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   32259篇
  免费   431篇
  国内免费   377篇
测绘学   1242篇
大气科学   2454篇
地球物理   6246篇
地质学   11391篇
海洋学   2502篇
天文学   7670篇
综合类   149篇
自然地理   1413篇
  2021年   284篇
  2020年   282篇
  2019年   328篇
  2018年   834篇
  2017年   796篇
  2016年   1030篇
  2015年   603篇
  2014年   974篇
  2013年   1707篇
  2012年   1050篇
  2011年   1296篇
  2010年   1087篇
  2009年   1396篇
  2008年   1211篇
  2007年   1164篇
  2006年   1179篇
  2005年   980篇
  2004年   872篇
  2003年   868篇
  2002年   858篇
  2001年   771篇
  2000年   745篇
  1999年   674篇
  1998年   617篇
  1997年   630篇
  1996年   571篇
  1995年   535篇
  1994年   508篇
  1993年   429篇
  1992年   384篇
  1991年   412篇
  1990年   415篇
  1989年   393篇
  1988年   360篇
  1987年   437篇
  1986年   361篇
  1985年   447篇
  1984年   480篇
  1983年   462篇
  1982年   445篇
  1981年   357篇
  1980年   360篇
  1979年   313篇
  1978年   306篇
  1977年   294篇
  1976年   259篇
  1975年   255篇
  1974年   283篇
  1973年   305篇
  1972年   195篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Summary The change of rainfall totals with height has been the subject of much investigation in many parts of the world. The rates of increase or decrease vary with the climatic regime experienced. This study of Sri Lankan rainfall shows that the relationship between height and mean annual rainfall is complex. Examples of increases and decreases of rainfall with height are found. The climatic regime of the area is important in controlling the level of maximum rainfall.With 7 Figures  相似文献   
52.
Izvestiya, Physics of the Solid Earth - Abstract—Granular media determine the dynamics of many natural systems including faults in the Earth’s crust. The paper addresses the laboratory...  相似文献   
53.
A special system of canonical variables is considered. An algorithm for expanding the principal functions of Keplerian motion in new elements is presented. The advantage of the proposed system is a relatively small number of terms in the classical expansions of the unperturbed two-body problem. A method for expanding the time derivatives of the rectangular coordinates is proposed. Some estimates of the number of terms in the presented expansions have been obtained through numerical experiments.  相似文献   
54.
55.
The eleventh list of faint late M and carbon type stars detected on the plates of the First Byurakan Spectral Survey in zone -7° -3° and covering about 1000 square degrees is presented. From 126 detected stars, 88 are newly discovered objects: they are 6 carbon stars, 8 carbon star candidates, and 74 M-type stars; among the latter 38 (26 PSC + 12 FSC) are unclassified IRAS sources, and one object is an unclassified ROSAT source. Distances to the 6 newly discovered early-type carbon stars are estimated. Equatorial coordinates, red magnitudes, and spectral classes determined from the Palomar E-charts are provided. The lack of optical counterparts on Palomar O and E maps for two detected late M-type stars indicates a large variability in brightnesses of these objects (amplitude not smaller than 7.0 magnitude).  相似文献   
56.
57.
Effects of deforestation upon slopes in limestones and in volcanic rocks in the Benson River valley, northern Vancouver Island, have been investigated quantitatively. Postlogging soil erosion and vegetal regeneration success were assessed by measuring soil depth, percent bare rock and moss cover, and the numbers and diversity of trees, shrubs, and plants on 25 sampling sites, each containing ten measuring quadrats selected at random. Sixteen sites were on the Quatsino Formation, a well-karstified limestone, and nine on the Karmutsen Formation of basaltic lavas. Eight sites were of virgin forest, 16 were logged between 1970 and 1983, and one (on limestone) was logged in 1911. Both bedrock types were significantly affected by the cutting. There was greater loss of soil and an increase in bare rock on the limestones. Erosion was increased significantly by burning on the limestones but not on the volcanics. Within-group comparisons on the limestones determined that steeper slopes and harder burned areas suffered the most and are slowest to regenerate. Volume of timber on the 1911 site was 19 percent of that in similar uncut forest sites. It appears that complete recovery on the barren limestone slopes will require at least some centuries.  相似文献   
58.
It is suggested that the pattern of three-dimensional substorm current circuit varies significantly even during the lifetime of a single substorm. This gives rise to quite complex time variations of the magnetic field at low latitude stations even for relatively isolated substorms. To verify this, three-dimensional current models with time dependent spatial variations are used to simulate one type of complex low-latitude “substorm signature”. It is shown that the utmost care should be exercised in determining different substorm phases on the basis of such a signature. The results indicate also that, in certain longitudes in the evening sector, one should expect distinct differences in characteristics between positive bays observed on the ground and at the synchronous distance.  相似文献   
59.
When seismic thrust faults emerge on the ground surface,they are particularly damaging to buildings,bridges and lifelines that lie on the rupture path.To protect a structure founded on a rigid raft,a thick diaphragm-type soil bentonite wall(SBW) is installed in front of and near the foundation,at sufficient depth to intercept the propagating fault rupture.Extensive numerical analyses,verified against reduced–scale(1 g) split box physical model tests,reveal that such a wall,thanks to its high deformability and low shear resistance,"absorbs" the compressive thrust of the fault and forces the rupture to deviate upwards along its length.As a consequence,the foundation is left essentially intact.The effectiveness of SBW is demonstrated to depend on the exact location of the emerging fault and the magnitude of the fault offset.When the latter is large,the unprotected foundation experiences intolerable rigid-body rotation even if the foundation structural distress is not substantial.  相似文献   
60.
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