全文获取类型
收费全文 | 44040篇 |
免费 | 1318篇 |
国内免费 | 252篇 |
专业分类
测绘学 | 897篇 |
大气科学 | 2039篇 |
地球物理 | 10340篇 |
地质学 | 17057篇 |
海洋学 | 3904篇 |
天文学 | 9756篇 |
综合类 | 122篇 |
自然地理 | 1495篇 |
出版年
2022年 | 516篇 |
2021年 | 823篇 |
2020年 | 897篇 |
2019年 | 1078篇 |
2018年 | 2043篇 |
2017年 | 1969篇 |
2016年 | 2181篇 |
2015年 | 1115篇 |
2014年 | 1973篇 |
2013年 | 2694篇 |
2012年 | 2099篇 |
2011年 | 2370篇 |
2010年 | 2202篇 |
2009年 | 2310篇 |
2008年 | 2116篇 |
2007年 | 2253篇 |
2006年 | 1963篇 |
2005年 | 1088篇 |
2004年 | 1005篇 |
2003年 | 990篇 |
2002年 | 895篇 |
2001年 | 908篇 |
2000年 | 750篇 |
1999年 | 478篇 |
1998年 | 498篇 |
1997年 | 517篇 |
1996年 | 363篇 |
1995年 | 383篇 |
1994年 | 369篇 |
1993年 | 307篇 |
1992年 | 320篇 |
1991年 | 318篇 |
1990年 | 377篇 |
1989年 | 304篇 |
1988年 | 277篇 |
1987年 | 279篇 |
1986年 | 215篇 |
1985年 | 318篇 |
1984年 | 321篇 |
1983年 | 312篇 |
1982年 | 296篇 |
1981年 | 261篇 |
1980年 | 285篇 |
1979年 | 228篇 |
1978年 | 258篇 |
1977年 | 220篇 |
1976年 | 198篇 |
1975年 | 203篇 |
1974年 | 189篇 |
1973年 | 229篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
This paper deals with the formation processes and the palaeoenvironmental significance of relict slope deposits located on the uppermost part of the north Portugal mountains. For this purpose, seven key sites representative of the different lithofacies have been selected and analysed in detail. The data show that three main dynamic processes are responsible for the emplacement of regional fossil slope deposits: runoff, debris flows and dry grain flows. The ubiquity of these processes and the lack of frost‐related features or landforms do not support the existence of severe Pleistocene climates in this part of the lberian Peninsula as postulated by previous work. Pedological data gathered at one of the study sites show that a subalpine environment was probably present at 700–800 m altitude between 29 and 14 kyr. Using data from the Pyrenees Mountains, a 6.5 to 12°C depression in mean annual temperature has been tentatively postulated for this Pleniglacial period. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
12.
13.
The experimental data obtained in transit observations of the extrasolar planet HD 209458b and their comparison with theoretical inferences have led to the conclusions that HD 209458b (and other similar hot jupiters) is of a (mainly) hydrogen nature and that these objects probably possess strong magnetic fields. The results of the studies of HD 209458b and prospects for searches for the transits of other extrasolar planets are considered in detail. 相似文献
14.
The ordinary kriging method, a geostatistical interpolation technique, was applied for developing contour maps of design storm depth in northern Taiwan using intensity–duration–frequency (IDF) data. Results of variogram modelling on design storm depths indicate that the design storms can be categorized into two distinct storm types: (i) storms of short duration and high spatial variation and (ii) storms of long duration and less spatial variation. For storms of the first category, the influence range of rainfall depth decreases when the recurrence interval increases, owing to the increasing degree of their spatial independence. However, for storms of the second category, the influence range of rainfall depth does not change significantly and has an average of approximately 72 km. For very extreme events, such as events of short duration and long recurrence interval, we do not recommend usage of the established design storm contours, because most of the interstation distances exceed the influence ranges. Our study concludes that the influence range of the design storm depth is dependent on the design duration and recurrence interval and is a key factor in developing design storm contours. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
15.
