Motion in a Bose condensate: IX. Crow instability of antiparallel vortex pairs

The Gross-Pitaevskii (GP) equation admits a two-dimensional solitary wave solution representing two mutually self-propelled, anti-parallel straight line vortices. The complete sequence of such solitary wave solutions has been computed by Jones and Roberts (J. Phys. A, 15, 2599, 1982). These solutions are unstable with respect to three-dimensional perturbations (the Crow instability). The most unstable mode has a wavelength along the direction of the vortices of the same order as their separation. The growth rate associated with this mode is evaluated here, and it is found to increase very rapidly with decreasing separation. It is shown, through numerical integrations of the GP equation that, as the perturbations grow to finite amplitude, the lines reconnect to produce a sequence of almost circular vortex rings.Comment: Submitted to J. Phys. A: Math. Gen.; Corrected reference

Assessment of the environmental toxicity and carcinogenicity of tungsten-based shot.

The toxicity of elemental tungsten released from discharged shot was assessed against previous studies that established a 1% toxic threshold for soil organisms. Extremely heavy theoretical shot loadings of 69,000 shot/ha were used to generate estimated environmental concentrations (EEC) for two brands of tungsten-based shot containing 51% and 95% tungsten. The corresponding tungsten EEC values were 6.5–13.5 mg W/kg soil, far below the 1% toxic threshold. The same shot loading in water produced tungsten EEC values of 2.1–4.4 mg W/L, levels that are not toxic under experimental conditions. Pure tungsten has not been shown to exhibit carcinogenic properties when ingested or embedded in animal tissues, but nickel, with which it is often alloyed, has known carcinogenicity. Given the large number of waterfowl that carry shot embedded in their body, it is advisable to screen lead shot substitutes for their carcinogenic potential through intra-muscular implantation

Improvement of oral reports through the students' use of audio-visual aids

Author misnumbered thesis. Please note that there are TWO page 108s, but the continuity is the same. Thesis (Ed.M.)--Boston Universit

Electrostatic charging of non-polar colloids by reverse micelles

Colloids dispersed in a non-polar solvent become charged when reverse micelles are added. We study the charge of individual sterically-stabilized poly(methyl methacrylate) spheres dispersed in micellar solutions of the surfactants sodium bis(2-ethyl 1-hexyl) sulfosuccinate [AOT], zirconyl 2-ethyl hexanoate [Zr(Oct)$_{2}$], and a copolymer of poly(12-hydroxystearic acid)--poly(methyl methacrylate) [PHSA-PMMA]. Although the sign of the particle charge is positive for Zr(Oct)$_{2}$, negative for AOT, and essentially neutral for PHSA-PMMA the different micellar systems display a number of common features. In particular, we demonstrate that, over a wide range of concentrations, the colloid charge is independent of the number of micelles added and scales linearly with the colloid size. A simple thermodynamic model, in which the particle charge is generated by the competitive adsorption of both positive and negative micelles, is in good agreement with the experimental data

Distributions of the perceptual switches associated with viewing ambiguous stimuli

The ambiguous Necker cube stimulus appears to switch between two alternative perspectives, 'cube-up' and 'cube-down', such that the inter-switch interval gives percept duration. Percept frequency, plotted as a function of duration, is modelled by a gamma distribution, and the model fit is improved by discarding arbitrarily durations greater than three standard deviations above the mean (Borsellino et al, 1972 Kybernetik 10 139 - 144). Alternatively, good fits can be achieved by discarding the 'dead period' at the beginning of the distribution that represents the observer's decision time between a switch and the subsequent response with a key press that signals the switch (Martin, 1967 Papers in Psychology 1 2). Twenty-nine naïve observers recorded switches between cube-up and cube-down percepts. During a total observation period of 10 min, observers recorded up to 300 switches. We found that the best fits were generated with the gamma distribution when the 'dead period' was discarded. Arbitrary trimming of the tail, three standard deviations above the mean, had little impact on goodness-of-fit. This suggests that (i) the perceptual changes can be adequately modelled as a simple stochastic process, and (ii) the modelling is improved by accounting for the decision time between switch and response

Dishing the dirt: sediments reveal a famous early human cave site was also home to hyenas and wolves

Denisova Cave in Siberia\u27s Altai Mountains is one of the world\u27s most important archaeological sites. It is famous for preserving evidence of three early human groups: Neanderthals, early Homo sapiens, and a third group known as the Denisovans. Fossil bones, stone tools and ancient DNA gathered from the cave have told a story that is extremely significant for understanding the early chapters of human evolution in Asia, going back 300,000 years. But our new analysis of the cave\u27s dirt floor reveals that it was also frequented by hyenas, wolves, and even bears for much of its history

The African Landscape Through Space and Time

It is generally accepted that Cenozoic epeirogeny of the African continent is moderated by convective circulation of the mantle. Nevertheless, the spatial and temporal evolution of Africa's “basin-and-swell” physiography is not well known. Here we show how continental drainage networks can be used to place broad constraints on the pattern of uplift through space and time. First, we assemble an inventory of 710 longitudinal river profiles that includes major tributaries of the 10 largest catchments. River profiles have been jointly inverted to determine the pattern of uplift rate as a function of space and time. Our inverse model assumes that shapes of river profiles are controlled by uplift rate history and modulated by erosional processes, which can be calibrated using independent geologic evidence (e.g., marine terraces, volcanism and thermochronologic data). Our results suggest that modern African topography started to develop ∼30 Myr ago when volcanic swells appeared in North and East Africa. During the last 15–20 Myr, subequatorial Africa was rapidly elevated, culminating in the appearance of three large swells that straddle southern and western coasts. Our results enable patterns of sedimentary flux at major deltas to be predicted and tested. We suggest that the evolution of drainage networks is dominated by rapid upstream advection of signals produced by a changing pattern of regional uplift. An important corollary is that, with careful independent calibration, these networks might act as useful tape recorders of otherwise inaccessible mantle processes. Finally, we note that there are substantial discrepancies between our results and published dynamic topographic predictions