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   25619篇
  免费   397篇
  国内免费   264篇
测绘学   648篇
大气科学   2016篇
地球物理   5459篇
地质学   8994篇
海洋学   2041篇
天文学   5409篇
综合类   39篇
自然地理   1674篇
  2020年   140篇
  2019年   127篇
  2018年   302篇
  2017年   278篇
  2016年   427篇
  2015年   305篇
  2014年   444篇
  2013年   1197篇
  2012年   529篇
  2011年   789篇
  2010年   650篇
  2009年   923篇
  2008年   845篇
  2007年   812篇
  2006年   830篇
  2005年   721篇
  2004年   749篇
  2003年   717篇
  2002年   721篇
  2001年   593篇
  2000年   606篇
  1999年   574篇
  1998年   550篇
  1997年   569篇
  1996年   469篇
  1995年   469篇
  1994年   444篇
  1993年   411篇
  1992年   381篇
  1991年   331篇
  1990年   381篇
  1989年   297篇
  1988年   342篇
  1987年   382篇
  1986年   328篇
  1985年   483篇
  1984年   530篇
  1983年   537篇
  1982年   424篇
  1981年   424篇
  1980年   440篇
  1979年   383篇
  1978年   401篇
  1977年   346篇
  1976年   379篇
  1975年   344篇
  1974年   383篇
  1973年   365篇
  1972年   236篇
  1971年   188篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
941.
Navy Fan is a Late Pleistocene sand-rich fan prograding into an irregularly shaped basin in the southern California Borderland. The middle fan, characterized by one active and two abandoned “distributary” channels and associated lobe deposits, at present onlaps part of the basin slope directly opposite from the upper-fan valley, thus dividing the lower-fan/basin-plain regions into two separate parts of different depths. Fine-scale mesotopographic relief on the fan surface and correlation of individual turbidite beds through nearly 40 cores on the middle and lower fan provide data for evaluating the Late Pleistocene and Holocene depositional processes. Margin setting represents fan and/or source area  相似文献   
942.
943.
Airglow hydroxyl band intensities have been measured for bands in the Δν = 2, 3, 4and5 sequences within the wavelength range 0·6–2·3 μ and include corrections for atmospheric extinction. These intensities, along with recently calculated A ν′ v″ values (Murphy, 1971), have been used to derive the direct excitation rates q ν into the ν= 3, 4,..., 9 vibrational levels. The results show that excitation into the v=8and9 levels accounts for about 80 per cent of the total excitation into all levels, ν=0, 1,..., 9.  相似文献   
944.
Comparisons between the chemistry of impact debris, especially spherical droplets of impact-formed material, are made for lunar samples and for soil samples taken from terrestrial impact sites. Differences are assigned to differences in the chemistry of impacting bodies and in the surface rocks, and to the influence of the atmosphere in the terrestrial cases.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.  相似文献   
945.
The two available methods for determining the rms amplitude of the granular convective velocity field, namely the interpretation of line profiles, and direct measurements of velocity fluctuations in highly resolved spectra, give values ( 2 km/sec, and 0.4 km/sec, resp.) which are apparently inconsistent both in magnitude and in their dependence upon optical depth. We give both theoretical and observational evidence for the working hypothesis, that the best resolved spectra mainly show velocity fluctuations due to the oscillation of the solar atmosphere, whereas the contribution of the granular velocity field is greatly reduced because of atmospheric seeing and can be found only as a weak superposition to the oscillatory velocity field. Realistic assumptions for the typical size of the granulation (2.5) and for the seeing parameter (1), together with a simplified model of the granular velocity field, lead to correction factors of 30 to 40 between the true and observed amplitudes of the granular velocities.Mitteilungen aus dem Fraunhofer Institut, Nr. 95.  相似文献   
946.
This article examines the conditions under which the pressure-work and viscous dissipation terms should be retained in the energy balance relation for single (liquid water or vapor) and two-phase (liquid water and vapor) fluid flow through porous media. It is shown that if one wishes to retain the pressure-work term, then one must also keep the viscous dissipation term in the energy balance. Consideration of steady non-isothermal radial flow demonstrates that both pressure-work and viscous dissipation are liable to have negligibly small effects in single phase liquid water and in two-phase liquid-vapor systems. This conclusion is, however, not generally valid for pure vapor systems; in this case, pressure-work and viscous dissipation can produce significant variations in the computed reservoir response.  相似文献   
947.
948.
949.
Three special classes of equilibrium orientations of gyrostat satellites subject to gravitational torques have been treated in the literature. Here we find the set of all equilibria for a restricted class of gyrostat configurations. Those configurations for which the internal angular momentum vector (or the rotor axis) is aligned with a principal axis have been treated in a separate work, where it is shown that at one, and only one, rotor speed there exists a continuum of equilibrium orientations. When the rotor axis is moved away from a principal axis in such a way that it is contained in a plane formed by two principal axes, it is shown that the continuum disappears, and we have a new set of eight equilibrium orientations which have not previously been described. The stability of these orientations is then investigated using the Hamiltonian as a Liapunov testing function. For properly chosen satellite inertia ratios some of these orientations are stable, and might be used in future gravitygradient stabilized satellites.This research is sponsored by the United States Air Force under Project RAND-Contract No. F44620-67-C-0045-monitored by the Directorate of Operational Requirements and Development Plans, Deputy Chief of Staff, Research and Development, Hq. USAF. Views or conclusions contained in this study should not be interpreted as representing the official opinion or policy of the United States Air Force. The material presented here was originally published in RAND Corporation Memorandum RM-5921-PR. The author wishes to acknowledge his indebtedness to Dr. R. E. Roberson for helpful discussions, and for suggesting a research area, part of which is treated here.  相似文献   
950.
This article describes a method that determines the position of a point relative to a fixed boundary. Its advantage over the other existing algorithms described below is that in most cases it is necessary that only part of the algorithm be executed to determine the position of the point. A possible inconvenience of using this algorithm is that the contour boundary has to be entered as data. For problems where the boundary often changes the algorithm is not suitable in its present form. The applications envisaged here are to cases where the position of a very large number of points have to be determined relative to one fixed polygonal boundary whose shape is known in advance, for example, blanking out of contours in automatic contouring, or selecting drill holes which lie in the particular part of a mineral deposit.  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