A note on sufficient conditions for negative exponential population densities

The author presents some comments on the Mills-Muth model of urban spatial structure, which predicts that population density declines as distance to the urban center increases.     

Serial correlation in multiregional migration models

"In this paper, we outline the specification and estimation of a time series of multiregional net-migration equations subject to first-order serial correlation. We show that the necessary nonstochastic adding-up constraint, which requires that net migration in the system sum to zero, imposes restrictions on the serial-correlation coefficients. We estimate equations under these restrictions using data for the ten Canadian provinces for the period 1962-1985. The results confirm the significance of the serial-correlation coefficient and, hence, the importance of incorporating this correction in future time-series models of multiregional migration."     

The assumption of equilibrium in models of migration

In recent articles Evans (1990) and Harrigan and McGregor (1993) (hereafter HM) scrutinized the equilibrium model of migration presented in a 1989 paper by Schachter and Althaus. This model used standard microeconomics to analyze gross interregional migration flows based on the assumption that gross flows are in approximate equilibrium. HM criticized the model as theoretically untenable, while Evans summoned empirical as well as theoretical objections. HM claimed that equilibrium of gross migration flows could be ruled out on theoretical grounds. They argued that the absence of net migration requires that either all regions have equal populations or that unsustainable regional migration propensities must obtain. In fact some moves are inter- and other are intraregional. It does not follow, however, that the number of interregional migrants will be larger for the more populous region. Alternatively, a country could be divided into a large number of small regions that have equal populations. With uniform propensities to move, each of these analytical regions would experience in equilibrium zero net migration. Hence, the condition that net migration equal zero is entirely consistent with unequal distributions of population across regions. The criticisms of Evans were based both on flawed reasoning and on misinterpretation of the results of a number of econometric studies. His reasoning assumed that the existence of demand shifts as found by Goldfarb and Yezer (1987) and Topel (1986) invalidated the equilibrium model. The equilibrium never really obtains exactly, but economic modeling of migration properly begins with a simple equilibrium model of the system. A careful reading of the papers Evans cited in support of his position showed that in fact they affirmed rather than denied the appropriateness of equilibrium modeling. Zero net migration together with nonzero gross migration are not theoretically incompatible with regional heterogeneity of population, wages, or amenities.     

Incorporating the effects of spatial structure in discrete choice models of migration

Discrete-choice theory and logit models are evaluated for their usefulness in analyzing migration patterns in a zonal system. The authors "argue that spatial effects and more specifically the relative location of zones are not taken into account in such analyses. We, therefore, introduce a measure of spatial structure and advocate its usage as a predictor of migration in such models. In an example of intrametropolitan migration in Toronto [Canada], we demonstrate that this variable is not only significant but also it improves the performance of all the other variables with the greatest impact on the distance between zones. In addition, inclusion of this variable improves the overall performance of the model in terms of residuals."     

On the role of amenities in models of migration and regional development

"The role of location-specific amenities in human migration decisions, and subsequently regional development, is explored. A framework is developed which motivates a new assessment of existing alternative models of regional development, indicating the need for additional modeling efforts which focus upon amenities as critical elements in such analyses. The approach hinges upon the notion that amenity values are capitalized into wages, rents, or other local prices. This process of capitalization enables researchers to explore the implicit value that society places upon amenities, which can then be used in assessing future regional-development trends in a more comprehensive manner."     

Effects of commune-internal migration in Finland on the spatial distribution of population and on dwelling house utilization

"The article is based on a programme of research which has been pursued for some years with the purpose of describing the extent and spatial orientation of commune-internal migration [in Finland], a topic about which very little has previously been known, relating this form of migration to total population movement and to dwelling house requirements and utilization. The analysis yields information on the dynamics of population changes occurring in areas on the margins of permanent settlement, while at the same time taking into consideration the effects of regional and in particular housing policy decisions upon the spatial distribution of population."     

Oxygen isotopic evidence of residence and migration in a Greek colonial population on the Black Sea

Oxygen isotope analysis was performed on the enamel of the first and third permanent molars of 60 individuals excavated from the Kalfata-Budjaka necropolis associated with the ancient Greek colony of Apollonia Pontica (5th–3rd centuries BC) on the Black Sea coast of Bulgaria. The objective of this study was to examine the geographic origins and residential histories of these individuals and to compare the isotopic data with archaeological evidence derived from the burials in an attempt to distinguish ‘locals’ from ‘non-locals’. The analysis revealed that 55 of the 60 individuals sampled were either locally born or came from areas outside of Apollonia with isotopically similar δ¹⁸O values. Five individuals were identified as ‘non-local’, and their isotope values suggest that they originated from areas with higher δ¹⁸O_w of precipitation, most likely the Aegean region. Unfortunately, the archaeological evidence associated with these five skeletons provides no clues as to their place of origin.     

A note on physical properties of musket fire

While the use of ballistics data and principles to clarify historical experience of firearms is to be encouraged, their application is not always straightforward. This paper emphasizes the need for care. An example is given of too sanguine reliance on period studies, specifically of muzzle velocity, and of inadequate appreciation of the physics, in particular of the Magnus force.     

Numerical models of petroleum migration via buoyancy-driven porosity waves in viscously deformable sediments

The apparent ability of petroleum to migrate rapidly through low permeability sediments in sedimentary basins has led to many questions about the manner of its transport. One possible explanation is suggested from observations of the compaction of viscously deformable porous media. These systems have been found in some cases to give rise to regions of locally elevated liquid fraction, in the form of fluid‐filled porosity waves that can ascend at rates much greater than that of the background flow. Previous research on the phenomenon has focused on its implications for magma transport, but recognition of the fact that the compaction of viscous porous media can take place in sedimentary basins has suggested the possibility that porosity waves could similarly be important for hydrocarbon transport. The purpose of the present study was to test this hypothesis by quantifying the transport that would occur as the result of porosity waves initiated during the conversion of kerogen to petroleum. A one‐dimensional numerical model was constructed solving equations for the mechanics of viscous compaction and for the kinetics of reactions describing the formation of petroleum from kerogen. The results showed that porosity waves would develop readily in viscously deformable regions of sedimentary basins, but would not necessarily provide enhanced transport over that of the background flow regime. In order for the waves to achieve this enhanced transport, they must develop high amplitudes, i.e. high porosities relative to the background porous medium. To achieve the high wave amplitudes, the background porosity must be very low in absolute terms. In addition, high kerogen contents are needed in the source layer, and the source layer needs to be buried rapidly to a high temperature region of the oil window. Considerable uncertainty exists as to the value of the matrix shear viscosity of sediments in basins. However, the wave volumetric transport capacity was not found to be significantly altered as a result of variations in the value of this parameter. The physical form of the waves was strongly altered by the matrix shear viscosity, with higher values leading to lower amplitudes and generation frequency, but higher wavelengths. Thus the waves become less recognizable physically at higher values of the matrix shear viscosity. As the waves ascend to higher stratigraphic levels, where the porosity is higher, they gradually lose their physical definition and become absorbed into the background.