全文获取类型
收费全文 | 83437篇 |
免费 | 1318篇 |
国内免费 | 1343篇 |
专业分类
测绘学 | 2585篇 |
大气科学 | 6096篇 |
地球物理 | 16388篇 |
地质学 | 32546篇 |
海洋学 | 6279篇 |
天文学 | 15005篇 |
综合类 | 2280篇 |
自然地理 | 4919篇 |
出版年
2021年 | 551篇 |
2020年 | 653篇 |
2019年 | 748篇 |
2018年 | 5962篇 |
2017年 | 5243篇 |
2016年 | 4045篇 |
2015年 | 1178篇 |
2014年 | 1605篇 |
2013年 | 2896篇 |
2012年 | 2705篇 |
2011年 | 4996篇 |
2010年 | 4040篇 |
2009年 | 4915篇 |
2008年 | 4257篇 |
2007年 | 4774篇 |
2006年 | 2400篇 |
2005年 | 1928篇 |
2004年 | 2173篇 |
2003年 | 2151篇 |
2002年 | 1850篇 |
2001年 | 1476篇 |
2000年 | 1374篇 |
1999年 | 1218篇 |
1998年 | 1238篇 |
1997年 | 1208篇 |
1996年 | 966篇 |
1995年 | 923篇 |
1994年 | 818篇 |
1993年 | 747篇 |
1992年 | 678篇 |
1991年 | 661篇 |
1990年 | 705篇 |
1989年 | 635篇 |
1988年 | 576篇 |
1987年 | 668篇 |
1986年 | 645篇 |
1985年 | 815篇 |
1984年 | 885篇 |
1983年 | 859篇 |
1982年 | 782篇 |
1981年 | 734篇 |
1980年 | 705篇 |
1979年 | 646篇 |
1978年 | 649篇 |
1977年 | 609篇 |
1976年 | 582篇 |
1975年 | 587篇 |
1974年 | 546篇 |
1973年 | 572篇 |
1972年 | 375篇 |
排序方式: 共有10000条查询结果,搜索用时 20 毫秒
41.
W. H. Jefferys G. F. Benedict P. D. Hemenway P. J. Shelus R. L. Duncombe 《Celestial Mechanics and Dynamical Astronomy》1985,37(3):299-305
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed. 相似文献
42.
Summary. Asymptotic ray theory (ART) fails in transition regions near critically reflected, bottom glancing or caustic-forming rays in a vertically inhomogeneous layered earth. These deficiencies are repaired here by replacing the transitional ray fields with guided modes plus truncation remainders. Exact ray-mode equivalences and their high-frequency asymptotic approximations are formulated, and their validity and efficiency are verified by numerical comparisons for SH motion in a two-layer earth model comprised of an inhomogeneous sediment above an homogeneous semi-infinite bedrock. 相似文献
43.
A numerical scheme is developed in order to simulate fluid flow in three dimensional (3‐D) microstructures. The governing equations for steady incompressible flow are solved using the semi‐implicit method for pressure‐linked equations (SIMPLE) finite difference scheme within a non‐staggered grid system that represents the 3‐D microstructure. This system allows solving the governing equations using only one computational cell. The numerical scheme is verified through simulating fluid flow in idealized 3‐D microstructures with known closed form solutions for permeability. The numerical factors affecting the solution in terms of convergence and accuracy are also discussed. These factors include the resolution of the analysed microstructure and the truncation criterion. Fluid flow in 2‐D X‐ray computed tomography (CT) images of real porous media microstructure is also simulated using this numerical model. These real microstructures include field cores of asphalt mixes, laboratory linear kneading compactor (LKC) specimens, and laboratory Superpave gyratory compactor (SGC) specimens. The numerical results for the permeability of the real microstructures are compared with the results from closed form solutions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
44.
Electron impact excitation rates for transitions in the S v ion, calculated with theR-matrix code, are used to derive the electron temperature sensitive emission line ratiosR
1 =I(854.8 Å)/I(786.9 Å),R
2 =I(852.2 Å)/I(786.9 Å),R
3 =I(849.2 Å)/I(786.9 Å), andR
4 =I(1199.1 Å)/I(786.9 Å), which are found to be significantly different from previous estimates. A comparison of the present results with observational data for a sunspot obtained with the Harvard S-055 spectrometer on boardSkylab reveals generally good agreement between theory and experiment, except in the case ofR
1, which is probably due to blending in the 854.8 Å feature. The possible effects of Lyman continuum absorption on the observed line ratios is briefly discussed. 相似文献
45.
M.S. Hanner E. Tedesco A.T. Tokunaga G.J. Veeder D.F. Lester F.C. Witteborn J.D. Bregman J. Gradie L. Lebofsky 《Icarus》1985,64(1):11-19
The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ~2 × 105 g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ~0.04 at 1.25 μm and ~0.05 at 2.2 μm. 相似文献
46.
47.
AGGLOMERATION AND RADIATION EFFECT OF THE PULL OF URBANIZATION 总被引:2,自引:1,他引:1
QIJin-li 《中国地理科学(英文版)》2003,13(3):224-227
In order to explore the train of thought for China‘s urbanizing development and coordinated rural eco-nomic development, and to find good ways of solving rural problems through urbanization, this paper absorbs the push-and-pull forces theory and the systematic dynamic theory in the traditional population migration theories, views urbanization as a dynamic system, makes research on the push-and-pull mechanism of urbanization. The pulling power of urbanization is analyzed according to two aspects, the agglomeration effect and the radiation effect of cities. The agglomeration effect provides continuous propelling force for urbanization, and the radiation effect further accelerates the urbanization process by pushing forward the development of rural economy. Of course, the slow de-velopment of urbanization can result in the hindrance to rural economic development. 相似文献
48.
49.
HUXue-lian CHENGCheng-qi WUDe-wen MAAi-nai 《中国地理科学(英文版)》2003,13(1):84-90
Based on the analysis of the development of GIS technology and application,this paper brought forward the concept of GoGIS,namely Cooperative GIS ,CoGIS is GIS facing group-users and supporting human-human interaction,which makes it differ from the former GISs,Then,the characteristics of general Computer Spport Cooperative Work (CSCW)applications and the complexity of Geographic Information Science were analyzed,and the conclusion the CoGIS was not a simple GIS layer on CSCW was reached,Further,this paper gaver the hierarchical architecture of CoGIS,and analyzed the coperative platform in detail from the following:1)basic elements;2) collaboration patterns;3) cooperation control mechanism;4) synchronization mechanism;5) security and 6) group communication and so on.With those,the problems about GIS applications are discussed,such as 1)distributed multi-source GIS information and knowledge sharing platform;2)the fusion and visualization of GIS information;3)virtual reality cooperative modeling;4) dymamic simulation;5)expert system and 6) decision-making.Finally,this paper analyzed CoGIS application mode in brief. 相似文献
50.