全文获取类型
收费全文 | 8932篇 |
免费 | 347篇 |
国内免费 | 97篇 |
专业分类
测绘学 | 206篇 |
大气科学 | 616篇 |
地球物理 | 2082篇 |
地质学 | 3159篇 |
海洋学 | 824篇 |
天文学 | 1417篇 |
综合类 | 19篇 |
自然地理 | 1053篇 |
出版年
2021年 | 89篇 |
2020年 | 119篇 |
2019年 | 122篇 |
2018年 | 169篇 |
2017年 | 168篇 |
2016年 | 237篇 |
2015年 | 202篇 |
2014年 | 206篇 |
2013年 | 505篇 |
2012年 | 265篇 |
2011年 | 323篇 |
2010年 | 311篇 |
2009年 | 382篇 |
2008年 | 344篇 |
2007年 | 314篇 |
2006年 | 356篇 |
2005年 | 264篇 |
2004年 | 325篇 |
2003年 | 290篇 |
2002年 | 287篇 |
2001年 | 198篇 |
2000年 | 194篇 |
1999年 | 157篇 |
1998年 | 165篇 |
1997年 | 121篇 |
1996年 | 125篇 |
1995年 | 125篇 |
1994年 | 131篇 |
1993年 | 123篇 |
1992年 | 117篇 |
1991年 | 113篇 |
1990年 | 107篇 |
1989年 | 99篇 |
1988年 | 91篇 |
1987年 | 135篇 |
1986年 | 107篇 |
1985年 | 180篇 |
1984年 | 191篇 |
1983年 | 149篇 |
1982年 | 134篇 |
1981年 | 147篇 |
1980年 | 115篇 |
1979年 | 130篇 |
1978年 | 123篇 |
1977年 | 113篇 |
1976年 | 99篇 |
1975年 | 99篇 |
1974年 | 73篇 |
1973年 | 92篇 |
1972年 | 56篇 |
排序方式: 共有9376条查询结果,搜索用时 109 毫秒
41.
42.
John R. Percy 《Experimental Astronomy》1994,5(1-2):121-128
The role of amateurs and small-scale equipment in astronomical photometry is reviewed. The value of suitably selected projects for both research and educational purposes is stressed. Recent technical developments are noted, and a brief extrapolation to possible future arrangements included.Keynote address at the conference 相似文献
43.
Cooper John F. Christian Eric R. Richardson John D. Wang Chi 《Earth, Moon, and Planets》2003,92(1-4):261-277
Times for accumulation of chemically significant dosages on icy surfaces of Centaur, Kuiper Belt, and Oort Cloud objects from plasma and energetic ions depend on irradiation position within or outside the heliosphere. Principal irradiation components include solar wind plasma ions, pickup ions from solar UV ionization of interstellar neutral gas, energetic ions accelerated by solar and interplanetary shocks, including the putative solar wind termination shock, and galactic cosmic ray ions from the Local Interstellar Medium (LISM). We present model flux spectra derived from spacecraft data and models for eV to GeV protons at 40 AU, a termination shock position at 85 AU, and in the LISM. Times in years to accumulate dosages ~100 eV per molecule are computed from the spectra as functions of sensible surface depth less than one centimeter at unit density.The collisional resurfacing model of Luu and Jewitt is reconsidered in thecontext of depth-dependent dosage rates from plasma, suprathermal,and higher energy protons, and global exposure, by micrometeoroiddust grain impacts, of moderately irradiated red material below athin crust of heavily irradiated neutral material. This material should be more visible on dynamically `cold’ objects in the ~40 AU region. 相似文献
44.
This study uses 2 years of data from a detailed weekly water sampling programme in a 11·4 km2 upland peat catchment in the Northern Pennines, UK. The sampling comprised precipitation, soil‐water samples and a number of streams, including the basin outlet. Samples were analysed for: pH, conductivity, alkalinity, Na, K, Ca, Mg, Fe, Al, Total N, SO4, Cl and colour. Principal component analysis (PCA) was used to identify end‐members and compositional trends in order to identify controls on the development of water composition. The study showed that the direct use of PCA had several advantages over the use of end‐member mixing analysis (EMMA) as it combines an analysis of mixing and evolving waters without the assumption of having to know the compositional sources of the water. In its application to an upland peat catchment, the study supports the view that shallow throughflow at the catotelm/acrotelm boundary is responsible for storm runoff generation and shows that baseflow is controlled by cation exchange in the catotelm and mixing with a base‐rich groundwater. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
45.
46.
In an attempt to discover planetary eclipses, we observed the open cluster NGC 7086. On one night we observed the cluster and standard stars through the B and V filters, enabling us to make a colour-magnitude diagram. Plots of the differential magnitudes were inspected for variability, but no planetary eclipses were found. New variable stars were discovered and their light curves show some of them to be eclipsing binary stars. The location on the colour-magnitude diagram of some of the variable stars is consistent with cluster membership. 相似文献
47.
Lance B. Morrissey Philippe Janvier Simon J. Braddy John P. Bennett Susan B. Marriott Peter R. Tarrant 《Geology Today》2006,22(2):66-67
Armoured jawless fish, or 'ostracoderms', lived 450–360 million years ago, and display unusual morphologies, unlike any modern fish group. Since they left no living descendants, their mode of swimming has, until recently, remained speculative, although this is a crucial question as the first true pectoral fins evolved within the 'ostracoderms'. The discovery of the oldest-known fish trails, from the Early Devonian (400 million year old) 'Lower Old Red Sandstone' of south-east Wales offers new insights into the swimming behaviour of these early fish, notably the osteostracan 'ostracoderms' (or cephalaspids), whose horseshoe-shaped head and paddle-shaped pectoral fins have remained a functional riddle. 相似文献
48.
49.
C. S. M. Turney K. Van Den Burg S. Wastegrd S. M. Davies N. J. Whitehouse J. R. Pilcher C. Callaghan 《第四纪科学杂志》2006,21(4):335-345
High‐precision correlation of palaeoclimatic and palaeoenvironmental records is crucial for testing hypotheses of synchronous change. Although radiocarbon is the traditional method for dating late Quaternary sedimentary sequences, particularly during the last glacial–interglacial transition (LGIT; 15–9 ka), there are inherent problems with the method, particularly during periods of climate change which are often accompanied by major perturbations in atmospheric radiocarbon content. An alternative method is the use of tephras that act as time‐parallel marker horizons. Within Europe, numerous volcanic centres are known to have erupted during the LGIT, providing considerable potential for high‐precision correlation independent of past radiocarbon fluctuations. Here we report the first identification of the Vedde Ash and Askja Tephra in Ireland, significantly extending the known provenance of these events. We have also identified two new horizons (the Roddans Port Tephras A and B) and tentatively recognise an additional horizon from Vallensgård Mose (Denmark) that provide crucial additional chronological control for the LGIT. Two phases of the Laacher See Tephra (LST) are reported, the lower Laacher See Tephra (LLST) and probably the C2 phase of the Middle Laacher See Tephra (MLST‐C2) indicating a more northeasterly distribution of this fan than reported previously. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
50.
John Logan 《Ground water》1986,24(3):382-382
Abstract. Execution time of many programs can be markedly shortened by (a) defining frequently used constants in assignment statements or placing them in arrays and by (b) avoiding or minimizing the use of exponentials. 相似文献