全文获取类型
收费全文 | 37331篇 |
免费 | 295篇 |
国内免费 | 216篇 |
专业分类
测绘学 | 739篇 |
大气科学 | 1730篇 |
地球物理 | 7080篇 |
地质学 | 14714篇 |
海洋学 | 3530篇 |
天文学 | 8845篇 |
综合类 | 102篇 |
自然地理 | 1102篇 |
出版年
2022年 | 488篇 |
2021年 | 711篇 |
2020年 | 756篇 |
2019年 | 828篇 |
2018年 | 1702篇 |
2017年 | 1551篇 |
2016年 | 1645篇 |
2015年 | 620篇 |
2014年 | 1423篇 |
2013年 | 2051篇 |
2012年 | 1624篇 |
2011年 | 1908篇 |
2010年 | 1795篇 |
2009年 | 2002篇 |
2008年 | 1769篇 |
2007年 | 1983篇 |
2006年 | 1745篇 |
2005年 | 903篇 |
2004年 | 819篇 |
2003年 | 815篇 |
2002年 | 747篇 |
2001年 | 767篇 |
2000年 | 638篇 |
1999年 | 423篇 |
1998年 | 443篇 |
1997年 | 476篇 |
1996年 | 342篇 |
1995年 | 359篇 |
1994年 | 341篇 |
1993年 | 290篇 |
1992年 | 286篇 |
1991年 | 299篇 |
1990年 | 340篇 |
1989年 | 274篇 |
1988年 | 260篇 |
1987年 | 260篇 |
1986年 | 192篇 |
1985年 | 303篇 |
1984年 | 306篇 |
1983年 | 296篇 |
1982年 | 276篇 |
1981年 | 250篇 |
1980年 | 269篇 |
1979年 | 203篇 |
1978年 | 243篇 |
1977年 | 205篇 |
1976年 | 177篇 |
1975年 | 184篇 |
1974年 | 171篇 |
1973年 | 206篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Using the basic Boussinesq's equation, the expression for the vertical stress distribution (σz) underneath any point on the ground surface due to a general triangular loaded region in a preferred orientation with a linearly varied loading has been successfully derived. When the triangle is not in a preferred orientation, a simple axis transformation is required and the expression will be equally applicable. Based on this expression, σz due to an arbitrarily shaped loaded foundation can simply be determined by first triangulating the loaded area and summing up the contributions from each generated triangular region. The procedures for triangulating and calculating the stress distribution can be simply automated through computer programs. 相似文献
73.
V.V. Orlov A.V. Petrova V.G. Tarantaev 《Monthly notices of the Royal Astronomical Society》2001,325(1):133-137
The intrinsic, three-dimensional shapes of small galaxy groups, containing between three and eight members, are evaluated using three different statistics: (i) the mean sum of square sines of angles in all possible triangles formed by members of the group; (ii) the variance of square paired separations in the group; (iii) the axial ratio of a rectangle containing the group. The mean values of these parameters and their rms deviations are calculated for observed galaxy groups and simulated groups with members that are distributed randomly within prolate or oblate spheroids. Comparison of observational data and simulations shows that the observed galaxy groups have shapes consistent with the projected shapes of prolate or oblate spheroids with axial ratios of 3:1, regardless of their multiplicity, but inconsistent with the projected shapes of spherical objects. 相似文献
74.
A very long series of photographic observations of the comet Hale-Bopp has been made during January–April 1997 at the double
astrograph (400/2000) of the Main Astronomical Observatory (Kyiv, Ukraine). Some of the cometary photos were obtained with
two wide-band filter combinations. One of these combinations isolates C2 emission, another — the nearby dust continuum. The images were digitized by means of AMDPH-XY machine and then calibrated
following the standard procedure. After subtraction of the dust continuum the distribution of surface brightness in the C2 emission coma of comet Hale-Bopp was studied. We found an asymmetric brightness distribution both pre- and post-perihelion.
On 21.77 April 1997 a secondary brightness peak is found at the distance of 1.03 × 105 km from the nucleus. It is possible that this peak is related to the extended source of the C2 molecules.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
75.
The distribution of perihelion distances of nearly periodic comets was studied using different assumptions about comet discovery probabilities. It was shown that the concentration of perihelia at small heliocentric distances cannot be explained solely by the influence of visibility conditions. 相似文献
76.
77.
G. Cevolani G. Bortolotti L. Foschini C. Franceschi G. Grassi G. Trivellone A. Hajduk V. Porubčan 《Earth, Moon, and Planets》1995,68(1-3):247-255
Continuous radio-wave monitoring of the Geminid activity in December 1992 and 1993 by using a forward scatter (FS) bistatic radar over the Bologna-Lecce baseline (700 km) in Italy, reveals peculiar structural aspects of the stream in terms of signal amplitude-rate and duration-rate dependence. The observational results of the Geminid display obtained in the two consecutive years with differentiated peak levels of transmitted power, exhibit different time distributions of underdense meteors against the signal received power. Both sets of the data relative to the peak activity in December 12–14, show reflection properties of Geminids which are atypical if compared with echoes from cometary-type showers, with really high echo counts at mid-upper levels of the peak received power. An assymmetric curve of activity of the Geminids complex is evidenced, with the peak flux of smaller particles occurring earlier than that of larger ones. 相似文献
78.
R. Vílchez-Gómex G. A. Bruzual R. Pelló R. Domínguez 《Astrophysics and Space Science》2001,276(2-4):757-764
We have developed model predictions for the morphological distribution of cluster galaxies as a function of the cluster-centric
distance and the local galaxy density, using a semi-analytical code. This code allows us to obtain magnitudes and colours
for cluster galaxies at different redshifts, and thus to study in detail the evolution of the colour–magnitude relation of
specific distant clusters.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
79.
Theoretical line ratios involving 2s
2
S - 3p
2
P, 2p
2
P - 3s
2
S, and 2p
2
S - 3d
2
D transitions inCiv between 312 and 420 Å are presented. A comparison of these with solar active region observational data obtained during a rocket flight by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals good agreement between theory and experiment, with discrepancies that average only 22%. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations, and also resolves discrepancies found previously when the theoretical results were compared with solar data from the S082A instrument on boardSkylab. The potential usefulness of theCIV line ratios as electron temperature diagnostics for the solar transition region is briefly discussed. 相似文献
80.
C. Debi Prasad Ashok Ambastha Nandita Srivastava Sushanta C. Tripathy Mona J. Hagyard 《Journal of Astrophysics and Astronomy》1997,18(1):39-55
Super-active region NOAA 6555 was highly flare productive during the period March 21st–27th, 1991 of its disk passage. We
have st udied its chromospheric activity using high spatial resolution Hα filtergrams taken at Udaipur along with MSFC vector
magnetograms. A possible relationship of flare productivity and the variation in shear has been explored. Flares were generally
seen in those subareas of the active region which possessed closed magnetic field configuration, whereas only minor flares
and/or surges occurred in subareas showing open magnetic field configuration. Physical mechanisms responsible for the observed
surges are also discussed. 相似文献