全文获取类型
收费全文 | 95252篇 |
免费 | 1523篇 |
国内免费 | 761篇 |
专业分类
测绘学 | 2386篇 |
大气科学 | 7196篇 |
地球物理 | 19447篇 |
地质学 | 31882篇 |
海洋学 | 8508篇 |
天文学 | 21368篇 |
综合类 | 195篇 |
自然地理 | 6554篇 |
出版年
2021年 | 685篇 |
2020年 | 811篇 |
2019年 | 868篇 |
2018年 | 1696篇 |
2017年 | 1610篇 |
2016年 | 2199篇 |
2015年 | 1444篇 |
2014年 | 2173篇 |
2013年 | 4735篇 |
2012年 | 2335篇 |
2011年 | 3424篇 |
2010年 | 3043篇 |
2009年 | 4267篇 |
2008年 | 3775篇 |
2007年 | 3607篇 |
2006年 | 3502篇 |
2005年 | 2986篇 |
2004年 | 3030篇 |
2003年 | 2858篇 |
2002年 | 2756篇 |
2001年 | 2437篇 |
2000年 | 2364篇 |
1999年 | 2007篇 |
1998年 | 2018篇 |
1997年 | 2000篇 |
1996年 | 1738篇 |
1995年 | 1648篇 |
1994年 | 1495篇 |
1993年 | 1366篇 |
1992年 | 1303篇 |
1991年 | 1154篇 |
1990年 | 1355篇 |
1989年 | 1194篇 |
1988年 | 1100篇 |
1987年 | 1295篇 |
1986年 | 1181篇 |
1985年 | 1436篇 |
1984年 | 1625篇 |
1983年 | 1523篇 |
1982年 | 1403篇 |
1981年 | 1396篇 |
1980年 | 1171篇 |
1979年 | 1150篇 |
1978年 | 1165篇 |
1977年 | 1099篇 |
1976年 | 1009篇 |
1975年 | 963篇 |
1974年 | 962篇 |
1973年 | 974篇 |
1972年 | 600篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
401.
402.
Summary. This paper is concerned with an examination of the possibility that there might exist a small scale of convective circulation beneath the oceanic lithosphere. Recent suggestions that this might be the case have been made in an effort to understand why the bathymetry of the sea-floor deviates from the prediction of boundary layer theory for ages in excess of about 100 Ma. The energy source which sustains the secondary motion is supposed to be found in the steep temperature gradient near the planetary surface which is itself presumably maintained by the large-scale convective circulation associated with plate creation and destruction. Here we investigate the extent to which the temperature dependence of viscosity may act so as to stabilize the upper boundary layer against disruption by such secondary instability. If the viscosity profile is monotonie and the asymptotic upper mantle viscosity is about 1022 poise, as suggested by post-glacial rebound data, then the existence of the second scale is extremely unlikely. On the other hand, if a sufficiently pronounced low viscosity zone does exist under old sea-floor then the development of such a second scale cannot be ruled out completely. Some recently obtained geophysical evidence is reviewed which suggests that this is unlikely to be the case. 相似文献
403.
Summary Future research directions for agricultural geography were the subject of debate in Area in the late 1980s, and the subsequent application of political economy ideas undoubtedly revived interest in agricultural research. This paper argues that agricultural geography contains greater diversity than the dominant political economy discourse would suggest. It reviews 'other' areas of agricultural research such as policy, post- productivism, people, culture and animals, presenting future suggestions which should ensure that agricultural research continues revitalized rather than redundant into the next millennium. 相似文献
404.
Definition and measurement of salinity in salt lakes 总被引:4,自引:0,他引:4
Salinity is the most important chemical attribute of athalassic salt lakes. Even so, some confusion persists of what salinity means and how to measure it. For sal lakes, salinity is best defined as the sum total of all ion concentrations, or total ion concentration. Ideally, it is recommended that salinities be expressed on a mass per mass basis and as ppt (parts per thousand). Direct measurements of salinity can only be derived from full ionic analyses. Indirect measurements can be derived by determinations of density, conductivity, freezing point depression and total dissolved solids or matter. 相似文献
405.
