A test for the coincident economic and spatial clustering of business enterprises

This paper uses input-output data combined with point process modeling techniques to test whether enterprises linked within nominal buyer-supplier chains have a greater propensity to cluster in space than manufacturing enterprises in general. The methodology controls for the general tendency of firms to seek locations in concentrated agglomerations and isolates the influence of firm interdependence on spatial clustering. Our findings suggest that there is indeed an association between economic linkages and geographic clustering in our study area, but only for some types of economic clusters, mainly those that are comprised mainly of more knowledge-based or technology-intensive sectors. In general, we endeavor to show that spatial analytical methods hold considerable promise for conducting rigorous tests of industrial location questions. Received: 9 September 1998 / Accepted: 12 December 1999     

Progressive crustal contamination of the Bushveld Complex: evidence from Nd isotopic analyses of the cumulate rocks

We report the first Nd isotopic data on the cumulate rocks of the Bushveld Complex, South Africa. We analysed 17 whole-rock samples covering 4700 m of stratigraphy through the Lower, Critical and Main Zones of the intrusion at Union Section, north-western Bushveld Complex. The basal ultramafic portions of the complex have markedly higher ɛNd(T) (−5.3 to −6.0) than the gabbronoritic Main Zone (ɛNd(T) −6.4 to −7.9). The rocks of the Upper Critical Zone have intermediate values. These results are in agreement with new Nd isotope data on marginal rocks and sills in the floor of the complex that are generally interpreted as representing chilled parental magmas, and with published Sr isotopic data, all of which show a larger crustal component in the upper part of the intrusion. In contrast, the concentrations of many highly incompatible trace elements are decoupled from the isotopic signatures. The basal portions of the complex have higher ratios of incompatible to compatible trace elements than the upper portions. The variations of isotopic and trace-element compositions are interpreted in terms of a change in the nature of the crustal material that contaminated Bushveld magmas. Those magmas that fed into the lower part of the complex had assimilated a relatively small amount of incompatible trace-element-rich partial melt of upper crust, whereas magmas parental to the upper part of the complex had assimilated a higher proportion of the incompatible trace-element-poor residue of partial melting. Received: 5 October 1999 / Accepted: 7 July 2000     

Collision Probability for Earth-Crossing Asteroids Using Orbital Ranging

We introduce new techniques for the computation of the collision probability for Earth-crossing asteroids in the case of short observational arcs and/or small numbers of observations. The techniques rely on the orbital element probability density computed using statistical orbital ranging. We apply the techniques to the Earth-crossing asteroid 1998 OX₄with non-vanishing collision probability in numerous close approaches after the year 2012 (inclusive). We study the invariance of the collision probability in transformations between different orbital element sets, and develop a Spearman rank correlation measure for the validity of the linear approximation. We introduce an optimized, fast version of the statistical ranging method.     

Observation of infrared lines in a prominence at 1–5 microns

We have observed 11 hydrogen and 3 helium lines in a quiescent prominence; none have been previously observed except for the He 10830 line. In contrast to Zirker's results based on longer-wavelength infrared lines, we find no net velocity shift, Doppler widths narrower than his by more than 30%, and a hydrogen excitation temperature 3 times higher. From the line intensity method, we find a helium temperature of 5300(±200) K and a hydrogen temperature of 10 900(±1900) K, in reasonable agreement with the findings of visible line studies. We conclude that helium lines are formed in the central region while hydrogen lines are formed around the edges of the prominence.Visiting Astronomer at the National Solar Observatory, a division of the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation.