首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   421篇
  免费   20篇
  国内免费   6篇
测绘学   15篇
大气科学   50篇
地球物理   133篇
地质学   130篇
海洋学   58篇
天文学   28篇
综合类   7篇
自然地理   26篇
  2023年   2篇
  2022年   2篇
  2021年   8篇
  2020年   6篇
  2019年   2篇
  2018年   21篇
  2017年   16篇
  2016年   25篇
  2015年   23篇
  2014年   19篇
  2013年   29篇
  2012年   16篇
  2011年   42篇
  2010年   29篇
  2009年   20篇
  2008年   25篇
  2007年   16篇
  2006年   17篇
  2005年   16篇
  2004年   11篇
  2003年   11篇
  2002年   8篇
  2001年   8篇
  2000年   2篇
  1999年   7篇
  1998年   6篇
  1997年   6篇
  1996年   6篇
  1995年   4篇
  1994年   2篇
  1992年   3篇
  1991年   2篇
  1990年   4篇
  1989年   4篇
  1988年   3篇
  1987年   2篇
  1984年   4篇
  1983年   3篇
  1981年   2篇
  1980年   1篇
  1979年   3篇
  1978年   1篇
  1975年   1篇
  1974年   1篇
  1971年   1篇
  1970年   1篇
  1967年   1篇
  1966年   1篇
  1965年   1篇
  1962年   1篇
排序方式: 共有447条查询结果,搜索用时 250 毫秒
91.
92.
93.
94.
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   
95.
96.
97.
98.
99.
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   
100.
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   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号