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   37440篇
  免费   969篇
  国内免费   1052篇
测绘学   948篇
大气科学   2875篇
地球物理   7667篇
地质学   13817篇
海洋学   3341篇
天文学   8159篇
综合类   240篇
自然地理   2414篇
  2022年   291篇
  2021年   474篇
  2020年   457篇
  2019年   497篇
  2018年   914篇
  2017年   877篇
  2016年   1050篇
  2015年   723篇
  2014年   1049篇
  2013年   1871篇
  2012年   1345篇
  2011年   1789篇
  2010年   1566篇
  2009年   2023篇
  2008年   1702篇
  2007年   1770篇
  2006年   1701篇
  2005年   1226篇
  2004年   1141篇
  2003年   1038篇
  2002年   1005篇
  2001年   846篇
  2000年   827篇
  1999年   672篇
  1998年   717篇
  1997年   690篇
  1996年   572篇
  1995年   564篇
  1994年   479篇
  1993年   421篇
  1992年   421篇
  1991年   386篇
  1990年   457篇
  1989年   374篇
  1988年   356篇
  1987年   438篇
  1986年   346篇
  1985年   432篇
  1984年   531篇
  1983年   451篇
  1982年   452篇
  1981年   403篇
  1980年   419篇
  1979年   360篇
  1978年   347篇
  1977年   341篇
  1976年   311篇
  1975年   297篇
  1974年   313篇
  1973年   341篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
The American whaling industry rose from humble beginnings off Long island to become an international giant. In its peak year, 1846, 735 ships and 70,000 people served the industry. As whale stocks and reserves decreased, whalers were forced to go farther and farther from their New England home ports. Voyages became longer, and risks on required return-on-investment became higher. The easy money of Atlantic and Pacific whaling was no more: the only remaining profitable ventures were to Arctic and Antarctic waters. Many ships returned empty, if at all. in 1871, most of the Arctic whaling fleet was crushed by early winter ice and lost. This calamity, in conjunction with the long-term diminishing whale stocks, the diversion of investment capital to more profitable ventures, and the discovery, development, and refinement of abundant petroleum crude oil, struck the death blow to the American whaling industry. By 1890, only 200 whaling vessels were at work, and by 1971, no American commercial whaling ship sailed the world's oceans.It is apparent that no single event caused the final, rapid decline. However, a single calamity, in an already stressed industry, that was self-insured and commercially interlinked, precipitated the end. Today's American petroleum industry, although adopting some principles of the American whaling industry, also has embraced other activities such as work process reengineering and customer alliances, which may preempt, or postpone, a similar catastrophic demise.  相似文献   
52.
The equatorial photospheric rotation rate has been observed on 14 days in 1978–1980. The resulting rotation rate, = 14.14±0.04°/day, is 2% slower than the rate as observed for long-lived sunspots.Stationed at Kitt Peak National Observatory.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.  相似文献   
53.
54.
55.
Lithology and Mineral Resources - The chemical composition and Nd isotope systematics were obtained for mudrocks (mudstones) from sections of the Siberian hypostratotype of the Riphean and Vendian...  相似文献   
56.
57.
Only a few very young landforms are the result of currently operating geomorphic processes. Because the time scale for landscape evolution is much longer than the time scale for late Cenozoic climate changes, almost all landscapes are palimpsests, written over repeatedly by various combinations of climate-determined processes. Relict glacial and periglacial landforms are widely identified in mid-latitude regions that have been traditionally described as having been shaped by the “normal” processes of fluvial erosion. Less confidently, deeply weathered regolith and associated relict landforms in the middle and high latitudes are attributed to early Tertiary warmth. However, assemblages of geomorphic processes specific to certain climatic regions, like faunal and floral assemblages, cannot be translated across latitude, so in spite of the many books about the geomorphology of specific modern climate regions, there are few sources that discuss former warm high-latitude, or cold low-latitude, low-altitude geomorphic processes that have no modern analogs. Students and teachers alike who attempt to interpret landforms by extrapolating modern climatic conditions to other latitudinal zones will find their outlook broadened, and they become better prepared to consider the geomorphic impacts of global climate change.  相似文献   
58.
59.
A zoobenthos submodel is included in an ecosystem model. The new model is used to assess the role of zoobenthos in the phosphorus exchange at the water–bed interface, in particular, the phosphorus cycle in the water body ecosystem is described more accurately. The calculated zoobenthos distribution over the bed of a lake can be used in modeling the lake ichthyocenoses.  相似文献   
60.
Relationships of mineralized microbiota with the content of polycyclic aromatic hydrocarbons in hydrothermal systems are considered. It has been established that the mineralized microbiota can serve as an indicator of hydrothermal hydrocarbon flows in present-day and ancient deposits.  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