Using the basic Boussinesq's equation, the expression for the vertical stress distribution (σz) underneath any point on the ground surface due to a general triangular loaded region in a preferred orientation with a linearly varied loading has been successfully derived. When the triangle is not in a preferred orientation, a simple axis transformation is required and the expression will be equally applicable. Based on this expression, σz due to an arbitrarily shaped loaded foundation can simply be determined by first triangulating the loaded area and summing up the contributions from each generated triangular region. The procedures for triangulating and calculating the stress distribution can be simply automated through computer programs. 相似文献
16.
In many areas of engineering practice, applied loads are not uniformly distributed but often concentrated towards the centre of a foundation. Thus, loads are more realistically depicted as distributed as linearly varying or as parabola of revolution. Solutions for stresses in a transversely isotropic half‐space caused by concave and convex parabolic loads that act on a rectangle have not been derived. This work proposes analytical solutions for stresses in a transversely isotropic half‐space, induced by three‐dimensional, buried, linearly varying/uniform/parabolic rectangular loads. Load types include an upwardly and a downwardly linearly varying load, a uniform load, a concave and a convex parabolic load, all distributed over a rectangular area. These solutions are obtained by integrating the point load solutions in a Cartesian co‐ordinate system for a transversely isotropic half‐space. The buried depth, the dimensions of the loaded area, the type and degree of material anisotropy and the loading type for transversely isotropic half‐spaces influence the proposed solutions. An illustrative example is presented to elucidate the effect of the dimensions of the loaded area, the type and degree of rock anisotropy, and the type of loading on the vertical stress in the isotropic/transversely isotropic rocks subjected to a linearly varying/uniform/parabolic rectangular load. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
17.
This paper presents a method that incorporates a non‐associated flow rule into the limit analysis to investigate the influence of the dilatancy angle on the factor of safety for the slope stability analysis. The proposed method retain's the advantage of the upper bound method, which is simple and has no stress involvement in the calculation of the energy dissipation and the factor of safety. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
18.
19.
M. V. Sazhin V. E. Zharov T. A. Kalinina 《Monthly notices of the Royal Astronomical Society》2001,323(4):952-964
Parallax measurements allow distances to celestial objects to be determined. Together with measurements of their position on the celestial sphere, they give a full three-dimensional picture of the location of the objects relative to the observer. The distortion of the parallax value of a distant source affected by weak microlensing is considered. This means that the weak microlensing leads to distortion of the distance scale. The gravitational deflection causes a retrograde apparent motion of the image, which is revealed as a negative parallax. It is shown that the distortions may significantly change the parallax values when they amount to several microseconds of arc. In particular, at this level many measured values of parallaxes should be negative. 相似文献
20.
V. Garnier D. Ohnenstetter G. Giuliani P. Blanc D. Schwarz 《Mineralogy and Petrology》2002,76(3-4):179-193
Summary ?Mong Hsu rubies of the “trapiche” type are sporadically seen in the gem market. However, they have never been described in
the field. The study of the nature of solid inclusions, the variation of trace-element contents, as well as the cathodoluminescence
behaviour of six “trapiche” rubies permit the conclusion that these rubies crystallised in the same geological environment
(marble-type deposit) as the normal rubies from Mong Hsu: (1) Cr and V are the main chromophorous elements in both ruby types;
they act, together with Ti, as activators or quenchers for cathodoluminescence; (2) calcite, dolomite, rutile, mica, diaspore,
apatite, chlorite, and feldspar are solid inclusions found in both ruby types; (3) the presence of bastn?site in trapiche
ruby and fluorite in non-trapiche ruby indicates the circulation of F-bearing fluids during ruby deposition; (4) the distribution
of trace-element contents in the crystal is similar for both ruby types. In the Cr2O3 vs. Fe2O3 and Cr2O3 vs. Fe2O3/TiO2 diagrams, the population fields of Mong Hsu “trapiche” and non-“trapiche” rubies overlap. They are distinct from those of
rubies and sapphires hosted in basalts from South-east Asia.
Received October 30, 2001; revised version accepted March 25, 2002 相似文献