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   260596篇
  免费   5565篇
  国内免费   4081篇
测绘学   7136篇
大气科学   19487篇
地球物理   54790篇
地质学   91320篇
海洋学   21724篇
天文学   56478篇
综合类   1180篇
自然地理   18127篇
  2021年   2358篇
  2020年   2693篇
  2019年   2941篇
  2018年   3669篇
  2017年   3298篇
  2016年   5829篇
  2015年   4317篇
  2014年   7034篇
  2013年   14359篇
  2012年   6760篇
  2011年   8178篇
  2010年   7178篇
  2009年   9854篇
  2008年   8649篇
  2007年   8070篇
  2006年   9851篇
  2005年   7881篇
  2004年   7673篇
  2003年   7168篇
  2002年   6798篇
  2001年   6051篇
  2000年   5994篇
  1999年   5219篇
  1998年   5257篇
  1997年   5049篇
  1996年   4683篇
  1995年   4438篇
  1994年   4112篇
  1993年   3858篇
  1992年   3642篇
  1991年   3610篇
  1990年   3776篇
  1989年   3529篇
  1988年   3308篇
  1987年   3855篇
  1986年   3421篇
  1985年   4227篇
  1984年   4747篇
  1983年   4430篇
  1982年   4328篇
  1981年   3937篇
  1980年   3653篇
  1979年   3520篇
  1978年   3491篇
  1977年   3285篇
  1976年   3048篇
  1975年   2962篇
  1974年   2932篇
  1973年   3086篇
  1972年   2029篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
101.
In connection with the celebration of the centennial of the Institute of Electrical and Electronics Engineers (UEEE) in 1984, the history of electrical engineering in relation to the world's oceans is traced, with emphasis on the 20-year period from the 1960's to the present. Note is taken of the breadth of the cross-disciplinary interests involved, as exemplified by the active participation of a score of IEEE Societies and Professional Groups, how they organized to cooperate, the periodic meetings and annual conferences they sponsored, the publications they undertook, the tokens of recognition they adopted, and how they adapted to changes in emphasis to reflect shifts in the areas of technology served. Finally, an assessment is made of the future of the recently organized IEEE Oceanic Engineering Society.  相似文献   
102.
The development of coal forests during the Carboniferous is one of the best-known episodes in the history of life. Although often reconstructed as steamy tropical rainforests, these ancient ecosystems were a far cry from anything we might encounter in the Amazon today. Bizarre giant club-mosses, horsetails and tree ferns were the dominant plants, not flowering trees as in modern rainforests. At their height, coal forests stretched all the way from Kansas to Kazakhstan, spanning the entire breadth of tropical Pangaea. Most of what we know of their biodiversity and ecology has been quite literally mined out of the ground through two centuries of hard labour. Without coal mining, our knowledge would be greatly impoverished. Over the past few years, we've been exploring underground coal mines in the United States, where entire forested landscapes have been preserved intact over huge areas. Never before have geologists had the opportunity to walk out through mile upon mile of fossilized forest. In this feature article, we describe some of our recent explorations and attempt to shed new light on these old fossils.  相似文献   
103.
The Mangshan Plateau is located on the south bank of the Huang He (Yellow River) just west of the city of Zhengzhou, well outside the Loess Plateau in central China. Mixing models of the grain‐size data indicate that the loess deposits are mixtures of three loess components. Comparison of the mixing model with existing models established for a series of loess–palaeosol sequences from the Loess Plateau indicates that the Mangshan loess has been supplied from a proximal dust source, the Huang He floodplain, during major dust outbreaks. The high accumulation rates, the composition of the loess components, and especially the high proportions of a sandy loess component support this. Owing to the exceptionally high accumulation rates, the Mangshan grain size, magnetic susceptibility and carbonate records provide a high‐resolution archive of environmental and climate change. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   
104.
In 1997, IGCP Project No. 410 was established to appraise known records of Ordovician biotas, in order to evaluate one of the greatest-ever diversifications of life on Earth, between 489 and 443 million years ago. Data collection and analysis of biodiversity were coordinated through seven regional teams, and a global“clade” team.  相似文献   
105.
106.
Rostov State Pedagogical Institute; All-Union Scientific Research Institute Gradient. Translated from Astrofizika, Vol. 33, No. 3, pp. 379–393, November–December, 1990.  相似文献   
107.
The interaction of free convection with thermal radiation of the oscillatory flow past a vertical plate is studied. The Rosseland approximation is used to describe the radiative heat flux in the energy equation.  相似文献   
108.
A transformation of the differential equations of motion of the two-body problem in the spherical coordinates to oscillator form is derived. It is shown that the independent variable transformation dt/ds=r2 is a transformation which makes the oscillator form possible.  相似文献   
109.
Shelf-mounted Ocean Thermal Energy Conversion (OTEC) plants require installation of cold-water pipes (CWP) on slopes of40degto depths of 1000 m. In addition, tower platforms containing OTEC power systems may be located on lesser sloped terrain near shore and exposed to special environmental loading problems affecting foundation design. Shelf-mounted installations require careful attention to site selection and geotechnical considerations for foundation integrity on sloped surfaces. This paper primarily discusses research associated with cold-water pipe and foundation installations on steep slopes, although research continues on tower platforms located on the shelf. At least five nations are in various stages of development of OTEC systems for island applications. Each of their systems is either shelf mounted or land based and requires that a large diameter cold-water pipe be installed on a steep slope to provide cold water from 1000-m depths. In addition to the installation and deployment of the large cold-water pipe, the most significant problem is the design and installation of suitable foundations that will last for several decades. To date there is very little experience in the offshore industry for large installations on steep slopes. A major scale-model research project is underway on the slopes of the island of Hawaii. A section of pipe 2.4 m in diameter and 24 m long was installed using combination concrete foundations and joints. The pipe and foundations are fully instrumented to measure environmental loading forces due principally to currents and waves. Environmental measurements will also be taken in the test area. The measurement data will be used to validate available analytical models for subsequent use in aiding industry in providing more cost-effective designs for OTEC pipes and foundations.  相似文献   
110.
It is shown that the kinematical parameters associated with the congruence formed by tachyonic motion can be defined in the manner of Greenberg, but not that of Ehlers. The space-like counterpart of Raychaudhuri's equation has also been obtained.  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