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   89522篇
  免费   1598篇
  国内免费   1396篇
测绘学   2976篇
大气科学   7166篇
地球物理   18138篇
地质学   32444篇
海洋学   6766篇
天文学   16840篇
综合类   2274篇
自然地理   5912篇
  2020年   488篇
  2019年   502篇
  2018年   5477篇
  2017年   4757篇
  2016年   3838篇
  2015年   1242篇
  2014年   1505篇
  2013年   3297篇
  2012年   2432篇
  2011年   4813篇
  2010年   3851篇
  2009年   5082篇
  2008年   4422篇
  2007年   4592篇
  2006年   2536篇
  2005年   2341篇
  2004年   2658篇
  2003年   2481篇
  2002年   2222篇
  2001年   1835篇
  2000年   1811篇
  1999年   1529篇
  1998年   1519篇
  1997年   1492篇
  1996年   1283篇
  1995年   1226篇
  1994年   1100篇
  1993年   1009篇
  1992年   957篇
  1991年   814篇
  1990年   1013篇
  1989年   861篇
  1988年   762篇
  1987年   941篇
  1986年   832篇
  1985年   1036篇
  1984年   1209篇
  1983年   1145篇
  1982年   1034篇
  1981年   1007篇
  1980年   860篇
  1979年   825篇
  1978年   877篇
  1977年   800篇
  1976年   766篇
  1975年   705篇
  1974年   710篇
  1973年   719篇
  1972年   446篇
  1971年   387篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
The First Law of Thermodynamics is developed from fundamentals for open, non-equilibrium systems of seawater in motion, exchanging salt and freshwater internally and with their surroundings, and varying continuously in temperature, pressure, and salinity. The aim is clarity and consistency of concepts – and precision in the accompanying vocabulary. Particular attention is given to the way in which salinity variation plays out in the logical structures. The arbitrary constants in the thermodynamic potentials and the various First Law equations are highlighted, in order to remove them, and to recover the physically meaningful content. When this is done, it is seen that salinity variations have little consequence in application of the First Law to the ocean, apart from affecting values of coefficients.  相似文献   
992.
The results of a comprehensive field trial of nearly all commercially available directional wave measurement systems at the Edda field in the North Sea during winter 1985-86 are presented. The results summarize the accuracy of the principal engineering wave parameters from each system and the dependence on sea state. Limiting factors on system performance and operational problems are also included in the assessment. Overall experience has been good with systems utilizing widely different measurement principles returning consistent results.  相似文献   
993.
Sedimentary facies of cores from the eastern side of the Rockall Bank and the Feni Drift (55°–58°N) have been studied. They confirm the existence of slumping which contributes only in small part to drift build-up.

These slumps occurred at different episodes during last Glacial and early Post-Glacial periods. They are sandwiched in between more important turbidites, and are found in places where the drift lies against the slope and so is acted upon by continental-rise sedimentary processes. The sea level rise periods with ice melting seems to favour gravity flow release.  相似文献   

994.
995.
The relationship between the depth and trail attained by a towed submerged vehicle and the magnitude and direction of the forces on it has been examined theoretically, for cases involving faired cables.It is shown that when the body force is varied the body in equilibrium moves on an almost circular path as if the cable were straight, and that depth changes depend primarily upon the change in the component of the body force that acts at right angles to the line joining the top and bottom of the cable.Examples are given which demonstrate that for bodies towed at large depression angles the use of water brakes is more effective in changing depth than the use of lifting wings.The data is presented so that a rapid preliminary assessment can be made of any proposed system to obtain the best combination of speed and scope for achieving a given depth with a body having known lift and drag coefficients.  相似文献   
996.
997.
998.
Petroleum hydrocarbons were investigated in Mytilus edulis and Macoma balthica at six intertidal sites in Port Valdez, Alaska after three to five years of oil terminal operation. Information about the concentrations of total hydrocarbons, unresolved complex mixture, normal alkanes, isoprenoids, olefins, hopanes and polycyclic aromatic hyrocarbons shows that petroleum accumulation in these bivalve mollusks is greatest at the Valdez municipal boat harbor and a crude oil terminal which has been permitted to discharge 170 kg oil day−1. Accumulation two- to ten-fold less were observed at 3 km from the terminal and boat harbor. The temporal trends in petroleum hydrocarbon concentrations in M. edulis and M. balthica reflect the effects of selective retention and depuration of different hydrocarbon classes as well as variation in hydrocarbon sources. It is not clear whether the petroleum concentrations in Port Valdez bivalves had reached steady state by 1982, the final sampling time reported here.  相似文献   
999.
1000.
Winter-spring phytoplankton blooms in Dabob Bay, Washington   总被引:4,自引:2,他引:4  
Scientific investigations in Dabob Bay, Washington State, USA, have been extensive since the early 1960s, but phytoplankton blooms have been studied mostly with regard to chlorophyll concentrations and little is known about the phytoplankton species themselves. Here we provide information on the species present, their abundances during blooms, their contribution to organic carbon concentrations and the ability of some phytoplankton species to produce toxic aldehydes that may impact metazoan grazers.Multiple blooms of phytoplankton, dominated by diatoms, occurred in the late winter-early spring period, with depth-integrated chlorophyll levels ranging from <20 to 230 mg m−2 and peaks in February and April. The major bloom species included Skeletonema costatum, Thalassiosira spp. and Chaetoceros spp; Phaeocystis cf. pouchetii occurred in 2002 and 2004. Other taxa or groups of organisms that were sometimes abundant included unidentified small flagellates <10 μm in size and unidentified heterotrophic dinoflagellates. Large diatoms usually comprised most of the cell carbon, but a large, heterotrophic dinoflagellate, identified only as Gyrodinium “tear” because of its shape, was a major contributor to the microplankton carbon when present even in small numbers. Five Thalassiosira species and S. costatum were found to produce polyunsaturated aldehydes (PUA) that are known to affect copepod reproduction and hatching success. Our findings are similar to the few previous studies in the last four decades that included phytoplankton species and suggest long-term similarities and relative stability in the phytoplankton species present and their timing in Dabob Bay.  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