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   86334篇
  免费   927篇
  国内免费   1438篇
测绘学   2933篇
大气科学   5753篇
地球物理   16703篇
地质学   34269篇
海洋学   6517篇
天文学   16585篇
综合类   2359篇
自然地理   3580篇
  2022年   602篇
  2021年   891篇
  2020年   938篇
  2019年   1034篇
  2018年   6691篇
  2017年   5818篇
  2016年   4837篇
  2015年   1328篇
  2014年   2236篇
  2013年   3423篇
  2012年   3322篇
  2011年   5384篇
  2010年   4501篇
  2009年   5233篇
  2008年   4491篇
  2007年   5093篇
  2006年   2755篇
  2005年   1926篇
  2004年   1959篇
  2003年   1916篇
  2002年   1735篇
  2001年   1447篇
  2000年   1323篇
  1999年   1026篇
  1998年   1009篇
  1997年   1022篇
  1996年   850篇
  1995年   829篇
  1994年   786篇
  1993年   651篇
  1992年   629篇
  1991年   650篇
  1990年   693篇
  1989年   607篇
  1988年   576篇
  1987年   651篇
  1986年   521篇
  1985年   705篇
  1984年   739篇
  1983年   725篇
  1982年   669篇
  1981年   583篇
  1980年   605篇
  1979年   493篇
  1978年   521篇
  1977年   474篇
  1976年   419篇
  1975年   415篇
  1974年   430篇
  1973年   488篇
排序方式: 共有10000条查询结果,搜索用时 917 毫秒
381.
382.
The muon flux at the South Pole was measured for five zenith angles, 0°, 15°, 35°, 82.13° and 85.15° with a scintillator muon telescope incorporating ice Cherenkov tank detectors as the absorber. We compare the measurements with other data and with calculations.  相似文献   
383.
384.
A high resolution numerical sonagram of Pc-1 magnetic record has shown that single narrow lines of Pc-1 spectra produce lower frequency (“red-shifted”) satellite lines more than twice as often as their higher frequency (“violet”) counterparts. The violet satellites usually emerge only when at least two ancestor lines are present simultaneously.  相似文献   
385.
This paper and subsequent Paper II are an investigation of the annihilation line formation in gamma-ray bursts based on the assumption of positron production in a strong magnetic field (because of one-photon absorption of hard gamma-quanta radiated in the neutron star hot polar spot). We discuss a two-photon annihilation line in this paper. It is shown that if the star magnetic field is greater than 3×1012 G, the relative flux in the line depends solely on the hardness of the continuum and is, as a rule, less than or about 10–20% of the total flux. This is consistent with the spectral data recorded by ‘Venera-11’ and ‘Venera-12’ space probes. The annihilation region formation above the hot polar spot is discussed, and positron density and annihilation region dimensions are estimated.  相似文献   
386.
U–Th–Pb analyses of zircons from six granites and one metasediment collected in the accretionary Central belt of Taimyr, Arctic Siberia, demonstrate that Neoproterozoic (c. 900 Ma) granites intrude late Mesoproterozoic/early Neoproterozoic amphibolite facies metamorphic rocks. This is the first time in the Mamont–Shrenk region that Neoproterozoic ages have been recognized for these lithologies, previously thought to be Archaean/Palaeoproterozoic in age. The Mamont–Shrenk Terrane (MST) represents a Grenvillian age (micro?) continent intercalated with younger Neoproterozoic ophiolites during thrusting and accreted to the northern margin of the Siberian craton sometime before the late Vendian. Basement to the MST may have been derived from the Grenvillian belt of east Greenland. Viable tectonic reconstructions must allow for an active margin along northern Siberia (modern day coordinates) in the middle Neoproterozoic.  相似文献   
387.
388.
V. P. Singh 《水文研究》2001,15(4):671-706
The history of the kinematic wave theory and its applications in water resources are traced. It is shown that the theory has found its niche in water resources and its applications are so widespread that they may well constitute what may be termed ‘kinematic wave hydrology’. Few theories have been applied in hydrology and water resources as extensively as the kinematic wave theory. This theory, however, is not without limitations and when it is applied they must be so recognized. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   
389.
The source IRAS 03134 + 5958 identified by Iyengar & Verma (1984) on the Palomar Observatory Sky Survey (POSS) prints with a nonstellar optical object with [P – R]≃ 5.3 ± 1.5 is near the edge of Lynds dark cloud No. 1384 and is either embedded in or behind the cloud. The galactic latitude of this source (b II = 2‡.3), its positionvis-a-vis the Lynds dark cloud, its nonstellar appearance, high [P – R] colour and its far-infrared spectrum, all suggest the possibility of its being a Herbig-Haro (HH) object. To test this possibility we undertook measurements of its proper motion and variability (two of the characteristic properties of HH objects). These yield μa = (3.6 ± 2.3) arcsec/century and μδ= (−1.2 ± 2.0) arcsec/century for its proper motion. The source reveals large variation in brightness between 1950 and 1954. Optical line studies of the source are required to confirm its classification as an HH object.  相似文献   
390.
An analytical model for the constant-head pumping test is developed for a partially penetrating well that has a finite thickness skin, and intersects a single vertical fracture. In the model, the fracture is fully confined and flow occurs only in the fracture. The model is developed using Laplace transform and finite Fourier transform methods. The model is to be used for analysing well test data from vertically fractured media and for verifying numerical models. Dimensionless curves are used to study the effects of a finite thickness skin and a partially penetrating wellbore. In the presence of a finite thickness skin, or a partially penetrating wellbore, a typical flow response for the constant-head pumping test has three distinct periods of flow corresponding to small-, intermediate- and large-time. Small- and large-time approximations are presented for the model. For tests where the wellbore is fully penetrating, or the partial penetration ratio is known, these approximations can be used to analyse field data.  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