全文获取类型
收费全文 | 72561篇 |
免费 | 976篇 |
国内免费 | 822篇 |
专业分类
测绘学 | 2042篇 |
大气科学 | 5116篇 |
地球物理 | 13701篇 |
地质学 | 25865篇 |
海洋学 | 6576篇 |
天文学 | 17221篇 |
综合类 | 229篇 |
自然地理 | 3609篇 |
出版年
2022年 | 428篇 |
2021年 | 772篇 |
2020年 | 828篇 |
2019年 | 908篇 |
2018年 | 2007篇 |
2017年 | 1916篇 |
2016年 | 2380篇 |
2015年 | 1290篇 |
2014年 | 2242篇 |
2013年 | 3814篇 |
2012年 | 2412篇 |
2011年 | 3102篇 |
2010年 | 2775篇 |
2009年 | 3607篇 |
2008年 | 3077篇 |
2007年 | 3154篇 |
2006年 | 2971篇 |
2005年 | 2203篇 |
2004年 | 2235篇 |
2003年 | 2115篇 |
2002年 | 2004篇 |
2001年 | 1747篇 |
2000年 | 1693篇 |
1999年 | 1414篇 |
1998年 | 1453篇 |
1997年 | 1342篇 |
1996年 | 1123篇 |
1995年 | 1089篇 |
1994年 | 1012篇 |
1993年 | 850篇 |
1992年 | 838篇 |
1991年 | 785篇 |
1990年 | 879篇 |
1989年 | 715篇 |
1988年 | 708篇 |
1987年 | 753篇 |
1986年 | 674篇 |
1985年 | 870篇 |
1984年 | 956篇 |
1983年 | 852篇 |
1982年 | 829篇 |
1981年 | 705篇 |
1980年 | 692篇 |
1979年 | 658篇 |
1978年 | 636篇 |
1977年 | 551篇 |
1976年 | 495篇 |
1975年 | 521篇 |
1974年 | 463篇 |
1973年 | 490篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
The open cluster An King 13 (α(1950) = 00h07m5 δ (1950) = +60°56') was measured in the UBV-system. An King 13 contained 80 stars up to the apparent magnitude mv = 16.4 mag. The distance of the cluster is 1730 pc. The diameter of An King 13 is 7 pc. The luminocity function was determinated. 相似文献
912.
Elvis A. Shukla Jagdish Prasad M. S. S. Nagaraju Rajeev Srivastava D. L. Kauraw 《Journal of the Indian Society of Remote Sensing》2009,37(1):129-137
Soil resource characterization of Dhamni micro-watershed in Chandrapur district of Maharashtra was carried out using IRS-1D
LISS-III data in conjunction with field survey and ancillary data. The study indicates that nearly 84.2 per cent of the total
geographical area of the watershed is under cultivation. Forest (mainly degraded) occupy only 4.5 per cent area Whereas wasteland
with scrub cover 9.4 per cent area of the watershed. Nine soil series were tentatively identified and mapped as soil series
association in to five mapping units. These soils belong to order Inceptisol, Vertisol and Mollisol. Except the soils of wasteland
with scrub, other soils are moderately suitable for pigeonpea and soybean and have average to good productivity. 相似文献
913.
Nonnegative matrix factorization (NMF) is a recently developed linear unmixing technique that assumes that the original sources and transform were positively defined. Given that the linear mixing model (LMM) for hyperspectral data requires positive endmembers and abundances, with only minor modifications, NMF can be used to solve LMM. Traditionally, NMF solutions include an iterative process resulting in considerable execution times. In this letter, we provide two novel algorithms aimed at speeding the NMF through parallel processing: the first based on the traditional multiplicative solution and the second modifying an adaptive projected gradient technique known to provide better convergence. The algorithms' implementations were tested on various data sets; the results suggest that a significant speedup can be achieved without decrease in accuracy. This supports the further use of NMF for linear unmixing. 相似文献
914.
Aref M.O.AL-JABALI Abdo S.AL-MAQTARY Hussein AL-AKHALI Mohammed HAZAEA Fadel AL-AGHBARI 《东北亚地学研究》2009,(1)
According to topography of Yemen,most areas and villages are located at obligated crest,toe of mountain and under cliffs.Therefore Al-Huwayshah consisting of Tawilah sandstone group is characterized by steep slope reach to 90° in some areas.This area is affected by strong tectonic movements and faults that occurred during the geological epochs.This effect enhances to find out fractures and joints as well as the rocks become brittle and ready to slide depending on the position of area.And there are some frac... 相似文献
915.
916.
G. T. R. Droop 《Geological Journal》1988,23(4):339-339
917.
918.
We present and analyse the sunspot observations performed by Franz I.C. Hallaschka in 1814 and 1816. These solar observations were carried out during the so-called Dalton minimum, around the maximum phase of Solar Cycle 6. These records are very valuable because they allow us to complete observational gaps in the collection of sunspot group numbers, improving the coverage for this epoch. We have analysed and compared the observations made by Hallaschka with the records made by other contemporary observers. Unfortunately, the analysis of the sunspot areas and positions showed that they are too inaccurate for scientific use. We conclude, however, that the sunspot counts made by Hallaschka are similar to those made by other astronomers of that time. The observations by Hallaschka confirm a low level of solar activity during the Dalton minimum. 相似文献
919.
We investigate the behaviour of a charged isotropic model with conformal symmetry. The relationship between the gravitational potentials arising from the conformal condition is used to generate a new class of exact solutions to the Einstein-Maxwell equations. A specific form of the electric field intensity and the metric potential is required to avoid a singularity at the centre. We can find simple elementary functions for the matter variables and the potentials with realistic profiles. The causality conditions, stability conditions and energy conditions are satisfied. Masses, radii, central densities and surface redshifts are generated, and the values are consistent with the compact stars 4U 1538-52 and PSR J1614-2230. 相似文献
920.
N. C. Wickramasinghe Dayal T. Wickramasinghe Edward J. Steele 《Astrophysics and Space Science》2018,363(12):244
A growing body of evidence suggests the operation of life and life processes in comets as well in larger icy bodies in the solar system including Enceladus. Attempts to interpret such data without invoking active biology are beginning to look weak and flawed. The emerging new paradigm is that life is a cosmic phenomenon as proposed by Hoyle and Wickramasinghe (Lifecloud: the Origin of Life in the Galaxy, 1978) and first supported by astronomical spectroscopy (Wickramasinghe and Allen, Nature 287:518, 1980; Allen and Wickramasinghe, Nature 294:239, 1981; Wickramasinghe and Allen, Nature 323:44, 1986). Comets are the transporters and amplifiers of microbial life throughout the Universe and are also, according to this point of view, the carriers of viruses that contribute to the continued evolution of life. Comets brought life to Earth 4.2 billion years ago and they continue to do so. Space extrapolations of comets, Enceladus and possibly Pluto supports this point of view. Impacts of asteroids and comets on the Earth as well as on other planetary bodies leads to the ejection of life-bearing dust and rocks and a mixing of microbiota on a planetary scale and on an even wider galactic scale. It appears inevitable that the entire galaxy will be a single connected biosphere. 相似文献