全文获取类型
收费全文 | 253900篇 |
免费 | 4293篇 |
国内免费 | 3339篇 |
专业分类
测绘学 | 6759篇 |
大气科学 | 18772篇 |
地球物理 | 53160篇 |
地质学 | 87462篇 |
海洋学 | 20998篇 |
天文学 | 55657篇 |
综合类 | 983篇 |
自然地理 | 17741篇 |
出版年
2021年 | 2220篇 |
2020年 | 2566篇 |
2019年 | 2827篇 |
2018年 | 2984篇 |
2017年 | 2676篇 |
2016年 | 5407篇 |
2015年 | 4161篇 |
2014年 | 6870篇 |
2013年 | 14160篇 |
2012年 | 6247篇 |
2011年 | 7413篇 |
2010年 | 6453篇 |
2009年 | 9111篇 |
2008年 | 7983篇 |
2007年 | 7376篇 |
2006年 | 9589篇 |
2005年 | 7615篇 |
2004年 | 7511篇 |
2003年 | 7000篇 |
2002年 | 6645篇 |
2001年 | 5947篇 |
2000年 | 5906篇 |
1999年 | 5190篇 |
1998年 | 5221篇 |
1997年 | 5010篇 |
1996年 | 4661篇 |
1995年 | 4412篇 |
1994年 | 4087篇 |
1993年 | 3833篇 |
1992年 | 3616篇 |
1991年 | 3588篇 |
1990年 | 3755篇 |
1989年 | 3503篇 |
1988年 | 3294篇 |
1987年 | 3843篇 |
1986年 | 3403篇 |
1985年 | 4213篇 |
1984年 | 4727篇 |
1983年 | 4401篇 |
1982年 | 4308篇 |
1981年 | 3913篇 |
1980年 | 3634篇 |
1979年 | 3506篇 |
1978年 | 3479篇 |
1977年 | 3277篇 |
1976年 | 3044篇 |
1975年 | 2959篇 |
1974年 | 2917篇 |
1973年 | 3078篇 |
1972年 | 2026篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Simultaneous wind, wave, and current data during 21 storms spanning four winters at Tromsøflaket (230 m depth) were analysed to determine joint probabilities of occurrence. Waves were measured with a Waverider Buoy, winds with a recording anemometer onboard a vessel and currents at up to five depths with Aandera RCM-4 current meters. Measured currents were filtered to separate tidal currents from residual currents. In most of the statistical analyses, the actual current profiles were replaced with a simplified “equivalent” profile (constant above 50 m depth) whose magnitude was chosen to give the same drag load on a single pile as the actual profile when both profiles were combined with wave orbital velocities. The data suggest that the equivalent current is weakly correlated with the wave height. Within the range of the data analysed, an in-line equivalent residual current of 30 cm sec−1 is adequate for design drag force calculations.The data presented here are used to illustrate a procedure for data analysis and are not suitable for use as design criteria. 相似文献
992.
R. Von Herzen 《Applied Ocean Research》1985,7(3):175-176
993.
The results of the uptake of plutonium 239 from sediment by a marine polychaete wormNereis diversicolor is reported in the present paper. A comparison is made of the relative importance of two possible uptake pathways, sediment and seawater by which this worm may obtain its plutonium body-burden. Under the laboratory conditions used for the comparison of those two routes it would appear thatNereis obtains greater than 98% of its body activity from seawater. 相似文献
994.
995.
James N. Butler 《Marine Chemistry》1975,3(1):9-21
The rate of evaporative weathering of a component of crude oil residue is assumed to be proportional to its equilibrium vapor pressure and to the amount remaining in the sample. This model is tested using literature data on normal paraffin profiles of crude oil weathered artificially and gas chromatographic data on crude oil residues weathered on rocky shores. Semiquantitative agreement over the time range from 1 to 400 days is demonstrated. Application of the model to gas chromatograms of pelagic tar lumps collected at Station S near Bermuda in 1972 implies that most of these lumps were formed by fragmentation of much larger and older masses, some within less than one day of the time they were collected. The weathering of pelagic tar thus involves a great deal more physical fragmentation than was previously supposed. 相似文献
996.
997.
Ship hull drag reduction using bottom air injection 总被引:1,自引:0,他引:1
Robert Latorre Dr. 《Ocean Engineering》1997,24(2):161-175
The idea of bottom air injection to reduce ship hull resistance is not new. Early patents envisioned planing hull applications. Recent planing hull tests speed realized an increase of 7–12 knots. River barges and ship fitted with an air injection system results are presented to show a 10–15% reduction in the frictional resistance. Graphs for making initial estimates for displacement hulls with bottom air injection are presented. It is clear from these results that improvements in high speed planing catamarans and full form hull resistance can be realized by using bottom air injection. 相似文献
998.
A clamped square-plate is subjected to a uniform state of stress in the x-direction. The present study deals with the basic question of determining the variation of the lower natural frequencies of transverse vibration of a plate as a small diameter, central hole is made in it, introducing a stress concentration field. 相似文献
999.
R.E. Rossi P.M. Belles P.A.A. Laura S. La Malfa L. Ercoli D. Pasqua 《Ocean Engineering》1996,23(3):271-276
A numerical solution is obtained for the title problem by means of the popular finite element method. An experimental investigation of the problem is also presented for plates of square planform. The agreement between theoretical and experimental values is very good in the case of the fundamental frequency, and reasonably good when the second and third normal modes of transverse vibration are considered. It is concluded that beneficial effects are obtained in the sense that one generates a lighter structural element with higher fundamental frequency of transverse vibration. Hence, dynamic stiffening is achieved. 相似文献
1000.
Based on the MEGAPOLYGON data, it is shown that the large-scale elements of the thermohaline structure and the fields of currents in the baroclinic layer (down to 1200 m) are well coordinated. The circulation has a two-layer structure with a transitory layer at a depth of 1200 m. The topography of the isopycnic surface of the layer's core is formed under the influence of small-scale and mesoscale processes which realize the trans-frontal exchange.Translated by Mikhail M. Trufanov. 相似文献