Monthly gravity field estimates made by the twin Gravity Recovery and Climate Experiment (GRACE) satellites have a geoid height accuracy of 2 to 3 millimeters at a spatial resolution as small as 400 kilometers. The annual cycle in the geoid variations, up to 10 millimeters in some regions, peaked predominantly in the spring and fall seasons. Geoid variations observed over South America that can be largely attributed to surface water and groundwater changes show a clear separation between the large Amazon watershed and the smaller watersheds to the north. Such observations will help hydrologists to connect processes at traditional length scales (tens of kilometers or less) to those at regional and global scales.

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Most independent nations today were part of empires in 1945. Using bilateral trade data from 1948 to 2006, we examine the effect of independence on post-colonial trade. While there is little short-run effect on trade, after four decades trade with the metropole (colonizer) has contracted by about 65%. Hostile separations lead to large, immediate reductions in trade. We also find that trade between former colonies of the same empire erodes as much as trade with the metropole, whereas trade with third countries decreases about 20%. The gradual trade deterioration following independence suggests the depreciation of some form of trading capital.

We discuss the IAU resolutions B1.3, B1.4, B1.5, and B1.9 that were adopted during the 24th General Assembly in Manchester, 2000, and provides details on and explanations for these resolutions. It is explained why they present significant progress over the corresponding IAU 1991 resolutions and why they are necessary in the light of present accuracies in astrometry, celestial mechanics, and metrology. In fact, most of these resolutions are consistent with astronomical models and software already in use. The metric tensors and gravitational potentials of both the Barycentric Celestial Reference System and the Geocentric Celestial Reference System are defined and discussed. The necessity and relevance of the two celestial reference systems are explained. The transformations of coordinates and gravitational potentials are discussed. Potential coefficients parameterizing the post-Newtonian gravitational potentials are expounded. Simplified versions of the time transformations suitable for modern clock accuracies are elucidated. Various approximations used in the resolutions are explicated and justified. Some models (e.g., for higher spin moments) that serve the purpose of estimating orders of magnitude have actually never been published before.

We evaluate two alternative models of international trade in differentiated products. An increasing returns model where varieties are linked to firms predicts home market effects: increases in a country's share of demand cause disproportionate increases in its share of output. In contrast, a constant returns model with national product differentiation predicts a less than proportionate increase. We examine a panel of U.S. and Canadian manufacturing industries to test the models. Although we find support for either model, depending on whether we estimate based on within or between variation, the preponderance of the evidence supports national product differentiation. (JEL F12, F15)

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Abstract. Immigrants may expand trade with their country of origin, owing to superior knowledge of, or preferential access to, market opportunities. We test this proposition using Canadian trade data with 136 partners from 1980 to 1992. In an augmented gravity equation we find that a 10 per cent increase in immigrants is associated with a 1 per cent increase in Canadian exports to the immigrant's home country and a 3 per cent increase in imports. We also find that among the primary categories of immigrants, independants have the greater influence on trade. Immigration via the entrepreneur class appears to have less impact on trade than all other classes except refugees. Immigration et expansion du commerce international: des resultats econometriques pour le Canada. Les immigrants peuvent accroitre le niveau du commerce international avec leur pays d'origine a cause de leur meilleure connaissance des possibilit6s de commerce ou de leur acces pr6f6rentiel a ces march6s. Les auteurs mettent cette proposition au test en utilisant les donn6es sur le commerce international avec 136 partenaires de 1980 a 1992. Les r6sultats montrent qu'une augmentation de 10% des immigrants est associ6e a une augmentation de 1% dans les exportations canadiennes vers le pays d'origine des immigrants et a une augmentation de 3% dans les importations en provenance de ces pays. Il appert que ce sont les immigrants ind6pendants qui ont l'influence la plus grande sur le commerce international L'immigration sous la rubrique entrepreneur semble avoir un effet moindre sur le commerce international que celles de toutes les autres classes d'immigrants a l'exception des r6fugi6s.While international trade has increased relative to world GDP in the postwar era, cross-border trade flows remain at much lower levels than those that would prevail in a fully integrated world economy. Manufacturers in Canada, for instance, supply 60 per cent of the Canadian market despite the fact that Canada produces only 2.5 per cent of world GDP. With the dismantling of most formal barriers to international The authors are grateful to two anonymous referees, Clive Chapple, Don Devoretz, Elizabeth Ruddick, and Ed Safarian for their helpful suggestions. We thank Meng Zhang for her assistance in constructing the data set and Jennifer Cliff for helping us to understand the immigration application procedure. Canadian Journal of Economics Revue canadienne d'Economique, xxxI, No. I February f6vrier 1998. Printed in Canada Imprime au Canada 0008-4085 / 98 / 47-62 $1.50 ? Canadian Economics Association 48 Keith Head and John Ries trade, transportation and transaction costs emerge as principal explanations of why trade remains largely within nation...

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