全文获取类型
收费全文 | 96655篇 |
免费 | 1952篇 |
国内免费 | 761篇 |
专业分类
测绘学 | 2459篇 |
大气科学 | 7411篇 |
地球物理 | 19802篇 |
地质学 | 32423篇 |
海洋学 | 8706篇 |
天文学 | 21654篇 |
综合类 | 199篇 |
自然地理 | 6714篇 |
出版年
2021年 | 705篇 |
2020年 | 840篇 |
2019年 | 895篇 |
2018年 | 1747篇 |
2017年 | 1651篇 |
2016年 | 2266篇 |
2015年 | 1485篇 |
2014年 | 2232篇 |
2013年 | 4831篇 |
2012年 | 2421篇 |
2011年 | 3523篇 |
2010年 | 3127篇 |
2009年 | 4377篇 |
2008年 | 3873篇 |
2007年 | 3692篇 |
2006年 | 3577篇 |
2005年 | 3044篇 |
2004年 | 3096篇 |
2003年 | 2926篇 |
2002年 | 2825篇 |
2001年 | 2477篇 |
2000年 | 2400篇 |
1999年 | 2048篇 |
1998年 | 2074篇 |
1997年 | 2036篇 |
1996年 | 1764篇 |
1995年 | 1660篇 |
1994年 | 1511篇 |
1993年 | 1385篇 |
1992年 | 1315篇 |
1991年 | 1178篇 |
1990年 | 1359篇 |
1989年 | 1207篇 |
1988年 | 1104篇 |
1987年 | 1303篇 |
1986年 | 1185篇 |
1985年 | 1449篇 |
1984年 | 1642篇 |
1983年 | 1531篇 |
1982年 | 1413篇 |
1981年 | 1401篇 |
1980年 | 1181篇 |
1979年 | 1157篇 |
1978年 | 1169篇 |
1977年 | 1108篇 |
1976年 | 1016篇 |
1975年 | 971篇 |
1974年 | 966篇 |
1973年 | 982篇 |
1972年 | 605篇 |
排序方式: 共有10000条查询结果,搜索用时 890 毫秒
921.
The motion and the drift force of a floating OWC (oscillating water column) wave energy device in regular waves are studied taking account of the oscillating surface-pressure due to the pressure drop across the duct of the air chamber. The potential problem inside the chamber is formulated by making use of the Green integral equation associated with the Rankine-type Green function while the outer problem with the Kelvin-type Green function. The added mass, wave damping and excitation coefficients as well as the motion and drift force of the OWC device are calculated for various values of parameter related to the pressure drop. 相似文献
922.
A sonic well log was obtained within the basement complex of the Walvis Ridge during Deep Sea Drilling Project Leg 74. The top of the basement complex is characterized by smooth acoustic reflectors. The rocks recovered within the basement complex consist of basalts with intercalated sediments. According to the log ~-50% of the upper 75 m of basement are igneous rocks and the other 50% sedimentary. Sonobuoy results indicate that the ratio of sediments to basalt increases with depth for an additional 225 m until a typical oceanic velocity structure is observed. Paleontological results suggest that the processes forming this upper 300 m of the basement complex was accomplished within a short time interval. 相似文献
923.
M.J. Tucker 《Ocean Engineering》1982,9(3):259-270
The vertical response of spar buoys to waves is examined. The response is the product of a rather complex wave forcing function and the resonant response of the buoy. With compound spar buoys (that is, consisting of more than one section of different diameters) the forcing function has a zero at a frequency which is usually close to the resonant frequency, but which can be arranged to be somewhat higher with beneficial effects on the response. Only the effects of pressure and inertia are considered, since this paper is mainly concerned to clarify some specific general principles and it is difficult to include the effects of drag, which are complex. 相似文献
924.
Surface currents influenced by a wind-driven upwelling event in San Pedro Bay moved total suspended matter (TSM) confined
to the inner shelf on 19 April 1978 seaward, so that by 27 April surface TSM had increased over the outer shelf. Near-bottom
concentrations of TSM also increased across the shelf during this time. This is explained by sediment resuspended by large
surface waves being advected from the inner shelf seaward by the mean flow after this flow had turned from southeasterly to
southerly when upwelling ceased on 26 April. These complex shelf dynamics contribute to the off-shelf transport of mud to
the slope and deep basins. 相似文献
925.
Investigations of short term acute exposure of bay scallops, Argopecten irradians and two scallop predators, the oyster drill Urosalpinx cinerea and the common starfish Asterias forbesi, to oil, dispersant, and oil-dispersant mixtures (Kuwait Crude Oil and Corexit 9527) suggested that predator and prey have different lethal susceptibilities. Scallops were most sensitive to dispersant and dispersant mixed with oil, starfish were only sensitive to dispersant while the oyster drill seemed unaffected even though all were exposed to dilutions of identically prepared stock solutions. Scallops were least susceptible during winter months and most susceptible at summer temperatures. Treatment had less effect on predators than on scallops at summer temperatures. Sublethal concentrations of dispersant and oil-dispersant mixtures diminished the behavioural ability of scallops to recognise drills and starfish. The degree of effect increased with temperature. Predator detection of prey at the same concentrations was more complex. The feeding response or posturing reflex of starfish was significantly slowed by all treatments. In contrast, drills were unaffected in their recognition of scallop effluent in a choice chamber after treatment. 相似文献
926.
Design improvements on swim fins are most difficult, if not impossible, to assess quantitatively by swimmers alone. In order to make reliable measurements of swim fin performance, an instrumented mechanical swimming device was developed and operated in a water tank. Using this facility, the performance of the more popular conventional fins and that of a new design concept were evaluated. The comparative performance of the various fins was correlated with their physical characteristics. 相似文献
927.
928.
The various assumptions implicit in the calculation of acid dissociation constants (based on ionic medium standard states) from potentiometric titrations using a cell with liquid junction (i.e. a pH measuring cell) have been examined. It was concluded that results can be obtained having an accuracy commensurate with the experimental precision. It has been shown that although the precise composition of the medium is a function of the hydrogen ion concentration (because of the protolytic nature of some of the ions in the media, e.g., sulphate and fluoride), the effect of such variations in the medium composition can be compensated for when defining the activity of hydrogen ion on an ionic medium standard state by defining the concentration of hydrogen ion as: where βHSO4 and βHF are the relevant association constants and ST and FT are the total concentrations of sulphate and fluoride, respectively.This approach was used to obtain values for the ionic product of water (KW) in artificial seawater media at various temperatures and ionic strengths. These were fitted to give the equation (molal concentration units): where I is the formal ionic strength of the artificial seawater medium and T is the absolute temperature. The values obtained are in reasonable agreement with those found by previous workers. 相似文献
929.
B.P. Mensink C.C. ten Hallers-Tjabbes J. Kralt I.L. Freriks J.P. Boon 《Marine environmental research》1996,41(4):315-325
Several cruises were carried out on the Eastern Scheldt between September 1992 and March 1995 to study the frequency and appearance of imposex in the common whelk, Buccinum undatum L. Incidences of imposex were always > 90%. For the first time several stages of imposex in Buccinum undatum are reported. The evolution of imposex in the whelk seems to follow a similar route as in Hinia reticulata belonging to the same superfamily (Buccinacea). Tissue organotin concentrations of animals collected in February 1995 showed the highest organotin concentrations in the digestive gland and lowest in the foot of the animals. Concentrations of phenyltin compounds (up to 625 ngSn/g DW) were much higher than those of butyltin compounds (up to 40 ngSn/g DW). 相似文献
930.