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   82023篇
  免费   1432篇
  国内免费   2119篇
测绘学   2771篇
大气科学   6352篇
地球物理   16292篇
地质学   32240篇
海洋学   6063篇
天文学   14490篇
综合类   2397篇
自然地理   4969篇
  2022年   368篇
  2021年   598篇
  2020年   624篇
  2019年   673篇
  2018年   5845篇
  2017年   5095篇
  2016年   3976篇
  2015年   1302篇
  2014年   1594篇
  2013年   2898篇
  2012年   2848篇
  2011年   5139篇
  2010年   4159篇
  2009年   5097篇
  2008年   4301篇
  2007年   4775篇
  2006年   2524篇
  2005年   2063篇
  2004年   2200篇
  2003年   2073篇
  2002年   1848篇
  2001年   1455篇
  2000年   1387篇
  1999年   1166篇
  1998年   1221篇
  1997年   1152篇
  1996年   957篇
  1995年   897篇
  1994年   809篇
  1993年   719篇
  1992年   687篇
  1991年   624篇
  1990年   713篇
  1989年   608篇
  1988年   570篇
  1987年   712篇
  1986年   576篇
  1985年   734篇
  1984年   867篇
  1983年   785篇
  1982年   744篇
  1981年   705篇
  1980年   715篇
  1979年   615篇
  1978年   630篇
  1977年   572篇
  1976年   560篇
  1975年   547篇
  1974年   526篇
  1973年   546篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
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   
92.
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   
93.
F.A. Gifford  R.P. Hosker  K.S. Rao 《Icarus》1978,36(1):133-146
The spreading angle of a number of light and dark Martian streaks is determined from selected Mariner 9 images. The resulting frequency distributions of spreading half-angles have maxima at ~5° for light, and ~7° for dark streaks; however the dark streaks have a secondary maximum spreading angle at ~14°. The smaller values, which include most streaks, are interpreted as crater-wake spreading phenomena. The larger value, found in only a few dark streaks or “tails,” may result from atmospheric diffusion and subsequent deposition of material from isolated sources such as vents or blowouts. An atmospheric diffusion-deposition analysis is presented, assuming this streak origin, from which it is possible to deduce the eddy diffusivity, K, in Mars' boudary layer. Calculated K values are found to agree with various theoretical estimates. They lie in the range 107 and 109 cm2 sec?1 and exhibit the proper scale dependence. Thus it appears that, in addition to streak-derived wind direction patterns and speed information, it is possible in a few cases to derive information on Mars' boundary-layer turbulence from streak-spreading measurements.  相似文献   
94.
95.
96.
97.
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   
98.
99.
100.
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