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   67488篇
  免费   1209篇
  国内免费   698篇
测绘学   1975篇
大气科学   5389篇
地球物理   13900篇
地质学   23091篇
海洋学   5652篇
天文学   15424篇
综合类   179篇
自然地理   3785篇
  2021年   450篇
  2020年   511篇
  2019年   617篇
  2018年   1400篇
  2017年   1303篇
  2016年   1764篇
  2015年   1014篇
  2014年   1675篇
  2013年   3200篇
  2012年   1817篇
  2011年   2443篇
  2010年   2202篇
  2009年   3001篇
  2008年   2636篇
  2007年   2535篇
  2006年   2503篇
  2005年   1951篇
  2004年   1983篇
  2003年   1871篇
  2002年   1907篇
  2001年   1680篇
  2000年   1662篇
  1999年   1438篇
  1998年   1383篇
  1997年   1406篇
  1996年   1181篇
  1995年   1123篇
  1994年   1034篇
  1993年   902篇
  1992年   833篇
  1991年   846篇
  1990年   835篇
  1989年   822篇
  1988年   760篇
  1987年   905篇
  1986年   844篇
  1985年   933篇
  1984年   1140篇
  1983年   1063篇
  1982年   1009篇
  1981年   943篇
  1980年   824篇
  1979年   818篇
  1978年   846篇
  1977年   736篇
  1976年   666篇
  1975年   666篇
  1974年   694篇
  1973年   713篇
  1972年   475篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
The wavemaking capability and energy absorbing characteristics of a single submerged body, constrained to move horizontally in a surge mode, are examined. The body possesses both a vertical axis of symmetry and a horizontal plane of symmetry and is ‘thin’ in the sense that the body draft is small compared to the length. The behaviour of a single body subject to a constraint being placed upon the amplitude of its displacement and the properties of an array of interacting identical devices are also discussed.  相似文献   
993.
Three methods for the determination of dissolved organic carbon in seawater were compared. Samples were analysed using persulphate oxidation, high-temperature combustion, and ultraviolet photo-oxidation. The dissolved organic carbon content of the seawater samples ranged from 0.6 to 1.6 mg C/I. This study shows that results of high-temperature oxidation and photo-oxidation procedures differ by less than 5%, whereas results with persulphate oxidation are about 15% less than those obtained with the high-temperature oxidation. The relative merits of each of the oxidation techniques for the determination of organic matter in seawater are discussed.  相似文献   
994.
995.
996.
The decline of the herring stocks and the gadoid outburst   总被引:2,自引:1,他引:2  
  相似文献   
997.
Existing methods for the determination of the various forms of iodine present in seawater are laborious or unwieldy in use. This paper describes modified forms of the iodate and total iodine methods described by Barkley and Thompson (1960), a spectrophotometric procedure for iodate determination derived from Johannesson's (1958) work, and an automatic method for total iodine determination. Procedures for iodate and total-iodine determination which are suitable for use aboard ship, are recommended. Both procedures for iodate determination returned a standard deviation close to 0.5 μg/l when several replicate samples of a seawater containing approximately 30 μg/l of iodate-iodine were analysed. The automatic method for total iodine determination yielded a standard deviation of 0.8 μg/l by the repeated analysis of a seawater containing a total of approximately 50 μg/l of iodine. Suitable methods for the filtration and storage of seawater are also described.  相似文献   
998.
The Solomon Sea Plate was widely developed during late Oligocene, separating the proto-West Melanesian Arc from the proto-Trobriand Arc. Spreading in the Bismarck Sea and in the Woodlark Basin resulted from interaction between the Pacific and Australian Plates, specifically from the collision of the proto-West Melanesian Arc with north New Guinea, which occurred after arc reversal. This model explains the extensive Miocene, Pliocene, and Quaternary volcanism of the Papua New Guinea mainland as it related to southward subduction of the Trobriand Trough. Our interpreted plate motions are concordant with the geological evidence onshore and also with complex tectonic features in the Solomon Sea Basin Region.  相似文献   
999.
Abstract. Benthic metabolism and standing stocks were investigated in the deep Red Sea between 21o and 27oN, Activity was assessed by the determination of respiration rates with a shipboard method and by calculating oxygen consumption from the activity in the electron transport system. We attempted to compare results from different latitudes within the warm Red Sea and with data from cold Atlantic environments. Our investigations were part of an environmental risk assessment to evaluate future mining of metalliferous sediments from the Atlantis II Deep.  相似文献   
1000.
Abstract. .The reproductive cycle of the brittlestar Ophioderma brevispinum is described using histological and organ index data for a population in Massachusetts, U.S.A. The cycle consists of a one month mid-summer spawning phase followed by gametogenesis and gradual gonadal growth during the winter, and greatly accelerated gonadal growth from May to June. At the end of the spawning season, oogonia proliferate near the base of the ovary, and a continuous layer of spermatogonia lines the testis. As oocytes grow to a maximum diameter of 350 um, yolk granules accumulate and the cytoplasm becomes less basophilic. Prior to spawning, the testis becomes branched and sulcate, and a whorl of spermatozoa produced by columns of spermatids accumulates in the lumen. Comparisons between the reproductive cycles of different populations of O. brevispinum and its congeners support the hypothesis that temperature may be a critical exogenous factor, but definitely not the only factor, in the initiation and duration of the growth and spawning phases of the ophiuroid reproductive cycle.  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