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   892篇
  免费   33篇
  国内免费   21篇
测绘学   8篇
大气科学   121篇
地球物理   197篇
地质学   364篇
海洋学   30篇
天文学   195篇
综合类   4篇
自然地理   27篇
  2021年   13篇
  2020年   13篇
  2019年   11篇
  2018年   13篇
  2017年   13篇
  2016年   23篇
  2015年   27篇
  2014年   28篇
  2013年   43篇
  2012年   41篇
  2011年   42篇
  2010年   34篇
  2009年   40篇
  2008年   48篇
  2007年   31篇
  2006年   33篇
  2005年   36篇
  2004年   31篇
  2003年   23篇
  2002年   28篇
  2001年   19篇
  2000年   22篇
  1999年   19篇
  1998年   16篇
  1997年   8篇
  1996年   13篇
  1995年   12篇
  1994年   14篇
  1993年   12篇
  1992年   9篇
  1991年   18篇
  1990年   9篇
  1989年   8篇
  1988年   11篇
  1987年   8篇
  1986年   7篇
  1985年   8篇
  1984年   12篇
  1983年   13篇
  1981年   16篇
  1980年   13篇
  1979年   7篇
  1978年   12篇
  1977年   9篇
  1976年   13篇
  1975年   8篇
  1974年   6篇
  1973年   10篇
  1972年   6篇
  1960年   4篇
排序方式: 共有946条查询结果,搜索用时 984 毫秒
821.
Prevention of climatic change by changing human economic behaviour or compensation for climatically detrimental effects by technological fixes is not necessarily better than adaptation. In fact, there are good reasons to conclude that adaptation is the most rational political option, at the same time requiring least marginal action. The problems arising from CO2 all appear at present to be marginal ones which arise, and should be taken care of, for other reasons as well. With respect to CO2-induced changes we could not do better than do what should be done in any event for reasons of development policy.Editor's Note: This paper is, in part, a result of interactions at the U.S. Department of Energy's interdisciplinary workshop on Environmental and Societal Consequences of a Possible CO2-induced Climatic Change, Annapolis, Maryland, April, 1979. Although it is more of a discussion draft than a formal paper, it is being published rapidly and in its present form, for the reason it was written: to generate discussion, debate and exchange on the very important, yet hardly explored, question of societal response to CO2. Comments on this paper, or its subject, would be welcomed.  相似文献   
822.
Inman (find, 1966) is a single, relatively unweathered stone of 7.25 kg that contains fresh metal and only few weathering products away from fractures. It has a pronounced chondritic texture, with 38 vol % of the meteorite being made up of chondrules of virtually all textural types. The recalculated bulk analysis, particularly the ratios of Fetotal/SiO2 (0.46), Fe°/Fetotal (0.35), and Fe°/Ni° (6.67) and the contents of Fetotal (19.45%) and metallic nickel-iron (7.94%), indicate that Inman is an L-group chondrite. The pronounced chondritic texture; the compositional variabilities of olivine, pyroxene, chromite, and ilmenite; the presence of a fine-grained, nearly opaque matrix, glass and twinned monoclinic low-Ca pyroxene indicate that the chondrite belongs to petrologic type 3.  相似文献   
823.
In the following paper we argue that each wind-driving star in relative motion with respect to the ambient interstellar medium experiences a force exerted on its central wind-generating body. The exact magnitude of this force depends on the actual geometry of the counterflow configuration of stellar and interstellar winds for a particular kinematic situation which is especially sensitive to whether the interstellar flow is subsonic or supersonic. It will, however, be demonstrated here that this force is of an accelerating nature, i.e., it operates like a rocket-motor, as long as the peculiar motion of the wind-driving star with respect to the ambient interstellar medium remains subsonic.Here we use a specific analytical model to describe theoretically the specific counterflow configuration for the case of the solar system in a subsonic peculiar motion with respect to the local interstellar medium assuming irrotational and incompressible flows. We can work out a quantitative number for the accelerating force governing the Sun's motion at present. The net reaction force exerted on the solar body is then mediated by the asymmetric boundary conditions to which the distant solar wind field has to adapt.Next we study the indirect action of such a force on orbiting Keplerian objects like planets, planetesimals and comets. Since this force only influences the central solar body, but not the planets themselves, the problem is different from the treatment of a constant perturbation force perturbing the Keplerian orbits. We present a perturbation analysis treating the action of a corresponding position-dependent perturbation force resulting in secular changes of the orbital elements of Keplerian objects. It is found that changes are accumulating more rapidly in time the closer to the sun the orbiting bodies are. Main axis and perihelion distances are systematically increasing. Especially pronounced are changes in the perihelion position angle of the objects. For solar wind mass losses larger than the Sun's present value by a factor of 1000 (T-Tauri phase of the Sun,) the migration periods calculated for the planet Mercury are of the same order of magnitude as that for corresponding general relativistic migration.  相似文献   
824.
The predominant 1480 Ma granites and rhyolites of the St. Francois Mountains, southeastern Missouri, are intruded by mafic rocks. A 40Ar—39Ar study of some of these, the Skrainka Mafic Group, indicates an age of ~ 1240 Ma, significantly younger than the host rocks, significantly older than Grenville/Keweenawan age, and close to the age of similar rocks in Labrador.  相似文献   
825.
826.
827.
828.
829.
830.
Highly non-linear seismic trace inversion problems can be solved efficiently by an implementation of Tabu Search, a meta-heuristic method related to artificial intelligence. The implementation under consideration is a deterministic, global search that combines the advantages ofa local search, giving a quick descent to local misfit minima, with an ability to cross misfit barriers in the model space. Once Tabu Search has found an area of low misfit, it performs an extensive exploration of its deepest points. This property makes it possible to use Tabu Search for a semiquantitative resolution and uncertainty analysis of the inverse problem.  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