45,041 research outputs found
[Review of] Wyatt MacGaffey. Religion and Society in Central Africa: The BaKongo of Lower Zaire
MacGaffey, professor of anthropology at Haverford College, has based this carefully crafted book on twenty years of fieldwork and archival research. This is the first systematic study of BaKongo religion. But the study is far more than an analysis of the religion, as MacGaffey demonstrates how BaKongo social structure and power relationships are embedded in its very fabric. Dividing the study into three parts, MacGaffey first focuses on a discussion of BaKongo cosmology, then describes BaKongo ritual and power, and finally deals with issues of change in the BaKongo religion and society. Taking the perspective of the BaKongo themselves, MacGaffey explains the significance of BaKongo cosmology and how it is reflected in their myths and rituals, and in the life cycle of the BaKongo people themselves. The cosmology serves as the model upon which marriage alliances are based and the religion is the basis for the sacred and secular power held by priests and chiefs
Modeling Evolving Coronal Loops with Observations from STEREO, Hinode, and TRACE
The high densities, long lifetimes, and narrow emission measure distributions
observed in coronal loops with apex temperatures near 1 MK are difficult to
reconcile with physical models of the solar atmosphere. It has been proposed
that the observed loops are actually composed of sub-resolution ``threads''
that have been heated impulsively and are cooling. We apply this heating
scenario to nearly simultaneous observations of an evolving post-flare loop
arcade observed with the EUVI/\textit{STEREO}, XRT/\textit{Hinode}, and
\textit{TRACE} imagers and the EIS spectrometer on \textit{HINODE}. We find
that it is possible to reproduce the extended loop lifetime, high electron
density, and the narrow differential emission measure with a multi-thread
hydrodynamic model provided that the time scale for the energy release is
sufficiently short. The model, however, does not reproduce the evolution of the
very high temperature emission observed with XRT. In XRT the emission appears
diffuse and it may be that this discrepancy is simply due to the difficulty of
isolating individual loops at these temperatures. This discrepancy may also
reflect fundamental problems with our understanding of post-reconnection
dynamics during the conductive cooling phase of loop evolution.Comment: Revised version submitted to ApJ in response to referee's comment
Ab initio studies of structural instabilities in magnesium silicate perovskite
Density-functional simulations are used to calculate structural properties
and high-symmetry phonons of the hypothetical cubic phase, the stable
orthorhombic phase and an intermediate tetragonal phase of magnesium silicate
perovskite. We show that the structure of the stable phase is well described by
freezing in a small number of unstable phonons into the cubic phase. We use the
frequencies of these unstable modes to estimate transition temperatures for
cubic--tetragonal and tetragonal--orthorhombic phase transitions. These are
investigated further to find that the coupling with the strain suggests that
phonons give a better representation than rigid unit modes. The phonons of an
intermediate tetragonal phase were found to be stable except for two rotational
modes. The eigenvectors of the most unstable mode of each of the cubic and
tetragonal phases account for all the positional parameters of the orthorhombic
phase. The phase boundary for the orthorhombic--tetragonal transition
intersects possible mantle geotherms, suggesting that the tetragonal phase may
be present in the lower mantle.Comment: 16 pages, REVTEX, 7 postscript figures (Fig 1 very large, contact
Authors if required); submitted to Physics and Chemistry of Mineral
Retracts of vertex sets of trees and the almost stability theorem
Let G be a group, let T be an (oriented) G-tree with finite edge stabilizers,
and let VT denote the vertex set of T. We show that, for each G-retract V' of
the G-set VT, there exists a G-tree whose edge stabilizers are finite and whose
vertex set is V'. This fact leads to various new consequences of the almost
stability theorem. We also give an example of a group G, a G-tree T and a
G-retract V' of VT such that no G-tree has vertex set V'.Comment: 15 pages, 0 figures. Formerly titled "Some refinements of the almost
stability theorem". Version
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