全文获取类型
收费全文 | 39115篇 |
免费 | 580篇 |
国内免费 | 283篇 |
专业分类
测绘学 | 831篇 |
大气科学 | 2877篇 |
地球物理 | 7763篇 |
地质学 | 14517篇 |
海洋学 | 3324篇 |
天文学 | 8391篇 |
综合类 | 116篇 |
自然地理 | 2159篇 |
出版年
2022年 | 225篇 |
2021年 | 340篇 |
2020年 | 373篇 |
2019年 | 422篇 |
2018年 | 1278篇 |
2017年 | 1158篇 |
2016年 | 1107篇 |
2015年 | 675篇 |
2014年 | 1060篇 |
2013年 | 1784篇 |
2012年 | 1286篇 |
2011年 | 1609篇 |
2010年 | 1377篇 |
2009年 | 1789篇 |
2008年 | 1581篇 |
2007年 | 1571篇 |
2006年 | 1432篇 |
2005年 | 1563篇 |
2004年 | 1743篇 |
2003年 | 1455篇 |
2002年 | 1009篇 |
2001年 | 905篇 |
2000年 | 861篇 |
1999年 | 710篇 |
1998年 | 745篇 |
1997年 | 731篇 |
1996年 | 542篇 |
1995年 | 537篇 |
1994年 | 485篇 |
1993年 | 428篇 |
1992年 | 374篇 |
1991年 | 363篇 |
1990年 | 391篇 |
1989年 | 342篇 |
1988年 | 303篇 |
1987年 | 402篇 |
1986年 | 322篇 |
1985年 | 446篇 |
1984年 | 468篇 |
1983年 | 469篇 |
1982年 | 414篇 |
1981年 | 384篇 |
1980年 | 398篇 |
1979年 | 360篇 |
1978年 | 388篇 |
1977年 | 336篇 |
1976年 | 322篇 |
1975年 | 331篇 |
1974年 | 304篇 |
1973年 | 298篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
We study morphology and luminosity segregation of galaxies in groups. We analyze the two catalogs of (∼2×400) groups which
have been identified in the Nearby Optical Galaxy sample, by means of hierarchical and percolation `friends-of-friends' methods.
We find that earlier-type (brighter) galaxies are more clustered and lie closer to the group centers, both in position and
in velocity, than later-type (fainter) galaxies. Spatial segregations are stronger than kinematical segregations. These effects
are generally detected at the ≳3-sigma level, with the exception of morphological segregation in velocity, which is the weakest
effect. Our main results are confirmed by the analysis of statistically more reliable groups (with at least five members),
and are strengthened by the detection of segregation in both hierarchical and percolation catalogs. Luminosity segregation
is shown to be independent of morphology segregation. Our conclusions agree with a continuum of segregation properties of
galaxies in systems, from low-mass groups to massive clusters.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
62.
63.
64.
Of the impact craters on Earth larger than 20 km in diameter, 10-15% (3 out of 28) are doublets, having been formed by the simultaneous impact of two well-separated projectiles. The most likely scenario for their formation is the impact of well-separated binary asteroids. If a population of binary asteroids is capable of striking the Earth, it should also be able to hit the other terrestrial planets as well. Venus is a promising planet to search for doublet craters because its surface is young, erosion is nearly nonexistent, and its crater population is significantly larger than the Earth's. After a detailed investigation of single craters separated by less than 150 km and “multiple” craters having diameters greater than 10 km, we found that the proportion of doublet craters on Venus is at most 2.2%, significantly smaller than Earth's, although several nearly incontrovertible doublets were recognized. We believe this apparent deficit relative to the Earth's doublet population is a consequence of atmospheric screening of small projectiles on Venus rather than a real difference in the population of impacting bodies. We also examined “splotches,” circular radar reflectance features in the Magellan data. Projectiles that are too small to form craters probably formed these features. After a careful study of these patterns, we believe that the proportion of doublet splotches on Venus (14%) is comparable to the proportion of doublet craters found on Earth (10-15%). Thus, given the uncertainties of interpretation and the statistics of small numbers, it appears that the doublet crater population on Venus is consistent with that of the Earth. 相似文献
65.
66.
Piero F. Spinnato Michael Fellhauer Simon F. Portegies Zwart 《Monthly notices of the Royal Astronomical Society》2003,344(1):22-32
We study the efficiency at which a black hole or dense star cluster spirals in to the Galactic Centre. This process takes place on a dynamical friction time-scale, which depends on the value of the Coulomb logarithm (ln Λ). We determine the accurate value of this parameter using the direct N -body method, a tree algorithm and a particle-mesh technique with up to two million plus one particles. The three different techniques are in excellent agreement. Our measurement for the Coulomb logarithm appears to be independent of the number of particles. We conclude that ln Λ= 6.6 ± 0.6 for a massive point particle in the inner few parsec of the Galactic bulge. For an extended object, such as a dense star cluster, ln Λ is smaller, with a value of the logarithm argument Λ inversely proportional to the object size. 相似文献
67.
68.
69.
J. BERGER O. FEMENIAS J. C. C. MERCIER D. DEMAIFFE 《Journal of Metamorphic Geology》2005,23(9):795-812
The Limousin ophiolite is located at the suture zone between two major thrust sheets in the western French Massif Central. This ophiolitic section comprises mantle‐harzburgite, mantle‐dunite, wehrlites, troctolites and layered gabbros. It has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and/or pargasite‐bearing chloritites. With various thermobarometric methods, it is possible to show that the different varieties of amphibole have registered low‐P (c. 0.2 GPa) conditions with temperature ranging from high‐T, late‐magmatic conditions to greenschist–zeolite metamorphic facies. The abundance of undeformed metamorphic rocks (which is typical of the lower oceanic crust), the occurrence of Ca–Al (–Mg) metasomatism illustrated by the growth of Ca–Al silicates in veins or replacing the primary magmatic minerals, the P–T conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from Ocean Drilling Program and worldwide ophiolites are the main arguments for an ocean‐floor hydrothermal metamorphism in the vicinity of a palaeo‐ridge. Among the West‐European Variscan ophiolites, the Limousin ophiolites constitute an extremely rare occurrence that has not been involved in any HP (subduction‐related) or MP (orogenic) metamorphism as observed in other ophiolite occurrences (i.e. France, Spain and Germany). 相似文献
70.
Longitudinal and transverse dispersion in estuaries may be determined in principle by measuring the change in time of the concentration field of a water soluble dye. In practice, measurements may take the form of several sampling runs crossing the dye patch. The coefficients of dispersion are usually estimated from the second moments of the concentration field; the use of second moments, in particular, introduces errors because measurements at long times or distance are unduly emphasized.A self-consistent method, free of the disadvantages inherent in the use of higher moments, is described in which the distribution of concentration in space is related to the zeroth spatial moment of concentration. In this formulation, based upon sampling runs across the dye patch, the concentration field in the estuary may be directly and simply related to the dispersion coefficients in both the intercept and gradient of a linear graph.The methods are illustrated by the characterization of a series of experiments in the Inner Estuary of Milford Haven. 相似文献