ~t m accepted by some one-counter or finite-turn pushdown machine, must be at least as difficult as to decide whether it is regular. The regularity problem itself is analyzed in detail, and Stearns' dec~mon procedure for this as improved by one level of exponentiatlon Upper bounds, close to known lower bounds, are obtained for the succinctness with which a pushdown automaton, and various restrictions of it, can express equivalent fimte-state machines

C 4 photosynthesis in plants from cool temperate regions, with particular reference to Spartina townsendii

Abstract: GREEN plants may be divided into two groups, C3 and C4 species, depending on whether the first product of photosynthetic CO2 fixation is phosphoglycerate (C3) or oxaloacetate (C4). In all species studied to date the C4 metabolism is associated with either ‘Kranz’ leaf anatomy, low CO2 compensation points or low 13C/12C ratios. Included in the C4 category are many of the highest yielding and rapidly growing crops, most of which are believed to have originated in tropical and subtropical regions. C4 species so far studied, which are mostly of tropical origin, attain their highest photosynthetic rates at leaf temperatures of about 30 °C and above1,2 so that selective advantages derived from photosynthetic rate will only be evident in tropical and subtropical regions1,2. A recent compilation of the reported rates of photosynthesis and productivity of C3 and C4 crops3 showed that differences in maximum annual dry matter yields could be explained by differences in lengths of growing season. It thus remains to be shown that there is a selective advantage which is a direct consequence of photosynthetic capacity of C4 species or that there is an inherent physiological barrier to the adaptation of C4 species to a similar range of latitudes to C3 species.

Cell death in normal and malignant tissues.

Abstract: Publisher Summary Cell loss from tumors is a major factor controlling their growth. An equation attempting to act as a model for growth must include suitable expressions to allow for tumor cell loss. This chapter proposes the concept that tumors contain two populations—dividing and nondividing tumor cells—and the fraction of proliferating cells is being termed as the “growth fraction.” In normal renewal tissue in steady state, the whole of cell production is matched by an equal cell loss. The reasons for the loss of cells from the proliferating pool can be classified into a number of general headings; sometimes these causes act alone, but usually they act in concert. The intrinsic, physical displacement, and extrinsic factors operate continuously in varying degrees from the earliest stages of tumor recognition. This chapter reviews cell death in normal and malignant tissues. The moment of death coincides with the loss of a certain percentage of the cell organization or the interrelation among vital cell organelles. The difficulty that faces the experimentalist is the phase of irreversible injury prior to cell death. The moment and mode of death and the necrosis of the cell corpse may show widely different patterns according to the nature of the cell and the mode of its death. Electron microscopy can be helpful in suggesting the likely sequence that is leading to the death of the cells, and it is particularly valuable for studying the death of cells when they appear discretely in well-vascularized parts of a tumor. Time-lapse microcinematography provides a continuous record of the behavior and movements of cells in vitro. The advantage of using time-lapse cinematography to study cell death is that the whole sequence of changes is permanently recorded.

Mechanics of running by quail (Coturnix)

Abstract: A force platform has been used to obtain records of the forces exerted on the ground by running quail. These records, with cinematograph film taken simultaneously, have been used to determine the fluctuations of kinetic and potential energy occurring during a step. This gives a first estimate of the energy required for running. Graphs of force against length changes during a step have been calculated for the major leg muscles, making certain assumptions. These graphs provide a second estimate of the energy required for running, and also show the stresses and strains which probably occur in the muscles. The bending moments required to break the major limb bones have been determined, and compared with the bending moments calculated to occur in running. A mathematical discussion provides estimates of the energy cost of a range of techniques of running.

Ultra-high-modulus linear polyethylene through controlled molecular weight and drawing

Abstract: Recent work is presented on the plastic deformation of linear polyethylenes. This work demonstrates the importance of molecular weight and initial morphology in determining the drawing behavior, and shows that by appropriate choice of these two factors a substantial increase in draw ratio and hence stiffness can be achieved over conventionally-oriented polyethylenes. Under optimum conditions a modulus of approximately 700 Kbar was obtained at a draw ratio of ∼30. These very high modulus materials displayed an extensibility to break of at least 3 percent and a strength of about 4 Kbar. In some cases they also exhibited very high melting points (∼139°C) and exceptionally good thermal stability.

Studies on the biology of Protopolystoma xenopodis (Monogenoidea): the oncomiracidium and life-cycle

Abstract: Protopolystoma xenopodis (Price, 1943) Bychowsky, 1957 is a parasite of the African clawed toads, species of Xenopus. Adult worms in the host's urinary bladder produce up to 15 eggs/24 h; these are voided with the urine and hatch after 20–24 days (at 26 °C). The swimming oncomiracidia enter the cloaca of the toad and travel via the urinary ducts to the kidneys. Larval development in the kidneys lasts 8–12 weeks (at 22 °C) and the juvenile parasites then migrate to the urinary bladder where maturity is reached after a further 3–4 weeks, i.e. 3–4 months after invasion. Egg production is continuous and parasite longevity has been found to exceed1½ years. Rarely, the adult worms may undertake a return migration from bladder to kidneys.The morphology of the oncomiracidium is described and compared with available accounts of other polystomatid genera. Consideration of data for the genus Polystoma suggests that the arrangement of the epidermal ciliary cells is relatively constant at the inter-specific level. Comparison of Protopolystoma with Polystoma, Diplorchis and Oculotrema reveals that whilst the ciliary cell patterns are clearly related there are important differences in both cell number and disposition at the inter-generic level. Differences are also evident in the distribution of epidermal sensillae in Protopolystoma and Polystoma. Protopolystoma is distinguished from all other polystomatids infecting anuran hosts by the presence of two pairs of haptoral hamuli.

Kelvin–Helmholtz wave growth on cylindrical sheets

Abstract: A first-order analysis of Kelvin-Helmholtz wave growth on cylindrical sheets is carried out. It is demonstrated that the growth rate of both symmetric and antisymmetric waves increases significantly with reduction of the radius of the core.