全文获取类型
收费全文 | 258026篇 |
免费 | 5263篇 |
国内免费 | 3340篇 |
专业分类
测绘学 | 6903篇 |
大气科学 | 19141篇 |
地球物理 | 54069篇 |
地质学 | 89651篇 |
海洋学 | 21530篇 |
天文学 | 56240篇 |
综合类 | 1026篇 |
自然地理 | 18069篇 |
出版年
2021年 | 2226篇 |
2020年 | 2606篇 |
2019年 | 2854篇 |
2018年 | 3503篇 |
2017年 | 3268篇 |
2016年 | 5701篇 |
2015年 | 4221篇 |
2014年 | 6909篇 |
2013年 | 14241篇 |
2012年 | 6493篇 |
2011年 | 7855篇 |
2010年 | 6949篇 |
2009年 | 9517篇 |
2008年 | 8382篇 |
2007年 | 7869篇 |
2006年 | 9689篇 |
2005年 | 7725篇 |
2004年 | 7619篇 |
2003年 | 7134篇 |
2002年 | 6699篇 |
2001年 | 5984篇 |
2000年 | 5964篇 |
1999年 | 5210篇 |
1998年 | 5227篇 |
1997年 | 5029篇 |
1996年 | 4674篇 |
1995年 | 4426篇 |
1994年 | 4107篇 |
1993年 | 3839篇 |
1992年 | 3633篇 |
1991年 | 3599篇 |
1990年 | 3769篇 |
1989年 | 3519篇 |
1988年 | 3309篇 |
1987年 | 3845篇 |
1986年 | 3411篇 |
1985年 | 4226篇 |
1984年 | 4740篇 |
1983年 | 4412篇 |
1982年 | 4321篇 |
1981年 | 3931篇 |
1980年 | 3643篇 |
1979年 | 3513篇 |
1978年 | 3482篇 |
1977年 | 3285篇 |
1976年 | 3053篇 |
1975年 | 2962篇 |
1974年 | 2925篇 |
1973年 | 3080篇 |
1972年 | 2029篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
A.E. Lynas-Gray 《Astrophysics and Space Science》2004,291(3-4):197-203
Studies in extragalactic astronomy, galactic structure and the late stages of stellar evolution provide ample motivation for surveys of fields in the Galactic Halo. Apart from white dwarfs, blue stars had been regarded as luminous objects confined to star-forming regions in the Galactic Plane; finding them at high galactic latitudes attracted immediate interest, because their luminosities were intermediate between those of white dwarfs and blue Main Sequence stars. The study of blue stars away from the Galactic Plane was initiated by Greenstein; in due course effective temperatures (T e ff), surface gravities (log g) and abundances showed these stars form what appeared to be a blue extension of the known Horizontal Branch (HB) in the Hertzsprung–Russell Diagram. Extended Horizontal Branch (EHB) stars were identified with Extreme Horizontal Branch stars in globular clusters. It was realised that HB and EHB stars must have formed as a consequence of mass-loss on the Giant Branch, either at or before the helium flash. Mass-loss on the Giant Branch leading to the formation of EHB stars was considered more likely for stars in binary systems. The scene was then set for three decades of EHB star research. 相似文献
942.
A. P. M. Vaughan L. A. Dowling F. J. G. Mitchell S.‐E. Lauritzen A. M. McCabe P. Coxon 《第四纪科学杂志》2004,19(6):577-590
Organic‐rich deposits, uncovered during overburden removal from mantled gypsum karst at Knocknacran opencast gypsum mine, Co. Monaghan, are the best candidate to date for a last interglacial record in Ireland. The two till and organic‐rich deposits (preserved at different quarry elevations) were emplaced on to a Tertiary dolerite surface during high‐energy flood events and subsequently folded and faulted by movement towards sinkholes in underlying gypsum. Uranium–thorium disequilibrium dating suggests that the organic‐rich deposits in the upper section were hydrologically isolated at ca. 41 ka and those in the lower section at ca. 86 ka. Interpretation of the pollen content, although tentative because of the depositional and post‐depositional history of the material, suggests that the organic material originated in a warm stage possibly warmer than the post‐Eemian interstadials. The unusual setting of preservation may indicate that in situ, last interglacial deposits have generally been removed by erosion in Ireland. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
943.
