全文获取类型
收费全文 | 78682篇 |
免费 | 956篇 |
国内免费 | 674篇 |
专业分类
测绘学 | 2115篇 |
大气科学 | 5571篇 |
地球物理 | 14346篇 |
地质学 | 30619篇 |
海洋学 | 6730篇 |
天文学 | 17160篇 |
综合类 | 340篇 |
自然地理 | 3431篇 |
出版年
2022年 | 440篇 |
2021年 | 738篇 |
2020年 | 777篇 |
2019年 | 846篇 |
2018年 | 4066篇 |
2017年 | 3730篇 |
2016年 | 3315篇 |
2015年 | 1195篇 |
2014年 | 2049篇 |
2013年 | 3294篇 |
2012年 | 2938篇 |
2011年 | 4833篇 |
2010年 | 4272篇 |
2009年 | 5053篇 |
2008年 | 4216篇 |
2007年 | 4796篇 |
2006年 | 2571篇 |
2005年 | 2207篇 |
2004年 | 2161篇 |
2003年 | 2098篇 |
2002年 | 1908篇 |
2001年 | 1504篇 |
2000年 | 1441篇 |
1999年 | 1123篇 |
1998年 | 1175篇 |
1997年 | 1071篇 |
1996年 | 904篇 |
1995年 | 884篇 |
1994年 | 772篇 |
1993年 | 683篇 |
1992年 | 649篇 |
1991年 | 669篇 |
1990年 | 706篇 |
1989年 | 553篇 |
1988年 | 570篇 |
1987年 | 608篇 |
1986年 | 553篇 |
1985年 | 697篇 |
1984年 | 756篇 |
1983年 | 679篇 |
1982年 | 637篇 |
1981年 | 586篇 |
1980年 | 569篇 |
1979年 | 556篇 |
1978年 | 537篇 |
1977年 | 437篇 |
1976年 | 405篇 |
1975年 | 410篇 |
1974年 | 371篇 |
1973年 | 398篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
A mechanically-actuated locking-compass was designed for mounting on corers to record equipment orientation during the sampling process, thereby providing magnetic orientation data on the recovered deep-sea core. Locking occurs upon impact by the corer with the sea-floor; after retrieval, the device is easily unlocked and reset for another station. Compass construction makes use of commercially available parts, requires no welding and little machining, and thus allows inexpensive and easy assembly at sea. 相似文献
942.
943.
944.
945.
In inviscid shallow water models currents generated by spatially uniform winds are transient and are eliminated by Rossby and equatorial Kelvin waves excited at the eastern and western coasts, respectively. The inclusion of mixing processes can lead to an entirely different steady state in which currents are present. It is shown that the mixing of heat affects the steady state more than does the mixing of momentum because the waves that are important in the oceanic adjustment are divergent. 相似文献
946.
Estimation of the leeway drift of small craft 总被引:1,自引:0,他引:1
Small craft (<6·4 m) leeway is determined as a function of the wind speed in the range of 5–20 knots (3·6–10·3 m/sec). Leeway is calculated relative to the surface current by measurement of the separation distance of the small craft from a dyed patch of surface water at sea, using time-sequenced aerial photography. Leeway increases linearly with wind speed for small craft equipped with or without a sea anchor in the wind range studied. Leeway for small craft without sea anchor can be calculated from the equation UL = 0.07 UW + 0.04 where UW is the wind speed at 2 m elevation. Leeway for small craft drifted off the be calculated from the equation ULD = 0·05 UW − 0·12. The small craft drifted off the downwind direction in about 80% of the experiments. The drift angle is variable and difficult to predict. 相似文献
947.
T.J. Hirst M. Perlow Jr. A.F. Richards B.S. Burton W.J. van Sciver 《Ocean Engineering》1975,3(1):17-27
The first-generation University of Illinois gamma-ray transmission densitometer, designed for the in situ measurement of sediment bulk density, was modified by incorporating in the detector probe (1) an Americium-241 alpha particle pulser and an anti-walk gain stabilization control to maintain better temperature stability and (2) a small power supply and a IC preamplifier to eliminate the need for a high-voltage coaxial cable between the detector and external signal conditioning electronics package. This second-generation Lehigh University system has been successfully deployed since 1971 in routine use from ships and submersibles in the Atlantic and Pacific Oceans and the Gulf of Mexico. Results are presented of system operations to (1) measure bulk density over the range of 1.2–1.8 Mg/m3 in the Hudson Canyon, (2) penetrate 1.9 m into the seafloor in the San Diego trough and, (3) be lowered to a water depth of 3.6 km in the Gulf of Mexico. 相似文献
948.
949.
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
950.