全文获取类型
收费全文 | 37899篇 |
免费 | 800篇 |
国内免费 | 399篇 |
专业分类
测绘学 | 1422篇 |
大气科学 | 2865篇 |
地球物理 | 8392篇 |
地质学 | 13376篇 |
海洋学 | 2766篇 |
天文学 | 8497篇 |
综合类 | 173篇 |
自然地理 | 1607篇 |
出版年
2021年 | 407篇 |
2020年 | 392篇 |
2019年 | 407篇 |
2018年 | 1102篇 |
2017年 | 1026篇 |
2016年 | 1395篇 |
2015年 | 854篇 |
2014年 | 1260篇 |
2013年 | 2057篇 |
2012年 | 1361篇 |
2011年 | 1570篇 |
2010年 | 1332篇 |
2009年 | 1684篇 |
2008年 | 1427篇 |
2007年 | 1323篇 |
2006年 | 1347篇 |
2005年 | 1115篇 |
2004年 | 996篇 |
2003年 | 1009篇 |
2002年 | 965篇 |
2001年 | 873篇 |
2000年 | 848篇 |
1999年 | 731篇 |
1998年 | 705篇 |
1997年 | 697篇 |
1996年 | 609篇 |
1995年 | 601篇 |
1994年 | 581篇 |
1993年 | 476篇 |
1992年 | 433篇 |
1991年 | 452篇 |
1990年 | 469篇 |
1989年 | 430篇 |
1988年 | 394篇 |
1987年 | 468篇 |
1986年 | 404篇 |
1985年 | 492篇 |
1984年 | 516篇 |
1983年 | 509篇 |
1982年 | 491篇 |
1981年 | 397篇 |
1980年 | 398篇 |
1979年 | 348篇 |
1978年 | 351篇 |
1977年 | 332篇 |
1976年 | 285篇 |
1975年 | 292篇 |
1974年 | 308篇 |
1973年 | 343篇 |
1972年 | 221篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
61.
62.
K. R. Subramanian R. Ramesh M. S. Sundara Rajan Ch. V. Sastry 《Astrophysics and Space Science》1996,243(1):259-262
Observations of the Sun at two frequencies (51 and 77 MHz) using the East-West arm of the Gauribidanur Radio heliograph are presented. 相似文献
63.
The emergence of artificial neural network (ANN) technology has provided many promising results in the field of hydrology and water resources simulation. However, one of the major criticisms of ANN hydrologic models is that they do not consider/explain the underlying physical processes in a watershed, resulting in them being labelled as black‐box models. This paper discusses a research study conducted in order to examine whether or not the physical processes in a watershed are inherent in a trained ANN rainfall‐runoff model. The investigation is based on analysing definite statistical measures of strength of relationship between the disintegrated hidden neuron responses of an ANN model and its input variables, as well as various deterministic components of a conceptual rainfall‐runoff model. The approach is illustrated by presenting a case study for the Kentucky River watershed. The results suggest that the distributed structure of the ANN is able to capture certain physical behaviour of the rainfall‐runoff process. The results demonstrate that the hidden neurons in the ANN rainfall‐runoff model approximate various components of the hydrologic system, such as infiltration, base flow, and delayed and quick surface flow, etc., and represent the rising limb and different portions of the falling limb of a flow hydrograph. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
64.
65.
The Dergaon (H5) chondrite: Fall,classification, petrological and chemical characteristics,cosmogenic effects,and noble gas records 总被引:1,自引:0,他引:1
P. N. SHUKLA A. D. SHUKLA V. K. RAI S. V. S. MURTY N. BHANDARI J. N. GOSWAMI A. C. MAZUMDAR P. PHUKON K. DUORAH R. E. GREENWOOD I. A. FRANCHI 《Meteoritics & planetary science》2005,40(4):627-637
Abstract— A multiple fall of a stony meteorite occurred near the town of Dergaon in Assam, India, on March 2, 2001. Several fragments weighing <2 kg and a single large fragment weighing ~10 kg were recovered from the strewn field, which extended over several tens of square kilometers. Chemical, petrographic, and oxygen isotopic studies indicate it to be, in most aspects, a typical H5 chondrite, except the unusually low K content of ~340 ppm. A cosmic ray exposure of 9.7 Ma is inferred from the cosmogenic noble gas records. Activities of eleven cosmogenic radionuclides were measured. 26Al and 22Na activities as well as the 22Na/26Al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays. The low 60Co activity (<1 dpm/kg) is indicative of a small preatmospheric size of the meteorite. Cosmic ray heavy nuclei track densities in olivine grains range from ~106 cm?2 in samples from the largest fragment to approximately (4–9) × 105 cm?2 in one of the smaller fragments. The combined track, radionuclide, and noble gas data suggest a preatmospheric radius of ~20 cm for the Dergaon meteorite. 相似文献
66.
67.
Spectra of the central core and surrounding coma of Comet IRAS-Araki-Alcock (1983d) were obtained at 8–13 μm on 11 May and 2–4 μm on 12 May 1983. Spatially resolved measurements at 10 μm with a 4-arcsec beam showed that the central core was more than 100 times brighter than the inner coma only 8 arcsec away; for radially outflowing dust, the brightness ratio would be a factor of 8. The observations of the central core are consistent with direct detection of a nucleus having a radius of approximately 5 km. The temperature of the sunlit hemisphere was > 300 K. Spectra of the core are featureless, while spectra of the coma suggest weak silicate emission. The spectra show no evidence for icy grains. The dust producton rate on 11.4 May was ~ 105 g/sec, assuming that the gas flux from the dust-producing areas on the nucleus was ~ 10?5 g/cm2/sec. 相似文献
68.
Agostino Renda Daisuke Kawata Yeshe Fenner Brad K. Gibson 《Monthly notices of the Royal Astronomical Society》2005,356(3):1071-1078
The chemical evolution history of a galaxy hides clues about how it formed and has been changing through time. We have studied the chemical evolution history of the Milky Way (MW) and Andromeda (M31) to find which are common features in the chemical evolution of disc galaxies as well as which are galaxy-dependent. We use a semi-analytic multizone chemical evolution model. Such models have succeeded in explaining the mean trends of the observed chemical properties in these two Local Group spiral galaxies with similar mass and morphology. Our results suggest that while the evolution of the MW and M31 shares general similarities, differences in the formation history are required to explain the observations in detail. In particular, we found that the observed higher metallicity in the M31 halo can be explained by either (i) a higher halo star formation efficiency (SFE), or (ii) a larger reservoir of infalling halo gas with a longer halo formation phase. These two different pictures would lead to (i) a higher [O/Fe] at low metallicities, or (ii) younger stellar populations in the M31 halo, respectively. Both pictures result in a more massive stellar halo in M31, which suggests a possible correlation between the halo metallicity and its stellar mass. 相似文献
69.
70.
Attila Mészáros 《Astrophysics and Space Science》1985,111(2):399-405
Graviton may, in principle, have a small non-zero mass. In this paper the relevant theory of the massive graviton with six polarisations is developed. The drastic impact of a non-zero mass of the graviton on cosmology is also illustrated. 相似文献