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   53784篇
  免费   434篇
  国内免费   428篇
测绘学   1732篇
大气科学   4017篇
地球物理   9739篇
地质学   21897篇
海洋学   3911篇
天文学   11046篇
综合类   273篇
自然地理   2031篇
  2021年   279篇
  2020年   276篇
  2019年   325篇
  2018年   3682篇
  2017年   3427篇
  2016年   2532篇
  2015年   703篇
  2014年   989篇
  2013年   1690篇
  2012年   2069篇
  2011年   3851篇
  2010年   3510篇
  2009年   3846篇
  2008年   3138篇
  2007年   3753篇
  2006年   1217篇
  2005年   1450篇
  2004年   1210篇
  2003年   1280篇
  2002年   1101篇
  2001年   800篇
  2000年   778篇
  1999年   673篇
  1998年   617篇
  1997年   629篇
  1996年   568篇
  1995年   534篇
  1994年   506篇
  1993年   428篇
  1992年   384篇
  1991年   411篇
  1990年   411篇
  1989年   387篇
  1988年   359篇
  1987年   432篇
  1986年   361篇
  1985年   444篇
  1984年   478篇
  1983年   462篇
  1982年   444篇
  1981年   368篇
  1980年   380篇
  1979年   311篇
  1978年   306篇
  1977年   294篇
  1976年   262篇
  1975年   254篇
  1974年   281篇
  1973年   305篇
  1972年   194篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
401.
We have released an archive of all observational data of the VUV spectrometer Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on SOHO that have been acquired until now. The operational phase started with ‘first light’ observations on 27 January 1996 and will end in 2014. Future data will be added to the archive when they become available. The archive consists of a set of raw data (Level 0) and a set of data that are processed and calibrated to the best knowledge we have today (Level 1). This communication describes step by step the data acquisition and processing that has been applied in an automated manner to build the archive. It summarizes the expertise and insights into the scientific use of SUMER spectra that has accumulated over the years. It also indicates possibilities for further enhancement of the data quality. With this article we intend to convey our own understanding of the instrument performance to the scientific community and to introduce the new, standard FITS-format database.  相似文献   
402.
Hornblende-orthopyroxene-calcic pyroxene-plagioclase (andesine-labradorite) — magnetite-ilmenite±quartz assemblages are evidently the most common granulite facies representatives of basic rocks throughout the world, and they must represent a variance of more than one. Ramberg (1948) invoked an additional degree of freedom arising out of unequal fractionation of Mg and Fe in the ferromagnesian silicates in such rocks. This is examined in detail on the basis of chemical data on twentytwo hornblende-orthopyroxenecalcic pyroxene triads, half of them from the type charnockite area near Madras.Theoretical consideration of a magnesium-iron distribution model shows that in quartz-bearing hornblende pyroxene granulites, the Mg/Mg+Fe ratios of hornblende, orthopyroxene and calcic pyroxene are uniquely determined by temperature at constant pressure. But these assemblages contain quartz rarely and Mg-Fe distribution among the three can be best described by three exchange equilibria where, at constant temperature, there is a fixed relation between the Mg/Mg+Fe ratios of the minerals. Among these, the exchange equilibria between hornblende-orthopyroxene and hornblende-calcic pyroxene appear to be interdependent; however, they are known to be significantly modified by varying tetrahedral aluminium contents of hornblendes. Comparison of molecular Mg/(Mg+Fe) ratios of hornblendes and parent rocks strongly points to an absence of Mg-Fe compositional variance in the total system, a petrologically important corollary being that the hornblendes in these rocks are highly unlikely to be secondary after pyroxenes. The general variance of the assemblages is also discussed.  相似文献   
403.
Aftershocks have the potential to cause collapse of a structure that has been already damaged by the preceding main shock. Seismic safety of a structure should therefore be ascertained through a damage analysis using the anticipated main shock and few larger-aftershock motions. Simulation of aftershock motions needs characterization of the seismic hazard due to aftershocks, and therefore it will be useful to develop a conditional scaling model that can predict the response spectrum of an anticipated aftershock motion consistent with the design spectrum of the main shock motion anticipated at the same station. In this study an attempt is made to develop a conditional scaling model for the pseudo spectral velocity spectrum via linear regression analysis on the aftershock and main shock recordings for the 1999 Chi–Chi earthquake. It is shown that it may be possible to obtain a simpler and approximate version of the conditional model from an unconditional model. Damage-causing potential of a ground motion also depends on its strong motion duration (SMD) and therefore a conditional scaling model is developed for SMD of the aftershock motion in several narrow frequency-bands. The model is developed for the larger-aftershock motions and it is shown that a reasonable replacement of such a model may be obtainable directly from an unconditional model. Finally, a simple weighted averaging scheme is proposed to obtain the composite SMD from the SMDs for different frequency bands by using the pseudo spectral acceleration spectrum of the motion.  相似文献   
404.
405.
