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   91525篇
  免费   1079篇
  国内免费   760篇
测绘学   2363篇
大气科学   5968篇
地球物理   17063篇
地质学   35060篇
海洋学   7876篇
天文学   20818篇
综合类   379篇
自然地理   3837篇
  2022年   666篇
  2021年   1035篇
  2020年   1124篇
  2019年   1219篇
  2018年   4337篇
  2017年   3914篇
  2016年   3787篇
  2015年   1488篇
  2014年   2702篇
  2013年   4184篇
  2012年   3483篇
  2011年   5225篇
  2010年   4561篇
  2009年   5452篇
  2008年   4648篇
  2007年   5191篇
  2006年   3402篇
  2005年   2453篇
  2004年   2424篇
  2003年   2361篇
  2002年   2202篇
  2001年   1876篇
  2000年   1726篇
  1999年   1298篇
  1998年   1340篇
  1997年   1288篇
  1996年   1064篇
  1995年   1066篇
  1994年   936篇
  1993年   811篇
  1992年   806篇
  1991年   815篇
  1990年   877篇
  1989年   689篇
  1988年   712篇
  1987年   733篇
  1986年   640篇
  1985年   844篇
  1984年   910篇
  1983年   807篇
  1982年   773篇
  1981年   696篇
  1980年   700篇
  1979年   633篇
  1978年   649篇
  1977年   531篇
  1976年   499篇
  1975年   504篇
  1974年   450篇
  1973年   507篇
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
961.
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   
962.
963.
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   
964.
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   
965.
966.
A marine magnetic total-field survey of Hudson Bay was conducted from 1975 to 1978. Approximately 62 000 line-km of data, or 35 000 five-minute averages, were acquired. The data were corrected for secular variation by means of a polynomial in latitude, longitude, and time, first fitted to annual mean differences from nine observatories and then integrated over time. The data were corrected for diurnal and transient variations by means of a linear interpolation, over geomagnetic latitude, of running hourly mean values from two neighbouring magnetic stations (numbering seven in all). Crossover differences are analysed in terms of a mixture of normal distributions resulting from the intermittent transient variations, and indicate the effectiveness of the corrections. The variance of the data is reduced, as a result of the corrections, by 87% during quiet periods and by 53% during disturbed periods, the weighted variance being reduced by 66%. Navigational accuracy is estimated, from a novel method utilizing crossover differences and field gradients, to be approximately 300 m. Anomalies are presented in profile form, and discussed briefly.Contribution from the Earth Physics Branch No. 973.  相似文献   
967.
Ice effects have caused extensive damage. Enormous forces have been recorded. On the other hand, ice has been used as a structural material under various situations. To use ice beneficially and to assess the danger of ice forces, it is necessary to know the mechanical properties of ice.Results from the large number of ice tests carried out in laboratories show the difficulties of sampling and testing procedures. Most of the tests have not reproduced faithfully enough the conditions existing in nature. To obtain adequate results, laborious and slow samplings and preparations of test specimens have been necessary. Recently, extensive use has been made of in situ tests in order to reduce the number of factors whixh can induce errors in measurements. Large scale tests have simulated actual structures. General ice properties have become available from small-scale in situ tests. Since these tests give rapid results, a large number are already available with a good coverage of field conditions.A theoretical basis has been devised for these in situ tests which measure essentially the unconfined and confined compressive strengths of ice, and practical devices have been developed to execute these tests. A correlation of measurements from laboratory tests and in situ tests has established a link between natural and artificial environments. Brittle elastic and clastic theories have been applied, to find the best correlations for ice tests. Test results have been related to the mechanical properties as used in engineering practice.A very summary conclusion of results is that cold sea ice and cold fresh water ice are both very strong. However, depending on the load rate, ice can be quite brittle, which mitigates dangerous effects from ice action against massive structures.  相似文献   
968.
An acoustic locator—a sodar—is a unique instrument for getting the objective characteristics of the current state of the atmospheric boundary layer (ABL) owing to a combination of such properties as remoteness, mobility, resolution, and information content. This study demonstrates the capabilities of a sodar to obtain data on the second moments of the field of wind-velocity fluctuations, in particular, the profiles of momentum flux in the lower ABL, which are very important in practical applications. A corresponding method is described, and the results of its experimental verification and some examples of its application under the conditions of different ABL stratification are presented.  相似文献   
969.
Visualization of a collapsing vertical packet of patches in a laboratory experiment and the results of acoustical sounding under field conditions allowed us to discriminate the stage of its evolution which was previously unknown. It is defined by the variations of the horizontal dimensions of patches and their effective scattering surface with a period ofT=150–250/N, whereN is the Brunt-Väisälä frequency. The horizontal dimensions of patches can reach 40% of their final dimensions at the evolution stage discriminated.Translated by Mikhail M. Trufanov.  相似文献   
970.
An integrate approach will be discussed for investigations on oxidative stress in xenobiotic toxicity. While the analysis of individual antioxidants is useful for their sensitivity and to understand the mode of action of a stressor, the integration with the analysis of the total antioxidant capacity provides a more holistic assessment of the overall biological significance of such variations. TOSC has a greater predictive value on the health condition of the organisms and allows to discriminate the different role of specific ROS in oxidative stress syndrome.  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