全文获取类型
收费全文 | 39295篇 |
免费 | 6439篇 |
国内免费 | 7723篇 |
专业分类
测绘学 | 7588篇 |
大气科学 | 4886篇 |
地球物理 | 11168篇 |
地质学 | 15513篇 |
海洋学 | 4843篇 |
天文学 | 1970篇 |
综合类 | 3197篇 |
自然地理 | 4292篇 |
出版年
2024年 | 145篇 |
2023年 | 393篇 |
2022年 | 1071篇 |
2021年 | 1423篇 |
2020年 | 1508篇 |
2019年 | 1895篇 |
2018年 | 1426篇 |
2017年 | 1879篇 |
2016年 | 1813篇 |
2015年 | 2018篇 |
2014年 | 2454篇 |
2013年 | 2607篇 |
2012年 | 2507篇 |
2011年 | 2547篇 |
2010年 | 2170篇 |
2009年 | 2550篇 |
2008年 | 2574篇 |
2007年 | 2885篇 |
2006年 | 2735篇 |
2005年 | 2279篇 |
2004年 | 2109篇 |
2003年 | 1752篇 |
2002年 | 1516篇 |
2001年 | 1233篇 |
2000年 | 1191篇 |
1999年 | 1090篇 |
1998年 | 970篇 |
1997年 | 828篇 |
1996年 | 688篇 |
1995年 | 602篇 |
1994年 | 566篇 |
1993年 | 440篇 |
1992年 | 350篇 |
1991年 | 236篇 |
1990年 | 197篇 |
1989年 | 207篇 |
1988年 | 142篇 |
1987年 | 111篇 |
1986年 | 77篇 |
1985年 | 57篇 |
1984年 | 35篇 |
1983年 | 23篇 |
1982年 | 34篇 |
1981年 | 23篇 |
1980年 | 20篇 |
1979年 | 8篇 |
1977年 | 13篇 |
1971年 | 6篇 |
1954年 | 19篇 |
1897年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
61.
Ren Tinawi Marc Sarrazin Andr Filiatrault 《Soil Dynamics and Earthquake Engineering》1993,12(8):469-477
The 1990 edition of the National Building Code of Canada (Associate Committee of the National Building Code, National Research Council, Ottawa, 1990) makes a clear distinction between eastern and western Canada in terms of seismic acceleration and velocity zones. While it is well established that ground motions can be amplified significantly through loose clay deposits, no results are available that take into consideration the typical high frequency content of ground motions in eastern Canada. This paper develops ground amplification curves for clays having depths between 10 and 70 m excited by typical eastern Canadian ground motions scaled to two different values of peak horizontal accelerations. Simplified free-field spectral design curves, which could be used by structural designers, are proposed. The curves show that maximum spectral accelerations occur for structural periods between 0.2 and 0.5 s. In addition, soil depth does not appear to be an important parameter controlling the response of typical clay deposits in eastern Canada. 相似文献
62.
N.C. Hambly A.C. Davenhall M.J. Irwin H.T. MacGillivray 《Monthly notices of the Royal Astronomical Society》2001,326(4):1315-1327
In this, the third in a series of three papers concerning the SuperCOSMOS Sky Survey, we describe the astrometric properties of the data base. We describe the algorithms employed in the derivation of the astrometric parameters of the data, and demonstrate their accuracies by comparison with external data sets using the first release of data, the South Galactic Cap survey. We show that the celestial coordinates, which are tied to the International Celestial Reference Frame via the Tycho–2 reference catalogue, are accurate to better than ±0.2 arcsec at J , R ∼19,18 , rising to ±0.3 arcsec at J , R ∼22,21 , with positional-dependent systematic effects from bright to faint magnitudes at the ∼0.1-arcsec level. The proper motion measurements are shown to be accurate to typically ±10 mas yr−1 at J , R ∼19,18 , rising to ±50 mas yr−1 at J , R ∼22,21 , and are tied to zero using the extragalactic reference frame. We show that the zero-point errors in the proper motions are ≤1 mas yr−1 for R >17 , and are no larger than ∼10 mas yr−1 for R <17 mas yr−1 . 相似文献
63.
C. Jordan S.A. Sim A.D. McMurry M. Aruvel 《Monthly notices of the Royal Astronomical Society》2001,326(1):303-312
Observations of ε Eri (K2 V) have been made with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope . The spectra obtained show a number of emission lines which can be used to determine, or place limits on, the electron density and pressure. Values of the electron pressure are required in order to make quantitative models of the transition region and inner corona from absolute line fluxes, and to constrain semi-empirical models of the chromosphere. Using line flux ratios in Si iii and O iv a mean electron pressure of P e = N e T e =4.8×1015 cm−3 K is derived. This value is compatible with the lower and upper limits to P e found from flux ratios in C iii , O v and Fe xii . Some inconsistencies which may be because of small uncertainties in the atomic data used are discussed. 相似文献
64.
65.
Numerical methods are usually used for the computation of ephemerides with perturbations for the precise orbital determination of an artificial satellite. But their numerical stability will be encountered in a long arc. In this case the use the improved Encke special perturbation methods has been suggested. The results of this paper show that Encke's method does indeed have a certain effectiveness, but cannot yet completely resolve the numerical stability, and the more efficient method is to use the energy integral or its variational relation to control the growth of the along-track error in general numerical calculations so that the aim of stabilization can be achieved. 相似文献
66.
1 INTRODUCTIONGlobal change research involves much geo-objectsand geo-process, such as climate and environmentalchange, substance and energy cycling, land-use/land-cover change (LUCC), interactivity between human and nature, etc.. So it need cooperation frommany research communities including international research programs groups such as IGBP (InternationalGeosphere-Biosphere Programme IPCC (Intergovemmental Panel on Climate Change), IHDP (InternationalHuman Dimension Program o… 相似文献
67.
We have analysed short-exposure high-resolution images obtained on the William Herschel Telescope (WHT), Big Telescope Azimuthal (BTA) and CanadaFranceHawaii Telescope (CFHT) in order to investigate mechanical vibrations that are capable of reducing the high angular resolution allowed by speckle-interferometric methods. After filtering the photon noise we have computed power spectra of the image centres of gravity. In these spectra we have found vibration features that differ from one telescope to another and that vary in particular with the zenith angle. In the case of the WHT we discuss these results in terms of possible causes and in terms of degradation of the transfer function. We present some means to improve the data. 相似文献
68.
R. T. Schilizzi W. Aldrich B. Anderson A. Bos R. M. Campbell J. Canaris R. Cappallo J. L. Casse A. Cattani J. Goodman H. J. van Langevelde A. Maccafferri R. Millenaar R. G. Noble F. Olnon S. M. Parsley C. Phillips S. V. Pogrebenko D. Smythe A. Szomoru H. Verkouter A. R. Whitney 《Experimental Astronomy》2001,12(1):49-67
69.