全文获取类型
收费全文 | 132258篇 |
免费 | 1937篇 |
国内免费 | 942篇 |
专业分类
测绘学 | 3044篇 |
大气科学 | 9226篇 |
地球物理 | 26298篇 |
地质学 | 46463篇 |
海洋学 | 11911篇 |
天文学 | 29594篇 |
综合类 | 329篇 |
自然地理 | 8272篇 |
出版年
2021年 | 1285篇 |
2020年 | 1471篇 |
2019年 | 1607篇 |
2018年 | 3192篇 |
2017年 | 2993篇 |
2016年 | 3670篇 |
2015年 | 2078篇 |
2014年 | 3572篇 |
2013年 | 6815篇 |
2012年 | 3839篇 |
2011年 | 5209篇 |
2010年 | 4682篇 |
2009年 | 6156篇 |
2008年 | 5423篇 |
2007年 | 5330篇 |
2006年 | 5132篇 |
2005年 | 3929篇 |
2004年 | 3902篇 |
2003年 | 3672篇 |
2002年 | 3579篇 |
2001年 | 3137篇 |
2000年 | 3086篇 |
1999年 | 2563篇 |
1998年 | 2589篇 |
1997年 | 2438篇 |
1996年 | 2158篇 |
1995年 | 2104篇 |
1994年 | 1841篇 |
1993年 | 1742篇 |
1992年 | 1636篇 |
1991年 | 1559篇 |
1990年 | 1682篇 |
1989年 | 1484篇 |
1988年 | 1327篇 |
1987年 | 1604篇 |
1986年 | 1414篇 |
1985年 | 1729篇 |
1984年 | 2003篇 |
1983年 | 1906篇 |
1982年 | 1743篇 |
1981年 | 1648篇 |
1980年 | 1438篇 |
1979年 | 1401篇 |
1978年 | 1409篇 |
1977年 | 1263篇 |
1976年 | 1215篇 |
1975年 | 1183篇 |
1974年 | 1168篇 |
1973年 | 1217篇 |
1972年 | 747篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The question of what lies ahead is of particular concern for Latin Americanists. The last decade has witnessed a serious erosion of both the popularity of their specialty, and an equally troublesome reduction in employment opportunities. This paper uses Association of American Geographers (AAG) data bases to document the age-gender structure of contemporary Latin Americanist geographers, and projects likely compositional changes through the end of the century. 相似文献
992.
An ecotoxicological assessment is given to the state of small lakes in specially protected natural areas in Northwestern and Central European Russia, which suffer from aerotechnogenic acidification. The toxicity of water and bottom sediments is shown to increase with decreasing water pH. The most sensitive characteristics of aquatic organisms in different trophic groups are chosen, allowing the state of lakes to be monitored. 相似文献
993.
A fully relativistic electron maser is proposed for the explanation of certain non-thermal solar and stellar radio bursts. This mechanism (maser synchrotron) is based on a gyroresonant interaction between waves and electrons of high energies and uses the free energy contained in an electronic distribution function that peaks for energies around 1 MeV.By a calculation of the growth rates of the three electromagnetic modes, we show that the X-mode prevails for values of
p/
cup to 2 or 3. This result is very different from the standard cyclotron maser case where such values of
p/
clead to quench the X-mode growth. Hence, the synchrotron maser instability appears to be a direct and efficient amplification process for considerably larger physical conditions than the cyclotron maser. In addition, the radiation, emitted over the second gyroharmonic, freely propagates without a strong reabsorbtion at the 2
clayer. All these points can constitute major advantages of this mechanism in an astrophysical context.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986. 相似文献
994.
A. V. Mityukov A. M. Nikishin O. A. Almendinger S. N. Bolotov V. A. Lavrishchev N. K. Myasoedov E. V. Rubtsova 《Moscow University Geology Bulletin》2012,67(2):81-92
This work is based on the results of 2-D and 3-D surveys in the Tuapse Basin and field works in the Crimea-Caucasus region. Seven zones were outlined in the model of sedimentation starting from the eroded mountain structure of the Greater Caucasus down to deep water sediments of the Tuapse Basin. The slope and deep-water sediments of channels and fan complexes are characterized. Volume models of the sedimentation system in the Black Sea are shown for the first time. 相似文献
995.
996.
997.
The main analytical properties of the generalized Shkarofsky function and a numerical code for its computation are discussed. The results of a numerical analysis are compared with the results of an asymptotic analysis for parameter values relevant to the problem of whistler-mode propagation in the Earth's magnetosphere. This comparison allows us to specify the range of applicability of different approximations to the generalized Shkarofsky function, which have been used for the analysis of relativistic effects on whistler-mode propagation and instability. 相似文献
998.
999.
WILLIAM A. KOELSCH 《Geographical review》2012,102(4):510-524
The conventional narrative regarding the American reception of George Perkins Marsh, author of Man and Nature (1864), is that his work and ideas were “lost,”“forgotten,” or “neglected” until Lewis Mumford “rediscovered” him and introduced him to geographers at the University of California‐Berkeley through The Brown Decades (Mumford [1931] 1955) and until Carl Sauer made him known to the profession at large beginning in 1938. This article upends the conventional narrative by looking at earlier references to Marsh's later versions of Man and Nature, which were published as The Earth as Modified by Human Action from 1874 to 1907. Analysis reveals that a number of geographers and historians cited these editions between 1875 and the early 1950s. Examining the legend of loss and rediscovery suggests the value of methods utilized in reception studies for research on the history of geography. 相似文献
1000.
D. J. Heath 《Astrophysics and Space Science》1991,175(1):35-50
In this paper we review the gravitational instability theory of galaxy formation. Closed-form expressions for the rate of growth of density perturbations in the regime prior to recombination are presented in special cases in which the cosmological constant is non-zero. After recombination, the fate of perturbations in Lemaître universes is studied with the aid of exact solutions to the density perturbation equation orginally derived by Bonnor. 相似文献