Relationships among conductivity, temperature, total dissolved solids (TDS), and density were determined for hypersaline brine
(TDS, 83 g kg−1) from Mono Lake. A theoretical estimate of density based on ionic composition and calculated partial molar volumes compared
well with actual measurements (difference, 390 ppm). However, an empirically derived equation of state based on temperature
and conductivity provides the most accurate density estimates (residual errors, 190 ppm) and can be used with data collected
with CTD profilers. 相似文献
406.
Nitrate-nitrogen (nitrate-N) in groundwater is a potential problem in many parts of Kansas.From 1990 to 1998, 747 water samples were collected from domestic, irrigation, monitoring,and public water supply wells primarily from the western two-thirds of the state, and analyzedfor nitrate-N by the Kansas Geological Survey. Nitrate-N concentrations of the 747 samplesanalyzed range from 29% with less than or equal to 3 mg/L, 51% between 3 and 10 mg/L,and 20% greater than or equal to 10 mg/L. Factors that show a statistically significantrelationship with the occurrence of nitrate-N in Kansas groundwater in this assessment includegeographic area of the state, depth of well, and age of well. Nitrate-N levels of wells screenedin the High Plains aquifer in south-central Kansas showed a statistically higher concentrationthan samples collected from the High Plains aquifer in the western portion of the state.Comparison of nitrate-N with depth of well indicated that shallower wells throughout the stategenerally have higher nitrate-N values than deeper wells. Irrigation wells older than 1975showed statistically higher nitrate-N concentration than wells installed during and since 1975,possibly related to changes in well-construction practices and regulations that occurred in 1975. 相似文献
407.
We augment our scenario for the formation of astronomical objects from macroscopic superstrings by the assumption that the central matter keeps its identity in the fragmentation. From the condition that the angular momentum per mass squared of this matter should be less than the Kerr limit G/c, we obtain upper limits for the ratio of the mass of central black holes M(BH) to the mass M of the host object. This limit is M(BH)/M ≈ 0.001, and, expressed in observed quantities, approximately M(BH)/M ≈ σ2/(v · c) where σ is the r.m.s. velocity, v the rotational velocity and c the velocity of light. The valuesM(BH) agree with the observed behaviour both in order of magnitude and in the variation with velocity dispersion. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
408.
The Dynamical Attitude Model (DAM) is a simulation package developed to achieve a detailed understanding of the Gaia spacecraft attitude. It takes into account external physical effects and considers internal hardware components controlling the satellite. The main goal of the Gaia mission is to obtain extremely accurate astrometry, and this necessitates a good knowledge of Gaia’s behaviour as a spinning rigid body under the influence of various perturbations. This paper describes these perturbations and how they are modelled in DAM. 相似文献
409.
The Upper Middle Rhine Valley, granted the status of a World Heritage site, is well known for its unique inner narrow valley
of Quaternary age with its historical legacy of numerous medieval castles and old towns. Less known is that this has always
been a risk area of floods and gravitative mass movements. Up to the recent past, mainly ice floods caused enormous damage.
The inhabitants of the valley were well aware that they lived in a risk area, but they had learned to handle the flood hazard.
With the demise of ice floods over the last 40 years, due to climate change and because of the additional heating of the river
water by power plants, the awareness of flood hazards has been much diminished, in contrast to that of potential damage by
rockfalls and landslides which were also much feared in the past, though at the local level only. Still in the people’s memory
is the Kaub catastrophe of March 10, 1876, when 28 persons were killed by a landslide. Nowadays, even minor rockfalls are
a major threat, as they will affect the much-used traffic lines on both banks of the river, in particular the railroads. Therefore,
since 2002, on behalf of German Rail (Deutsche Bahn, DB), all problematic slopes have been protected by costly steel-ring nets, although they are an aesthetic problem by UNESCO
standards. The feeling of absolute safety created among the public is only subjective, though, as planners are well aware
of. Moreover, the impact of modern climate change on slope stability is nearly unknown. Therefore, it is still necessary to
develop a risk map for the narrow valley, with emphasis on gravitational hazards. 相似文献
410.