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   86371篇
  免费   1160篇
  国内免费   532篇
测绘学   1774篇
大气科学   5653篇
地球物理   16474篇
地质学   31063篇
海洋学   7941篇
天文学   20057篇
综合类   224篇
自然地理   4877篇
  2022年   622篇
  2021年   1052篇
  2020年   1120篇
  2019年   1255篇
  2018年   2569篇
  2017年   2371篇
  2016年   2734篇
  2015年   1372篇
  2014年   2643篇
  2013年   4529篇
  2012年   2879篇
  2011年   3720篇
  2010年   3402篇
  2009年   4299篇
  2008年   3754篇
  2007年   3872篇
  2006年   3587篇
  2005年   2544篇
  2004年   2489篇
  2003年   2315篇
  2002年   2295篇
  2001年   1992篇
  2000年   1982篇
  1999年   1549篇
  1998年   1609篇
  1997年   1490篇
  1996年   1263篇
  1995年   1262篇
  1994年   1052篇
  1993年   1025篇
  1992年   953篇
  1991年   986篇
  1990年   948篇
  1989年   829篇
  1988年   760篇
  1987年   893篇
  1986年   785篇
  1985年   960篇
  1984年   1087篇
  1983年   1043篇
  1982年   965篇
  1981年   902篇
  1980年   803篇
  1979年   759篇
  1978年   739篇
  1977年   628篇
  1976年   635篇
  1975年   622篇
  1974年   597篇
  1973年   663篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
The authors describe the effect of copper pollution on the mitochondria from various areas of the Torpedo marmorata central nervous system. A high percentage of swollen mitochondria has been observed in neurons from animals experimentally exposed to high level of copper (4 ppm). The correlation with age pigment granules is discussed.  相似文献   
112.
113.
Trends in emigration from Ireland over time are reviewed. "During the eighteenth and nineteenth centuries overseas migration to the United States and seasonal harvest migration to Britain were the main types of movement, but over the past 100 years the Irish have developed a special affinity for settling in British towns. Although the outflow was halted for a time during the 1970s, when return migration took over, the 1980s have seen a renewal of the exodus. This time, however, the character of the flow has changed from predominantly low-skill construction and factory workers to embrace better-educated emigrants, including many graduates. This shift reflects Ireland's changing position in the international market for labour."  相似文献   
114.
We observed near-Main-Sequence B-stars in the Magellanic Clouds with the 3.6 m telescope and CASPEC at La Silla. We obtained spectra of high resolution and high S/N-ratio. The stars are members of the blue globular clusters NGC 1818 (LMC) and NGC 330 (SMC). The spectra are used for differential abundance analyses using HER as galactic reference star. Apart from CNO the metals are underabundant by about a factor of three and four for the LMC and SMC star, respectively. The CNO pattern is interesting since in both stars oxygen is considerably more abundant than carbon.Based on observations collected at ESO, La Silla and on Calar Alto, Spain and on observations made with the International Ultraviolet Explorer.  相似文献   
115.
116.
117.
"Debates concerning the origins and development of the late nineteenth- to early twentieth-century declines in marital fertility and infant mortality in England and Wales have been centred largely on the material provided by answers to the ?special' questions in the 1911 census. In their published form these figures have restricted researchers to an examination of large scale geographic and social class differences in the levels and rates of decline of the two phenomena. This paper outlines research conducted on a sample of individual census returns from the 1911 census. From this data it becomes clear that for Victorian and Edwardian England ?where one lived' was rather more important than ?who one was' in determining both family building strategies and the survival of those children born."  相似文献   
118.
By considering a simple fluid model, we investigate the role of phase transitions in the ISM on the galaxy- scale gas dynamics. Cooling and heating timescales in the ISM are typically shorter than typical galactic rotation timescales, so the individual phases in the ISM can be assumed to be in temperature equilibrium with the radiation field. Using this approximation we can construct an equation of state which depends upon the average density and mass fractions in the individual phases. Previous studies suggest that there is an equilibrium phase fraction as a function of pressure. We incorporate evolution towards this equilibrium state as a relaxation term with a time to obtain equilibrium . We derive a condition in terms of a critical Mach number when one dimensional shocks should be continuous. For small values of the relaxation time we show that the relaxation term acts like a viscosity. We show with one dimensional simulations that increasing causes shocks to become smoother. This work suggests that phase changes can strongly effect the gas dynamics of the ISM across spiral arms and bars.  相似文献   
119.
120.
The possible correlation noted between the intrinsic quasar luminosity and the absorption line expulsion velocity is re-examined using homogeneous data sets for metal and L only line systems. The method of analysis is chosen to enable any reasonable form of correlation to be found. No correlation is detected at a confidence level >10%.  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