全文获取类型
收费全文 | 34890篇 |
免费 | 627篇 |
国内免费 | 478篇 |
专业分类
测绘学 | 927篇 |
大气科学 | 3238篇 |
地球物理 | 7442篇 |
地质学 | 12544篇 |
海洋学 | 2524篇 |
天文学 | 7227篇 |
综合类 | 149篇 |
自然地理 | 1944篇 |
出版年
2020年 | 238篇 |
2019年 | 235篇 |
2018年 | 636篇 |
2017年 | 620篇 |
2016年 | 892篇 |
2015年 | 591篇 |
2014年 | 890篇 |
2013年 | 1661篇 |
2012年 | 946篇 |
2011年 | 1176篇 |
2010年 | 1073篇 |
2009年 | 1445篇 |
2008年 | 1212篇 |
2007年 | 1056篇 |
2006年 | 1138篇 |
2005年 | 970篇 |
2004年 | 923篇 |
2003年 | 920篇 |
2002年 | 929篇 |
2001年 | 814篇 |
2000年 | 842篇 |
1999年 | 714篇 |
1998年 | 700篇 |
1997年 | 710篇 |
1996年 | 634篇 |
1995年 | 589篇 |
1994年 | 535篇 |
1993年 | 461篇 |
1992年 | 445篇 |
1991年 | 441篇 |
1990年 | 449篇 |
1989年 | 432篇 |
1988年 | 401篇 |
1987年 | 499篇 |
1986年 | 467篇 |
1985年 | 490篇 |
1984年 | 592篇 |
1983年 | 603篇 |
1982年 | 535篇 |
1981年 | 523篇 |
1980年 | 481篇 |
1979年 | 467篇 |
1978年 | 479篇 |
1977年 | 427篇 |
1976年 | 382篇 |
1975年 | 389篇 |
1974年 | 431篇 |
1973年 | 419篇 |
1972年 | 262篇 |
1971年 | 241篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
191.
192.
H.-E. Frhlich 《Astronomische Nachrichten》1982,303(2):97-103
In order to determine the quantitative influence of small amounts of dust on the luminosity profile of spherical galaxies, we represent an elliptical galaxy by a KING sphere filled with dust. The density of dust particles is assumed to be related to the density of stars according to a simple power law. Assuming that the observed central reddenings in NGC 4874 and NGC 4889 are really caused by dust, we estimate the optical depths in the centres of both galaxies to be about 0.75. 相似文献
193.
194.
195.
196.
197.
H.-E. Frhlich 《Astronomische Nachrichten》1985,306(3):101-105
Burnt-out Population III remnants are thought to have acted as protogalactic seeds. Once tepid pancakes have formed such seeds (≈106 M⊙) could be subject to considerable growing by quasi-stationary accretion. 相似文献
198.
H. A. Brouwer 《International Journal of Earth Sciences》1918,8(5-8):197-209
Ohne Zusammenfassung 相似文献
199.
H.J.B. Birks 《Quaternary Research》1980,14(1):60-86
The flora and vegetation of six ice-cored moraines of the Klutlan Glacier were analyzed in 65 plots by European plant-sociological techniques. The age of each plot was estimated from annual growth rings of shrubs or trees in the plots. Nine major vegetation types are distinguished: Crepis nana, Dryas drummondii, Hedysarum mackenzii, Hedysarum-Salix, Salix-Shepherdia canadensis, Picea-Salix, Picea-Arctostaphylos, Picea-Ledum, and Picea-Rhytidium. These contain plants aged 2–6, 9–23, 10–20, 24–30, 32–58, 58–80, 96–178, 177–240, and >163- >339 yr, respectively. Six other vegetation types are described from windthrow areas, drainage channels, volcanic tephra slopes, lake margins, fens, and drained lakes. The major vegetation types reflect a vegetational succession related to moraine age and stability, with the Crepis nana type as the pioneer vegetation developing through the other vegetation types to the Picea-Rhytidium type on the oldest moraines. Changes in species diversity and soil development, particularly humus accumulation, parallel the vegetational succession. This succession differs from patterns of revegetation of deglaciated landscapes in Alaska and British Columbia today and in Minnesota in late-Wisconsin times because of differences in climate, plant migration, and local ecology. 相似文献
200.
N. H. WOODCOCK 《Sedimentology》1979,26(1):83-99
The two main methods of estimating palaeoslope direction from slump structure orientation data are reviewed. The mean axis method takes the downslope direction as the perpendicular to the mean slump fold axis. The separation are method estimates the downslope direction by the bisector of a planar separation angle between groups of slump folds with opposite downplunge asymmetry. The advantages and disadvantages of each method are considered in detail, by reference first to idealized slump fold distributions and then to a compilation of sixty natural slump fold axis distributions, mostly from published data. Among the advantages of the mean axis method are the ease of deriving confidence limits for estimates and its applicability where fold asymmetry data are unavailable. The main advantage of the separation are method is its validity for strongly skewed fold axis samples. The statistical robustness of the mean axis method, however, due to its reliance on average properties of data, makes it more generally reliable than the separation arc method which depends on extreme properties of data. A procedure for determining palaeoslope directions is suggested which reflects the respective strengths of the two methods. 相似文献