全文获取类型
收费全文 | 4960篇 |
免费 | 205篇 |
国内免费 | 49篇 |
专业分类
测绘学 | 99篇 |
大气科学 | 403篇 |
地球物理 | 1267篇 |
地质学 | 1675篇 |
海洋学 | 468篇 |
天文学 | 775篇 |
综合类 | 18篇 |
自然地理 | 509篇 |
出版年
2022年 | 25篇 |
2021年 | 66篇 |
2020年 | 65篇 |
2019年 | 71篇 |
2018年 | 97篇 |
2017年 | 84篇 |
2016年 | 133篇 |
2015年 | 138篇 |
2014年 | 133篇 |
2013年 | 234篇 |
2012年 | 156篇 |
2011年 | 240篇 |
2010年 | 174篇 |
2009年 | 251篇 |
2008年 | 216篇 |
2007年 | 196篇 |
2006年 | 208篇 |
2005年 | 184篇 |
2004年 | 166篇 |
2003年 | 149篇 |
2002年 | 153篇 |
2001年 | 77篇 |
2000年 | 99篇 |
1999年 | 83篇 |
1998年 | 85篇 |
1997年 | 63篇 |
1996年 | 64篇 |
1995年 | 87篇 |
1994年 | 83篇 |
1993年 | 62篇 |
1992年 | 66篇 |
1991年 | 51篇 |
1990年 | 70篇 |
1989年 | 61篇 |
1988年 | 63篇 |
1987年 | 65篇 |
1986年 | 60篇 |
1985年 | 70篇 |
1984年 | 92篇 |
1983年 | 71篇 |
1982年 | 72篇 |
1981年 | 59篇 |
1980年 | 68篇 |
1979年 | 55篇 |
1978年 | 56篇 |
1977年 | 41篇 |
1976年 | 50篇 |
1975年 | 52篇 |
1974年 | 39篇 |
1973年 | 51篇 |
排序方式: 共有5214条查询结果,搜索用时 140 毫秒
21.
James Binney 《Journal of Astrophysics and Astronomy》1996,17(3-4):81-93
22.
23.
Jeremy Allington-Smith Graham Murray Robert Content George Dodsworth Bryan W. Miller James Turner Inger Jorgensen Isobel Hook 《Experimental Astronomy》2002,13(1):1-37
We describe the integral field unit (IFU) which converts the Gemini Multiobject Spectrograph (GMOS) installed on the Gemini-North telescope to an integral field spectrograph,which produces spectra over a contiguous field of view of 7 × 5 arcsec with spatial sampling of 0.2 arcsecover the wavelength range 0.4-1.0 μm.GMOS is converted to this mode by the remote insertion of the IFU into thebeam in place of the masks used for the multiobject mode. A separate fieldof half the area of the main field, but otherwise identical, is alsoprovided to improve background subtraction. The IFU contains 1500lenslet-coupled fibres and was the first facility of any type for integralfield spectroscopy employed on an 8/10 m telescope.We describe the design, construction and testing of the GMOS IFU and present measurements of the throughput both in the laboratory and at the telescope. We compare these with a theoretical prediction made before construction started. All are in good agreement with each other, with the on-telescope throughput exceeding 60% (averaged over wavelength). Finallywe show an example of data obtained during commissioning to illustrate the power of the device. 相似文献
24.
25.
26.
27.
28.
