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Summary By analogy with the calculation from data of angular momentum flow across latitude surfaces in the earth's atmosphere, the corresponding calculation is proposed for the case of our galaxy and possibly other spiral nebulae. A cylindrical wall of constant radius is used to replace the latitude wall, and the subject is developed through consideration of an ideal model. Gravitational torques enter the problem in addition to advective effects and possible other torques in actual systems. The relation of gravitational torques to the structure of ordinary spirals is discussed, and certain more special effects for the case of barred spirals are introduced and discussed. The connection of the subject with the classical theory of tidal evolution is pointed out.
Zusammenfassung Analog mit der Berechnung von Daten des Flusses des Momentes der Bewegungsgrösse über Breiten-Oberflächen in der Erd-Atmosphäre wird eine korrespondierende Berechnung für den Fall unseres Sternsystems und eventuell anderer spiraler Himmelskörper vorgeschlagen. Eine zylindrische Wand mit konstantem Radius wird benutzt, um die Breitenwand zu ersetzen, und dieses Problem wird entwickelt durch die Betrachtung eines idealen Modelles. Drehmomente der Schwerkraft kommen bei diesen Problem hinzu, zusätzlich der Wirkungen von Advektion und in aller Wahrscheinlichkeit anderer Drehmomente in den vorhandenen Systemen. Die Beziehung der Drehmomente der Schwerkraft in Bezug auf gewöhnliche Spiralen ist besprochen, und gewisse mehr spezialisierte Wirkungen für den Fall von Stangen-Spiralen werden eingeführt und besprochen. Der Zusammenhang dieses Problems mit der klassischen Theorie von Drehmomenten der Gezeiten ist nachgewiesen.
Sommario In analogia ai calcoli che, partendo da dati sperimentali, vengono fatti sul trasferimento del momento angolare fra zone dell'atmosfera a diverse latitudini, vengono qui proposti corrispondenti calcoli per la nostra galassia e possibilmente per altre nebulose a spirale. Viene usata una superficie cilindrica a raggio costante in sostituzione della superficie conica a latitudine costante; l'argomento è sviluppato secondo un modello ideale. La coppia gravitazionale viene presa in considerazione insieme agli effetti convettivi e ad altre possibili coppie. Viene discussa la relazione fra la coppia gravitazionale e la struttura di una spirale ordinaria; sono quindi introdotti e discussi certi effetti nel caso di spirali a sbarra. Infine viene messo in luce la relazione del presente argomento con la teoria classica della coppia dovuta alle marce.相似文献
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The molluscan macrofauna living in shallow muddy bottoms with the green algae Caulerpa prolifera was studied monthly between February 1994 and January 1996 in the inner Bay of Cádiz (SW Spain). The molluscan assemblage followed a similar pattern over the 2 years, displaying seasonal trends in species richness, abundance and structure. During the autumn and winter months, a decrease in abundance, species richness and diversity and an increase in evenness occurred. During the spring and summer months, the molluscan assemblage was characterised by an increase in species richness, abundance and diversity. These seasonal trends were supported statistically by the presence of significantly different groupings of seasonal samples in multivariate analyses.Despite human impacts in the bay (e.g. aquaculture activities, sewage), the presence of repetitive seasonal trends, based on the qualitative and quantitative data, indicates the stability of the molluscan assemblage over 2 years. Benthic characteristics from the inner Bay of Cádiz, such as shallow soft bottoms with the presence of macrophytes and current dynamics, seem to be key factors influencing the composition and seasonality of this molluscan assemblage. 相似文献
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Information coming from fishery monitoring, surveys and experimental fishing with participation of fishers was employed to determine the impact of an artisanal gear, ‘boliche’, on the biodiversity of the Ciénaga Grande de Santa Marta (CGSM), an estuarine lagoon on the Caribbean coast of Colombia. Fishery monitoring (catch data) included landings before (1968 and 1978) and after (1994–1996) the introduction of the boliche in the CGSM (1985), whereas surveys were conducted seasonally during 1993–1994. Fishing experiments involved evaluating different mesh sizes and the short-term effect of physical disturbance by the boliche. Monitoring suggested potential trophic effects of this fishing gear: the catch of large, long-lived, carnivorous species declined after the introduction of the boliche in the CGSM, whereas catch rates of smaller, shorter-lived, and lower trophic level species increased. Surveys revealed that the boliche retained 41 species. The by-catch made up 62% of the total catch and the remaining 38% involved the three target species Eugerres plumieri, Mugil incilis and Cathorops spixii. Selectivity experiments showed that 2.5 in. stretched mesh size gill nets caught more species than the 3.0-in. mesh. The smaller mesh also increased the risk of a critical reduction in the spawning stock of target species (notably E. plumieri); a situation that could affect the fish community if mesh sizes lower than 2.5 in. were intensively used. Suspended particulate matter significantly increased after fishing activity, with higher resuspension on mud-shells and mud substrata, whereas dissolved oxygen showed no appreciable changes after fishing operations. Notwithstanding, the activity of the boliche would generate sediment resuspension between 382 and 470 t day−1, which could lead to potential cascade impacts on water quality. We propose a framework of redundancy in management measures in order to simultaneously reach management and conservation goals. 相似文献