944.
We discuss the difficulties encountered when the Heisenberg-Kolmogoroff model for turbulence is applied to the large-scale turbulence in: (A) molecular clouds (specifically the velocity vs size relationship) and (B) stars (specifically, the estimate of convective fluxes).A new model for large-scale turbulence is, therefore, needed. 相似文献
945.
Unsteady two-dimensional flow of a viscous, incompressible and electrically conducting fluid which is confined on one side of a semi-infinite wall, in presence of a transverse magnetic field is investigated. The wall is initially at rest and then it is suddenly accelerated in its own plane with a velocity which is an arbitrary function of the time. Solution of the problem is obtained, for two particular cases, i.e., corresponding to two free-stream velocities, with the help of the finite difference approximation technique using the explicit method for uniformly accelerated motion of the wall. 相似文献
946.
Seismic reflection profiles and long- and medium-range sidescan sonar were used to investigate a salt diapir complex and area of slope instability near the base of the Continental Slope off North Carolina. Within the area of investigation three diapirs are bounded on their upslope side by a scarp 60 m high and 50 km long. The slope above the scarp is characterized by a series of shallow rotational normal faults. The bottom below the scarp is furrowed by slide tracks, which were probably carved by large blocks that broke off the scarp face and slid downslope leaving rubble and scree lobes.Extensive slumping in this area appears to be a result of uplift and faulting associated with salt intrusion, which has fractured and oversteepened the slope leading to instability and failure. Sharply defined slide tracks suggest that slope failure above the breached diapir complex is a continuing process, in contrast to much of the surrounding slope area where few instability features were observed. 相似文献
947.
948.
We report on a year's study of spatial and seasonal patterns of zooplankton abundance in Port Phillip and Westernport Bays, July 1982 to August 1983. These two bays, closely adjacent on the southern coast of Victoria, Australia, differ in several respects: Port Phillip is a nearly landlocked bay with a broad basin, while Westernport is an open tidal embayment with extensive mud and seagrass banks. Both bays have a resident zooplankton fauna distinct from that of Bass Strait. Although these resident communities have many species in common, patterns of abundance and dominance are quite different. We found that the holoplankton of Port Phillip was about half copepods, mostly Paracalanus indicus, with 23% Caldocera and 21% larvaceans. Westernport Bay zooplankton was dominated by Acartia tranteri, with no resident cladoceran fauna. Bass Strait species were more often found in Westernport than in Port Phillip Bay, but the resident community of Port Phillip Bay was more similar to that of Bass Strait than to that of Westernport.Although this study was undertaken in an exceptionally dry year, the available historical data show that the overall patterns found in 1982–1983 are typical for these bays. The differences in community composition probably relate to differences in depth profile, predator abundance, and suspended matter between the bays. 相似文献
949.
The problem of collapse of a dust ball has been studied in detail from the point of view of an external observer (O). It is seen that although there is no material pressure from the point of view of the comoving observer, there is a non-vanishing material pressure in the ball from the point of view of O. In the early stages of collapse (w.r.t. O), this pressure is positive everywhere inside the ball, while during the later stages, if one goes outwards from the centre towards the surface of the ball, this pressure is negative up to a certain value of the radial coordinate, and then changes its sign thereafter. On the basis of this pressure, one can understand the physics of the whole scenario of collapse w.r.t. O; in particular, the important feature is that in the ultimate stages an event horizon is formed asymptotically and there is no collapse to a point (as takes place from the point of view of the comoving observer). 相似文献
950.
Al. A. Suchkov 《Astrophysics》1985,23(1):452-458