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   49035篇
  免费   1128篇
  国内免费   1348篇
测绘学   1898篇
大气科学   3813篇
地球物理   9686篇
地质学   20008篇
海洋学   3465篇
天文学   7822篇
综合类   1907篇
自然地理   2912篇
  2022年   248篇
  2021年   407篇
  2020年   409篇
  2019年   438篇
  2018年   4430篇
  2017年   3843篇
  2016年   2882篇
  2015年   799篇
  2014年   894篇
  2013年   1551篇
  2012年   1842篇
  2011年   3563篇
  2010年   2744篇
  2009年   3339篇
  2008年   2812篇
  2007年   3232篇
  2006年   1373篇
  2005年   1133篇
  2004年   1240篇
  2003年   1156篇
  2002年   980篇
  2001年   710篇
  2000年   702篇
  1999年   545篇
  1998年   587篇
  1997年   546篇
  1996年   466篇
  1995年   454篇
  1994年   397篇
  1993年   333篇
  1992年   320篇
  1991年   307篇
  1990年   374篇
  1989年   293篇
  1988年   285篇
  1987年   352篇
  1986年   287篇
  1985年   330篇
  1984年   430篇
  1983年   366篇
  1982年   367篇
  1981年   336篇
  1980年   356篇
  1979年   294篇
  1978年   267篇
  1977年   267篇
  1976年   251篇
  1975年   224篇
  1974年   249篇
  1973年   262篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
Spectra of three high-redshift quasars discovered on low-dispersion objective prism plates were obtained. We present the emission line identifications and redshifts of the objects. Equivalent widths, line widths and low-resolution line profiles are given characterizing the quasar emission region. The Lyα and CIV equivalent widths indicate that the considered quasars are luminous objects at high redshift. Both these equivalent widths are too low by a factor 4, whereas the N V/Lyα ratio is 5 times larger than predicted by the photoionisation models. The line profiles are compared with logarithmic, electron-scattering and Gaussian profiles. In all the spectra rich absorption line systems are evident.  相似文献   
132.
Within the sizable literature on electricity demand relatively little attention is given to regional variations in electricity demand elasticities. This paper investigates 39 electric utility companies having a total of 93 separate operating regions in order to assess the nature of regional electricity demand elasticity variations. Residential, commercial, and industrial demand functions are estimated for each operating region using a flow adjustment model of demand. The resulting electricity price, measure of economic activity, and natural gas price elasticities vary regionally, but not in a systematic manner. The demand elasticities found in each sector range from highly inelastic to highly elastic with the pattern of elasticity variations exhibiting few spatial trends or relationships. Comparisons between operating region results and more aggregated statewide results indicate that statewide and national analysis of electricity demand masks much regional detail — a result which has important implications for energy policy making.  相似文献   
133.
134.
Summary Sets of virtual poles corresponding to sets of geomagnetic field values at equidistant points, lying along circles of latitude, were defined on the basis of the 1980 IGRF extending to degree and order 8. Certain places on the Earth's surface yield virtual poles lying very close together. The segments of the virtual pole paths, corresponding to these places, have a large curvature. The mentioned places of the Earth's surface are supposed to be areas with a low rate of the westward drift. They form continuous zones that show a certain relation to global-tectonics features.
uum ¶rt; 8- n¶rt;a ¶rt;a¶rt; aaumu n 1980 u u nmu uma n, mmmu nm uu n ma, a u¶rt;umam ¶rt; naa m ma nmu nua¶rt;am uma n, m uu ¶rt; ¶rt;. amu u, mmmu mu ma, um uu. ¶rt;naam, m nm ma nmu m amu, m m ana¶rt; ¶rt;a ua. u am n na, m naam a m uua mmuu.
  相似文献   
135.
136.
Further photometric and spectroscopic observations of a chain of galaxies in Centaurus are presented and discussed.  相似文献   
137.
Seismic reflection profiling in the South San Clemente Basin and the southern portion of the San Diego Trough has revealed at least six sedimentary units exhibiting varying stages of deformation. Four of the units are interpreted to be marine turbidites supplied by adjacent submarine canyons. Sediments comprising the Descanso Plain and correlative material within the South San Clemente Basin are attributed to a southerly source (Banda Canyon), while the more recent Quaternary turbidites from Coronado Canyon filled the southern San Diego Trough and then spilled over into South San Clemente Basin. The relatively high but intermittent rates of sedimentation, together with shifting sources and areas of deposition, have resulted in sedimentary units that were emplaced in comparatively short episodes but which were subjected to relatively continuous tectonic activity. Consequently, the sedimentary layers of each unit appear uniformly affected by deformation which increases in successively older units.  相似文献   
138.
139.
A thick (200 m) rock salt mass covering Late Archean granitoids was exposed for the first time in the Early Proterozoic volcanogenic-sedimentary formations in the Onega trough of the east Baltic Shield by the Onega parametric well. The mineral composition of salts, their geochemical features, and the isotopic composition of carbonate carbon and oxygen have been studied. After fluid inclusions present in salts, their metamorphism temperature and isotopic composition of helium and argon were determined. The obtained results give evidence of the fact that rock salts and magnesites associated with them were formed in an evaporate basin with participation of deep crustal processes. The age of the underlying granitoids (2.716 ± 9 Ma) is determined using the Pb—Pb method.  相似文献   
140.
Revegetation of disturbed land, particularly in arid environments, is often hindered by low seedling establishment. Information on seed biology and germination cues of keystone species is lacking, particularly in arid Australia; a major zone for mining developments. This study investigated seed characteristics and germination of 18 common species required for rehabilitation of disturbed areas at Shark Bay Salt in the Shark Bay World Heritage Area, Western Australia. Untreated seeds of seven species (Aphanopetalum clematideum, Atriplex bunburyana, Austrostipa elegantissima, Melaleuca cardiophylla, Pembertonia latisquamea, Rhagodia baccata, Salsola tragus) exhibited high germination percentages. Seeds of two species (Acacia tetragonophylla, Stylobasium spathulatum) had low imbibition, which increased with hot-water treatment, hence require scarification for germination. Gibberellic acid, smoke water and karrikinolide (a butenolide isolated from smoke) substantially increased germination percentages of three species (Anthocercis littorea, Diplolaena grandiflora, Solanum orbiculatum). Seeds of the remaining six species (Dioscorea hastifolia, Eremophila oldfieldii, Nitraria billardierei, Ptilotus exaltatus, Thryptomene baeckeacea, Zygophyllum fruticulosum) had low germination percentages regardless of treatment. Most species germinated equally well at 26/13 °C and 33/18 °C, however seven species had improved performance at 26/13 °C. This study is significant to land managers and conservation agencies with an interest in optimising germination of arid zone seeds for restoration.  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