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   86531篇
  免费   1429篇
  国内免费   539篇
测绘学   1788篇
大气科学   5704篇
地球物理   16649篇
地质学   31224篇
海洋学   7996篇
天文学   20084篇
综合类   226篇
自然地理   4828篇
  2022年   627篇
  2021年   1057篇
  2020年   1131篇
  2019年   1265篇
  2018年   2587篇
  2017年   2386篇
  2016年   2765篇
  2015年   1386篇
  2014年   2650篇
  2013年   4547篇
  2012年   2897篇
  2011年   3735篇
  2010年   3437篇
  2009年   4322篇
  2008年   3768篇
  2007年   3891篇
  2006年   3598篇
  2005年   2560篇
  2004年   2482篇
  2003年   2315篇
  2002年   2307篇
  2001年   1984篇
  2000年   1977篇
  1999年   1556篇
  1998年   1614篇
  1997年   1485篇
  1996年   1258篇
  1995年   1264篇
  1994年   1067篇
  1993年   1019篇
  1992年   955篇
  1991年   988篇
  1990年   958篇
  1989年   839篇
  1988年   768篇
  1987年   894篇
  1986年   786篇
  1985年   966篇
  1984年   1097篇
  1983年   1054篇
  1982年   964篇
  1981年   918篇
  1980年   811篇
  1979年   767篇
  1978年   745篇
  1977年   631篇
  1976年   637篇
  1975年   625篇
  1974年   600篇
  1973年   668篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
971.
972.
A series of eight tracer experiments were carried out over one year (1983) on a low energy, macro-tidal beach in South Wales, UK. Tracer movement was monitored and 30 pebbles adjacent to coloured tracer pebbles were measured. Measurements were of the A, B, and C axes, roundness, Maximum Projection Sphericity and Oblate-Prolate indices. Analysis shows that through time, the percentage rejection of these properties increased. It is theorised that the surface tracers are 'misfits' or 'rejects' which have not found their equilibrium position in the pebble facies.  相似文献   
973.
974.
This paper discusses the conclusion derived during a joint U.S—Turkey expedition on the R/VKrorr on the dramatic variations of the O2–H2S zone in the Black Sea observed over the last 20 years. This conclusion is shown to be invalid since it was based on a comparison of data obtained at two stations only (in 1969 and 1988) that was made in various regions and in different seasons. Examples are provided for the inter-seasonal and mesoscale variations of hydrophysical and hydrochemical characteristics in the O2–H2S zone which, as to their scales, are comparable to or even exceed the variations registered during the expedition ofKrorr.Translated by Mikhail M. Trufanov.  相似文献   
975.
Sea-surface microlayer samples were collected from six nearshore areas receiving different amounts of anthropogenic inputs. The samples were analyzed for selected trace metals, chlorinated hydrocarbons, and polycyclic aromatic hydrocarbons. The relative toxicities of the samples were determined with fish embryo bioassays.Contaminant concentrations generally increased from offshore to the inshore stations. Contaminant concentrations were several orders of magnitude higher in microlayer samples from the highly industrialized Los Angeles and Long Beach harbors compared to samples from a site 15km offshore. Microlayer samples from the inshore stations were significantly more toxic, and induced significantly more developmental abnormalities and chromosome aberrations, than samples from the offshore stations.  相似文献   
976.
The authors describe the effect of copper pollution on the mitochondria from various areas of the Torpedo marmorata central nervous system. A high percentage of swollen mitochondria has been observed in neurons from animals experimentally exposed to high level of copper (4 ppm). The correlation with age pigment granules is discussed.  相似文献   
977.
Our intent has been to develop rapid and sensitive bioassay procedures for laboratory and field applications based on hemolymph factors in animals such as the bay mussel, Mytilus edulis. We found that mussel hemolymph could be easily and repeatedly sampled from individual animals in sufficient amounts to allow analyses for lysozyme, proteolytic activity, total protein and hemocyte counts. In static tests there seemed to be graded responses indicating release of increasing amounts of the above factors into the hemolymph in roughly direct proportion to Cu2+ concentration at lethal and sublethal levels. We interpreted this to be evidence for increased lysosomal and cellular disruption as cupric ion exposure was increased or prolonged. At the same time, Cu2+ concentrations in the mussel hemolymph increased up to an order of magnitude above ambient in the range of 26 to 267 ppb. We felt that this was evidence for the release of Cu-binding proteins into the mussel hemolymph. Simultaneously with the above events the serum granulocytes increased in numbers above control values by factors of 3 or 4 in response to copper challenge while macrophage (terminology of Moore & Lowe, 1977) counts, including controls, declined precipitously.  相似文献   
978.
This paper discusses the possibility of measurement of the current velocity vector characteristics averaged over the time of continuous regular down-up probings from a moving carrier. The measurements can be carried out either from a moving vessel or from a helicopter, floating on or hovering above the sea surface. As an illustration for the suggested technique,in situ observations compiled by a free-component (±XYZ) VEGA-3 probe in the Guiana current region in March 1990 are provided. Comparison of the results with buoy data shows good agreement within the accuracy limits of the instruments employed.Translated by Vladimir A. Puchkin.  相似文献   
979.
CTD data are used to study the relationship between the temperature field fine structure and the location of the frontal zones and synoptic eddies in the region adjacent to the eastern coast of the Kamchatka peninsula. High levels of fine structure activity were observed in the frontal zone of the East Kamchatka current and in the peripheries of anticyclonic eddies. Estimates of the coefficient of the horizontal turbulent exchange and the transfrontal (lateral) heat flux are derived.Translated by Mikhail M. Trufanov.  相似文献   
980.
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