The Permian system of the Palaeozoic Erathem is divided into three series, the Early Permian Cisuralian Series, the Middle Permian Guadalupian Series, and the Late Permian Lopingian Series. The Cisuralian Series encompasses the Asselian to Kungurian stages which constitute the basal part of the Gondwana supersequence I. In India, they are represented lithostratigraphically by the Talchir, Karharbari, and Barakar formations. This paper presents the palynological results from the Barakar Formation of the Upper Cisuralian Series from Manuguru which lies in the southeastern part of the Godavari basin. The succession studied comprises 35 subsurface samples from bore hole 1007 represented by clay, shale, sandstone, and coal. The palynofloras in this sequence have a homogenous composition demonstrating that not many significant floral changes took place through the considered stratigraphic range. The entire sequence is characterized by the dominance of nonstriate bisaccate genus Scheuringipollenites and sub-dominance of striate bisaccate genus Faunipollenites(=Protohaploxypinus). The other pollen genera among the nonstriate bisaccates are Rhizomaspora, Primuspollenites, Ibisporites, and Platysaccus. The striate bisaccates include Striatites, Striatopodocarpites, and Stroterosporites. The taeniate taxa are represented by Lueckisporites and Lunatisporites. The common monosaccate genera include Caheniasaccites, Potoniesporites, and Barakarites. Spores are less common and include Latosporites, Brevitriletes, Horriditriletes, Microbaculispora, and Callumispora. They characterize the palynofloral composition of the Lower Barakar Formation. The correlation of this assemblage with some of the biostratigraphic palynozones proposed previously for the Cisuralian sequences of the Paraná Basin of South America, Kalahari Karoo Basin of South Africa, Ruhuhu Basin of Tanzania, East Africa as well as palynoassemblages from South Victoria Land and Dronning Maud Land, Antarctica and Collie Basin of west Australia point out to their Early Permian (Late Sakmarian–Early Artinskian) age. Palynomorphs such as Botryococcus sp., Tetraporinia sp., Balmeela sp. and Leiosphaeridia sp. are also recorded which suggest that these sediments were deposited during post-glacial near shore, cool and humid environment.  相似文献   
406.
Criteria were established for the estimation of the lanthanide composition of sedimentary complexes for the reconstruction of sedimentation conditions. The distribution of lanthanides was investigated in phosphorites and sedimentary rocks from the cover of the East European platform, and published data on the geochemistry of phosphorites from Eurasia were analyzed. Indicator lanthanide ratios were established for the determination of the climate and depth of sedimentation, and the possibility of the use of rare earth elements for the facies settings and transgression-regression cycles of sedimentation was demonstrated. The sedimentation conditions of the iron formations of the Proterozoic Kursk and Krivoi Rog sedimentary groups were inferred from the distribution of lanthanides: the hematite and magnetite quartzites are the deepest water complexes of the sedimentation profile; the climatic conditions of sedimentation were estimated as humid for shale subformations and humid-semihumid for iron formations; the depths of sedimentation in the Kursk basin was 50−300 m; the sources of iron were the material of weathering profiles and endogenous influx.  相似文献   
407.
408.
According to observations, drops freely falling in the air under the action of gravity are deformed and oscillate in a wide range of frequencies and scales. A technique for calculating surface axisymmetric oscillations of a deformed droplet in the linear approximation under the assumption that the amplitude and wavelength are small when compared to the droplet diameter is proposed. The basic form of an axisymmetric droplet is chosen from observations. The calculation results for surface oscillations agree with recorded data on the varying shape of water droplets falling in the air.  相似文献   
409.
The geochemistry of lake (Renstradträsket) and estuarine (Pieni Pernajanlahti Bay) sediment was investigated in a medium sized watershed draining to the Gulf of Finland, Baltic Sea. Catchment land-use types were compared and found similar. Sediment cores were dated using 210Pb- and 137Cs-chronologies and analyzed for Al, K, Cu, Zn, Fe, Mn, phosphorus fractions, TN, TC and biogenic silica (BSi). Differences between the sediment cores were studied by using linear regression analysis and principal components analysis (PCA). Despite similarities in catchment land-use and history, the sediment geochemical profiles of the sites varied significantly. Some of the differences could be related to differences in chemical sedimentation environment (lacustrine versus estuarine). TP concentration was found to be positively correlated with sediment iron content in estuarine sediment but negatively correlated with Fe in lake sediment. In the estuarine core sedimentary iron was not correlated to lithogenic potassium and aluminum but in the lake core the iron seemed to be lithogenic in origin, as suggested by the strong positive correlations (r 2 = 0.95–0.96) between these three variables. Most similarities among the cores were found in Al concentrations. Estuarine nutrient profiles appeared relatively monotonous compared to the lake core. This is probably due to more vigorous mixing of the sediments that may ensure more rapid and complete consumption of the organic matter deposited on the bottom of the estuary. Therefore the lake sediment appeared to preserve the historical record of eutrophication better. Biologically less active and more particle-bound materials like the trace metals Cu and Zn seemed to retain good records of anthropogenic impact also in the estuarine core. The study highlights the need to take the sedimentation environment into account when interpreting geochemical record.  相似文献   
410.
Zusammenfassung Es werden Gleichungen abgeleitet, mittels derer man aus dem Energiespektrum des Windes die Häufigkeit von Böen bestimmter Größe und bestimmter Mindestgröße, die Größe der häufigsten Böenspitze und einen mittleren Abstand zwischen zwei Böenstößen berechnen kann. Die Theorie wird an einem Beispiel erläutert.
Summary Equations are derived which allow the calculation of frequencies of gusts with a velocity equal or larger than a given amount, of the velocity of the most frequent gust, and of a mean interval between two gusts out of the power-spectrum of wind. The theory is illustrated by an example.

Résumé L'auteur établit des formules permettant de calculer les grandeurs suivantes en partant du spectre énergétique du vent: Fréquence des rafales de force ou de force maximale déterminées, force des rafales les plus fréquentes ainsi que le temps moyen séparant deux rafales. Il démontre sa théorie par un exemple.


Mit 4 Textabbildungen  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