881 research outputs found
Beads from the Hudson\u27s Bay Company\u27s Principal Depot, York Factory, Manitoba, Canada
There is no other North American fur trade establishment whose longevity and historical significance can rival that of York Factory. Located in northern Manitoba, Canada, at the base of Hudson Bay, it was the Hudson\u27s Bay Company\u27s principal Bay-side trading post and depot for over 250 years. The existing site of York Factory is the last of a series of three posts, the first of which was erected in 1684. Completed in 1792, York Factory III functioned as the principal depot and administrative center for the great Northern Department until the 1860s when its importance began to wane. It then entered a long period of decline which ended in 1957, when the post was finally closed. Subsequent archaeological work at the site has revealed many structural features and associated artifacts including a large and varied assemblage of beads, mostly glass, which are the subject of this report
The Beads of St. Eustatius, Netherlands Antilles
Archaeological excavations conducted on the Caribbean island of St. Eustatius over a seven-year period produced a wide array of 18th to early 20th-century beads of glass, coral and carnelian. Detailed descriptions of the recovered specimens are supplemented by information concerning their distribution, relative frequencies, color preference, temporal placement, origins, acquisition and use. Comparative site data are also provided
A universal mechanism for long-range cross-correlations
Cross-correlations are thought to emerge through interaction between particles. Here we present a universal dynamical mechanism capable of generating power-law cross-correlations between non-interacting particles exposed to an external potential. This phenomenon can occur as an ensemble property when the external potential induces intermittent dynamics of Pomeau-Manneville type, providing laminar and stochastic phases of motion in a system with a large number of particles. In this case, the ensemble of particle-trajectories forms a random fractal in time. The underlying statistical self-similarity is the origin of the observed power-law cross-correlations. Furthermore, we have strong indications that a sufficient condition for the emergence of these long-range cross-correlations is the divergence of the mean residence time in the laminar phase of the single particle motion (sporadic dynamics). We argue that the proposed mechanism may be relevant for the occurrence of collective behaviour in critical systems
A consistent approach for the treatment of Fermi acceleration in time-dependent billiards
The standard description of Fermi acceleration, developing in a class of
time-dependent billiards, is given in terms of a diffusion process taking place
in momentum space. Within this framework the evolution of the probability
density function (PDF) of the magnitude of particle velocities as a function of
the number of collisions is determined by the Fokker-Planck equation (FPE).
In the literature the FPE is constructed by identifying the transport
coefficients with the ensemble averages of the change of the magnitude of
particle velocity and its square in the course of one collision. Although this
treatment leads to the correct solution after a sufficiently large number of
collisions has been reached, the transient part of the evolution of the PDF is
not described. Moreover, in the case of the Fermi-Ulam model (FUM), if a
stadanrd simplification is employed, the solution of the FPE is even
inconsistent with the values of the transport coefficients used for its
derivation. The goal of our work is to provide a self-consistent methodology
for the treatment of Fermi acceleration in time-dependent billiards. The
proposed approach obviates any assumptions for the continuity of the random
process and the existence of the limits formally defining the transport
coefficients of the FPE. Specifically, we suggest, instead of the calculation
of ensemble averages, the derivation of the one-step transition probability
function and the use of the Chapman-Kolmogorov forward equation. This approach
is generic and can be applied to any time-dependent billiard for the treatment
of Fermi-acceleration. As a first step, we apply this methodology to the FUM,
being the archetype of time-dependent billiards to exhibit Fermi acceleration.Comment: 12 Pages, 7 figure
Hyperacceleration in a stochastic Fermi-Ulam model
Fermi acceleration in a Fermi-Ulam model, consisting of an ensemble of
particles bouncing between two, infinitely heavy, stochastically oscillating
hard walls, is investigated. It is shown that the widely used approximation,
neglecting the displacement of the walls (static wall approximation), leads to
a systematic underestimation of particle acceleration. An improved
approximative map is introduced, which takes into account the effect of the
wall displacement, and in addition allows the analytical estimation of the long
term behavior of the particle mean velocity as well as the corresponding
probability distribution, in complete agreement with the numerical results of
the exact dynamics. This effect accounting for the increased particle
acceleration -Fermi hyperacceleration- is also present in higher dimensional
systems, such as the driven Lorentz gas.Comment: 4 pages, 3 figures. To be published in Phys. Rev. Let
Scattering off an oscillating target: Basic mechanisms and their impact on cross sections
We investigate classical scattering off a harmonically oscillating target in
two spatial dimensions. The shape of the scatterer is assumed to have a
boundary which is locally convex at any point and does not support the presence
of any periodic orbits in the corresponding dynamics. As a simple example we
consider the scattering of a beam of non-interacting particles off a circular
hard scatterer. The performed analysis is focused on experimentally accessible
quantities, characterizing the system, like the differential cross sections in
the outgoing angle and velocity. Despite the absence of periodic orbits and
their manifolds in the dynamics, we show that the cross sections acquire rich
and multiple structure when the velocity of the particles in the beam becomes
of the same order of magnitude as the maximum velocity of the oscillating
target. The underlying dynamical pattern is uniquely determined by the phase of
the first collision between the beam particles and the scatterer and possesses
a universal profile, dictated by the manifolds of the parabolic orbits, which
can be understood both qualitatively as well as quantitatively in terms of
scattering off a hard wall. We discuss also the inverse problem concerning the
possibility to extract properties of the oscillating target from the
differential cross sections.Comment: 18 page
Statistical models for over-dispersion in the frequency of peaks over threshold data for a flow series.
In a peaks over threshold analysis of a series of river flows, a sufficiently high threshold is used to extract the peaks of independent flood events. This paper reviews existing, and proposes new, statistical models for both the annual counts of such events and the process of event peak times. The most common existing model for the process of event times is a homogeneous Poisson process. This model is motivated by asymptotic theory. However, empirical evidence suggests that it is not the most appropriate model, since it implies that the mean and variance of the annual counts are the same, whereas the counts appear to be overdispersed, i.e., have a larger variance than mean. This paper describes how the homogeneous Poisson process can be extended to incorporate time variation in the rate at which events occur and so help to account for overdispersion in annual counts through the use of regression and mixed models. The implications of these new models on the implied probability distribution of the annual maxima are also discussed. The models are illustrated using a historical flow series from the River Thames at Kingston
Comparing league formats with respect to match importance in Belgian football
Recently, most clubs in the highest Belgian football division have become convinced that the format of their league should be changed. Moreover, the TV station that broadcasts the league is pleading for a more attractive competition. However, the clubs have not been able to agree on a new league format, mainly because they have conflicting interests. In this paper, we compare the current league format, and three other formats that have been considered by the Royal Belgian Football Association. We simulate the course of each of these league formats, based on historical match results. We assume that the attractiveness of a format is determined by the importance of its games; our importance measure for a game is based on the number of teams for which this game can be decisive to reach a given goal. Furthermore, we provide an overview of how each league format aligns with the expectations and interests of each type of club
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