全文获取类型
收费全文 | 8676篇 |
免费 | 345篇 |
国内免费 | 105篇 |
专业分类
测绘学 | 207篇 |
大气科学 | 593篇 |
地球物理 | 2013篇 |
地质学 | 3155篇 |
海洋学 | 805篇 |
天文学 | 1300篇 |
综合类 | 19篇 |
自然地理 | 1034篇 |
出版年
2021年 | 88篇 |
2020年 | 113篇 |
2019年 | 117篇 |
2018年 | 172篇 |
2017年 | 169篇 |
2016年 | 237篇 |
2015年 | 200篇 |
2014年 | 207篇 |
2013年 | 491篇 |
2012年 | 263篇 |
2011年 | 326篇 |
2010年 | 296篇 |
2009年 | 372篇 |
2008年 | 346篇 |
2007年 | 300篇 |
2006年 | 333篇 |
2005年 | 249篇 |
2004年 | 313篇 |
2003年 | 277篇 |
2002年 | 278篇 |
2001年 | 185篇 |
2000年 | 180篇 |
1999年 | 151篇 |
1998年 | 146篇 |
1997年 | 107篇 |
1996年 | 128篇 |
1995年 | 122篇 |
1994年 | 141篇 |
1993年 | 115篇 |
1992年 | 116篇 |
1991年 | 115篇 |
1990年 | 107篇 |
1989年 | 91篇 |
1988年 | 94篇 |
1987年 | 131篇 |
1986年 | 108篇 |
1985年 | 171篇 |
1984年 | 193篇 |
1983年 | 155篇 |
1982年 | 137篇 |
1981年 | 141篇 |
1980年 | 113篇 |
1979年 | 130篇 |
1978年 | 123篇 |
1977年 | 105篇 |
1976年 | 101篇 |
1975年 | 96篇 |
1974年 | 69篇 |
1973年 | 89篇 |
1972年 | 49篇 |
排序方式: 共有9126条查询结果,搜索用时 15 毫秒
41.
42.
In an attempt to discover planetary eclipses, we observed the open cluster NGC 7086. On one night we observed the cluster and standard stars through the B and V filters, enabling us to make a colour-magnitude diagram. Plots of the differential magnitudes were inspected for variability, but no planetary eclipses were found. New variable stars were discovered and their light curves show some of them to be eclipsing binary stars. The location on the colour-magnitude diagram of some of the variable stars is consistent with cluster membership. 相似文献
43.
A high-velocity ionized outflow and XUV photosphere in the narrow emission line quasar PG1211+143 总被引:1,自引:0,他引:1
K. A. Pounds J. N. Reeves A. R. King K. L. Page P. T. O'Brien M. J. L. Turner 《Monthly notices of the Royal Astronomical Society》2003,345(3):705-713
We report on the analysis of a ∼60-ks XMM–Newton observation of the bright, narrow emission line quasar PG1211+143. Absorption lines are seen in both European Photon Imaging Camera and Reflection Grating Spectrometer spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionized outflow velocity of ∼24 000 km s−1 . The highest energy lines require a column density of N H ∼ 5 × 1023 cm−2 , at an ionization parameter of log ξ∼ 3.4 . If the origin of this high-velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius of ∼130 Schwarzschild radii, providing a natural explanation for the big blue bump (and strong soft X-ray) emission in PG1211+143. 相似文献
44.
Lance B. Morrissey Philippe Janvier Simon J. Braddy John P. Bennett Susan B. Marriott Peter R. Tarrant 《Geology Today》2006,22(2):66-67
Armoured jawless fish, or 'ostracoderms', lived 450–360 million years ago, and display unusual morphologies, unlike any modern fish group. Since they left no living descendants, their mode of swimming has, until recently, remained speculative, although this is a crucial question as the first true pectoral fins evolved within the 'ostracoderms'. The discovery of the oldest-known fish trails, from the Early Devonian (400 million year old) 'Lower Old Red Sandstone' of south-east Wales offers new insights into the swimming behaviour of these early fish, notably the osteostracan 'ostracoderms' (or cephalaspids), whose horseshoe-shaped head and paddle-shaped pectoral fins have remained a functional riddle. 相似文献
45.
46.
John Logan 《Ground water》1986,24(3):382-382
Abstract. Execution time of many programs can be markedly shortened by (a) defining frequently used constants in assignment statements or placing them in arrays and by (b) avoiding or minimizing the use of exponentials. 相似文献
47.
48.
Variables related to urban park awareness are identified and methods for relaxing assumptions of perfect information in park use models are discussed. Park awareness is related to park characteristics (age and degree of development of the park), population characteristics (race, age, length of residence, recreation participation), and distance. Park attributes are stronger predictors of both park awareness and use than is distance. These findings parallel similar research on the cognitive aspects of shopping decisions. 相似文献
49.
50.
Subaqueous pyroclastic flows and ignimbrites: an assessment 总被引:2,自引:0,他引:2
An assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary, hot gas-rich pyroclastic flows, mass-flows which resulted from the transformation of gassupported flows into water-supported ones, and secondary mass-flows carrying redeposited pyroclastic debris. Based on subaerial pyroclastic flows, the term pyroclastic flow should be restricted to demonstrably hot, gas-rich mass-flows of pyroclastic debris. Using this definition, very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described. In the majority of these cases, the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate. The only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the Ordovician nearshore, shallow marine ignimbrites of Ireland and Wales, and Miocene ignimbrites of southwest Japan, resulting from the passage of subaerially erupted pyroclastic flows into shallow water. Other possible examples are near-vent dense clast deposits in the Donzurobo Formation of Japan, possible submarine intra-caldera ponded ignimbrite successions in California and Wales, and near-vent pumiceous deposits of Ramsay Island, Wales. All other purported cases are either clearly the result of water-supported mass-flow transportation and deposition (debris avalanches, debris flows, turbidity currents), or lack adequate supporting evidence regarding the heat state or the palaeoenvironment. Only the shallow marine ignimbrites of Ireland and Wales show adequate evidence of welding, but even these could have been nearly wholly exposed above sea-level when welding occurred. We conclude that when pyroclastic flows enter water they are generally disrupted explosively and/or ingest water and transform into water-supported mass-flows, and we suggest the various scenarios in which this occurs. There is no evidence to suggest that welding in wholly subaqueous environments is common. 相似文献