全文获取类型
收费全文 | 75157篇 |
免费 | 1071篇 |
国内免费 | 636篇 |
专业分类
测绘学 | 2063篇 |
大气科学 | 5362篇 |
地球物理 | 15214篇 |
地质学 | 25379篇 |
海洋学 | 6673篇 |
天文学 | 17619篇 |
综合类 | 178篇 |
自然地理 | 4376篇 |
出版年
2021年 | 658篇 |
2020年 | 677篇 |
2019年 | 823篇 |
2018年 | 1714篇 |
2017年 | 1565篇 |
2016年 | 2042篇 |
2015年 | 1145篇 |
2014年 | 1887篇 |
2013年 | 3710篇 |
2012年 | 2104篇 |
2011年 | 2910篇 |
2010年 | 2672篇 |
2009年 | 3568篇 |
2008年 | 3040篇 |
2007年 | 3107篇 |
2006年 | 2895篇 |
2005年 | 2260篇 |
2004年 | 2288篇 |
2003年 | 2167篇 |
2002年 | 2168篇 |
2001年 | 1899篇 |
2000年 | 1775篇 |
1999年 | 1521篇 |
1998年 | 1518篇 |
1997年 | 1517篇 |
1996年 | 1254篇 |
1995年 | 1208篇 |
1994年 | 1118篇 |
1993年 | 999篇 |
1992年 | 907篇 |
1991年 | 900篇 |
1990年 | 900篇 |
1989年 | 917篇 |
1988年 | 839篇 |
1987年 | 973篇 |
1986年 | 879篇 |
1985年 | 1046篇 |
1984年 | 1197篇 |
1983年 | 1071篇 |
1982年 | 1010篇 |
1981年 | 1005篇 |
1980年 | 849篇 |
1979年 | 838篇 |
1978年 | 822篇 |
1977年 | 762篇 |
1976年 | 685篇 |
1975年 | 679篇 |
1974年 | 649篇 |
1973年 | 709篇 |
1972年 | 448篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
Pastoral production around artificial watering points in semi-arid environments affects the density and composition of plant communities. In the Kalahari desert of southwestern Africa, bush encroachment is often regarded as the major form of land degradation resulting from pastoral production around watering points. We investigated the OvaHerero pastoralists’ perceptions of the extent of vegetation change since the establishment of artificial watering points in the northern Kalahari desert of Namibia, and related this to ecological data on vegetation change. We determined the utility of woody vegetation to pastoralists’ livelihoods in terms of provision of construction material, fuel wood and browse. We quantified local knowledge of cattle browse consumption and correlated this with field data. We also assessed the purposes for which major livestock types were used. Our results showed that: (1) pastoral knowledge of bush encroachment and browse consumption was consistent with concurrently collected field data, (2) the current level of bush encroachment was perceived to be beneficial for pastoral production, and (3) cattle played an important role in the production of milk and milk by-products for domestic use, and served as a source of cash income, while sheep and goats were primarily kept for meat consumption. This result contrasts with historical studies that mainly portray cattle as a symbol of social status among OvaHerero pastoralists. 相似文献
212.
A special system of canonical variables is considered. An algorithm for expanding the principal functions of Keplerian motion in new elements is presented. The advantage of the proposed system is a relatively small number of terms in the classical expansions of the unperturbed two-body problem. A method for expanding the time derivatives of the rectangular coordinates is proposed. Some estimates of the number of terms in the presented expansions have been obtained through numerical experiments. 相似文献
213.
Assuming that a disk galaxy is composed of an ambient pervasive gas, small clouds, molecular clouds and stars, its evolution is studied through examining the interchange processes among them. Main results obtained are: (1) The star formation rate is directed by the formation process of molecular clouds. (2) Depending upon the parameters there may be three or four types of evolution of disk galaxies: the no star formation case, the active in the past and inactive at present star formation case, the burst-like star formation case and the very active in star formation case.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan between 30 September–6 October, 1984. 相似文献
214.
215.
216.
Kieran P. Donaghy 《Journal of Geographical Systems》2011,13(1):17-30
In the literatures of regional science, urban economics, and urban development planning, a working assumption is that individuals
respond to incentives and regulations, given their preferences. Models for planning and policy analyses are used to consider
what might occur if the incentives or regulations were different. In these models, however, preferences are usually assumed
to be given and stable, and agents are usually assumed to be homogeneous. This paper focuses on the implications of making
preferences in models of policy implementation endogenously determined and time varying heterogeneous agents. We consider
first the recent literature on intertemporal choice and preference change, which cuts across many disciplines, and more briefly
the literature on norm-regarding behavior. We then elaborate a simple model of transportation demand—from a static to a dynamic
orientation, from fixed and exogenously given preferences of strictly self-regarding agents to endogenously determined and
policy-induced preferences of heterogeneous agents—and illustrate its characteristics with simple numerical examples. 相似文献
217.
This paper presents an efficient methodology for computing constant‐ductility inelastic response spectra. The computation of constant‐ductility spectra involves numerical root‐finding algorithms to find the strongest structure providing a desired ductility response. Smooth inelastic structural behavior is modeled using a first‐order nonlinear differential equation and the transient structural response is solved using an implicit algorithm requiring Newton iterations at each time step. For structural models with smooth hysteretic behavior (not piece‐wise linear), a simple root‐finding method involving a combination of hyperbolic fits, linear interpolation, and Newton's method converges upon the highest strength (conservative) solution with a small number of iterations. The effect of the hysteretic smoothness on the occurrence of multiple roots is examined for two near‐fault and two far‐fault earthquake records, and for two measures of ductility and for normalized hysteretic energy. The results indicate how the smoothness of the hysteretic behavior affects ductility demand and constant‐ductility response spectra. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
218.
O. V. Ilyina N. S. Tychkov A. M. Agashev A. V. Golovin A. E. Izokh O. A. Kozmenko N. P. Pokhilenko 《Doklady Earth Sciences》2016,467(2):408-411
The results of the first study of the PGE distribution in deformed lherzolites of the Udachnaya kimberlite pipe (Yakutia) are presented here. The complex character of evolution of the PGE composition in the Deformed lherzolites is assumed to be the result of silicate metasomatism. At the first stage, growth in the amount of clinopyroxene and garnet in the rock is accompanied by a decrease in the concentration of the compatible PGE (Os, Ir). During the final stage, the rock is enriched with incompatible PGE (Pt, Pd) and Re possible due to precipitation of submicron-sized particles of sulfides in the interstitial space of these mantle rocks. 相似文献
219.
M.?B.?BellEmail author L.?W.?Avery J.?M.?MacLeod J.?P.?Vallée 《Astrophysics and Space Science》2011,333(2):377-388
Sensitive spectral observations made in two frequency bands near 6.0 and 17.6 GHz are described for Orion and W51. Using frequency
switching we were able to achieve a dynamic range in excess of 10,000 without fitting sinusoidal or polynomial baselines.
This enabled us to detect lines as weak as T
A
∼1 mK in these strong continuum sources. Hydrogen recombination lines with Δn as high as 25 have been detected in Orion. In the Orion data, where the lines are stronger, we have also detected a systematic
shift in the line center frequencies proportional to linewidth that cannot be explained by normal optical depth effects. 相似文献
220.