In recent years, high‐molecular‐weight anionic polyacrylamides (PAMs) have been tested on a variety of soils, primarily in temperate climates. However, little information is available regarding the effectiveness of PAM for preventing soil loss through runoff in tropical settings. Screening tests were performed using three negatively charged PAMs and one positively charged PAM on five Hawaii soils (two Oxisols, one Vertisol, and two Aridisols) to determine erosion loss, sediment settling, and aggregate stability. A laboratory‐scale rainfall simulator was used to apply erosive rainfall at intensities from 5 to 8·5 cm h?1 at various PAM doses applied in both dry and solution forms. Soil detachment due to splash and runoff, as well as the runoff and percolate water volumes, were measured for initial and successive storms. The impact of PAM on particle settling and aggregate stability was also evaluated for selected soil‐treatment combinations. Among the PAMs, Superfloc A‐836 was most effective, and significantly reduced runoff and splash sediment loss for the Wahiawa Oxisol and Pakini Andisol at rates varying between 10 and 50 kg ha?1. Reduced runoff and splash sediment loss were also noted for PAM Aerotil‐D when applied in solution form to the Wahiawa Oxisol. Significant reductions in soil loss were not noted for either the Lualualei Vertisol or the Holomua Oxisol. It is believed that the high montmorillonite content of the Lualualei Vertisol and the low cation‐exchange capacity of the Holomua Oxisol diminished the effectiveness of the various PAMs tested. The polymers were also found to enhance sediment settling of all soils and helped improve their aggregate stability. This screening study shows the potential use of PAM for tropical soils for applications such as infiltration enhancement, runoff reduction, and enhanced sedimentation of detention ponds. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
29.
The Mangala Valles system is an ∼
∼900 km fluvially carved channel system located southwest of the Tharsis rise and is unique among the martian outflow channels
in that it heads at a linear fracture within the crust as opposed to a collapsed region of chaos as is the case with the circum-Chryse
channels. Mangala Valles is confined within a broad, north–south trending depression, and begins as a single valley measuring
up to 350 km wide that extends northward from a Memnonia Fossae graben, across the southern highlands toward the northern
lowlands. Approximately 600 km downstream, this single valley branches into multiple channels, which ultimately lose their
expression at the dichotomy boundary. Previous investigations of Mangala Vallis suggested that many of the units mapped interior
to the valley were depositional, related to flooding, and that a minimum of two distinct periods of flooding separated by
tens to hundreds of millions of years were required to explain the observed geology. We use infrared and visible images from
the THermal EMission Imaging System (THEMIS), and topographic data from the Mars Orbiting Laser Altimeter (MOLA), to investigate
the nature of the units mapped within Mangala Vallis. We find that the geomorphology of the units, as well as their topographic
and geographic distribution, are consistent with most of them originating from a single assemblage of volcanic flow deposits,
once continuous with volcanic flows to the south of the Memnonia Fossae source graben. These flows resurfaced the broad, north–south
trending depression into which Mangala Vallis formed prior to any fluvial activity. Later flooding scoured and eroded this
volcanic assemblage north of the Mangala source graben, resulting in the present distribution of the units within Mangala
Vallis. Additionally, our observations suggest that a single period of catastrophic flooding, rather than multiple periods
separated by tens to hundreds of millions of years, is consistent with and can plausibly explain the interior geology of Mangala
Vallis. Further, we present a new scenario for the source and delivery of water to the Mangala source graben that models flow
of groundwater through a sub-cryosphere aquifer and up a fracture that cracks the cryosphere and taps this aquifer. The results
of our model indicate that the source graben, locally enlarged to a trough near the head region of Mangala, would have required
less than several days to fill up prior to any spill-over of water to the north. Through estimates of the volume of material
missing from Mangala (13,000–20,000 km3), and calculation of mean discharge rates through the channel system (∼
∼5 × 106 m3 s−1), we estimate that the total duration of fluvial activity through the Mangala Valles was 1–3 months. 相似文献
30.
On the practice of estimating fractal dimension 总被引:11,自引:0,他引:11
Coastlines epitomize deterministic fractals and fractal (Hausdorff-Besicovitch) dimensions; a divider [compass] method can be used to calculate fractal dimensions for these features. Noise models are used to develop another notion of fractals, a stochastic one. Spectral and variogram methods are used to estimate fractal dimensions for stochastic fractals. When estimating fractal dimension, the objective of the analysis must be consistent with the method chosen for fractal dimension calculation. Spectal and variogram methods yield fractal dimensions which indicate the similarity of the feature under study to noise (e.g., Brownian noise). A divider measurement method yields a fractal dimension which is a measure of complexity of shape. 相似文献