全文获取类型
收费全文 | 42024篇 |
免费 | 752篇 |
国内免费 | 367篇 |
专业分类
测绘学 | 1289篇 |
大气科学 | 3111篇 |
地球物理 | 8575篇 |
地质学 | 13903篇 |
海洋学 | 3658篇 |
天文学 | 10274篇 |
综合类 | 115篇 |
自然地理 | 2218篇 |
出版年
2021年 | 333篇 |
2020年 | 373篇 |
2019年 | 457篇 |
2018年 | 984篇 |
2017年 | 917篇 |
2016年 | 1192篇 |
2015年 | 677篇 |
2014年 | 1127篇 |
2013年 | 2031篇 |
2012年 | 1237篇 |
2011年 | 1631篇 |
2010年 | 1494篇 |
2009年 | 2041篇 |
2008年 | 1799篇 |
2007年 | 1825篇 |
2006年 | 1709篇 |
2005年 | 1288篇 |
2004年 | 1317篇 |
2003年 | 1192篇 |
2002年 | 1235篇 |
2001年 | 1089篇 |
2000年 | 1028篇 |
1999年 | 898篇 |
1998年 | 897篇 |
1997年 | 891篇 |
1996年 | 716篇 |
1995年 | 670篇 |
1994年 | 628篇 |
1993年 | 544篇 |
1992年 | 478篇 |
1991年 | 490篇 |
1990年 | 467篇 |
1989年 | 485篇 |
1988年 | 437篇 |
1987年 | 503篇 |
1986年 | 457篇 |
1985年 | 538篇 |
1984年 | 660篇 |
1983年 | 572篇 |
1982年 | 559篇 |
1981年 | 510篇 |
1980年 | 441篇 |
1979年 | 435篇 |
1978年 | 446篇 |
1977年 | 381篇 |
1976年 | 342篇 |
1975年 | 349篇 |
1974年 | 324篇 |
1973年 | 362篇 |
1972年 | 260篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
The equivalent width calculations for the fundamental vibration rotation band lines of HeH+ have been carried out for a non-DA white dwarf model with an effective temperature of 12 000 K. BothP andR branch lines with rotational quantum numbersJ=3 to 18 were included in the calculations. A search for these lines in helium rich white dwarfs is suggested. 相似文献
172.
C. S. Crawford A. C. Fabian P. Gandhi R. J. Wilman R. M. Johnstone 《Monthly notices of the Royal Astronomical Society》2001,324(2):427-442
We present observations of a sample of optically faint, hard X-ray sources of the kind likely to be responsible for much of the hard X-ray background. We confirm that such sources are easily detected in the near-infrared, and find that they have a featureless continuum suggesting that the active nucleus is heavily obscured. The infrared colours of the majority of the targets observed are consistent with absorbed elliptical host galaxies at z =1–2. It is likely that we are observing some of the brighter members of the important new class of X-ray type II quasars. 相似文献
173.
The cumulative probability distributions for stream order, stream length, contributing area, and energy dissipation per unit length of channel are derived, for an ordered drainage system, from Horton's laws of network composition. It is shown how these distributions can be related to the fractal nature of single rivers and river networks. Finally, it is shown that the structure proposed here for these probability distributions is able to fit the observed frequency distributions, and their deviations from straight lines in a log-log plot. 相似文献
174.
A dominant source of errors in swath bathymetry is acoustic interference. In 1989 the author published an analysis of these errors and predicted depth accuracies for a system which reduced their effect by averaging. This present paper shows how a considerable improvement in performance may be obtained by a variety of signal processing strategies that include the use of several widely spaced receivers and the elimination of the most unsatisfactory measurements before averaging. Simulations show how impressive sea bed profiles can be produced with a single ping, even at low signal-to-interference ratios 相似文献
175.
176.
177.
A.W.R. Bevan R. Hutchison A.J. Easton G.P. Durant C.M. Farrow 《Meteoritics & planetary science》1985,20(3):491-501
High Possil and Strathmore are the first (1804) and last (1917) meteorite falls, respectively, of the three recorded in Scotland. Olivine compositions and total Fe contents in High Possil (Fa25.2; 21.35 wt %) and Strathmore (Fa25.3; 20.6 wt %) confirm their classification as L-group chondrites (Mason, 1963), and the presence of abundant plagioclase feldspar shows that both chondrites belong to petrologic type 6. Both chondrites display thermal and mechanical alteration attributable to moderate shock-loading appropriate to facies c (High Possil) and c-d (Strathmore) (Dodd and Jarosewich, 1979). Incipient shock-melting of metal and troilite in both chondrites is the first described from Lc chondrites, and differences in the responses of metallic and silicate minerals to shock-loading are discussed. 相似文献
178.
Observations of the thermospheric wind at a mid-latitude station have been made using a Fabry-Perot interferometer to measure the Doppler shift of the nighttime OI emission at 630 nm. The results from 12 summer nights show that the zonal wind has a distinct feature associated with magnetic activity. The zonal wind first reverses and becomes westward. The maximum strength of the westward wind, its duration, and the maximum strength of the subsequent eastward wind all increase with increasing magnetic activity. The meridional wind is less consistent in its behaviour. It is normally equatorward but during magnetic activity it can increase, decrease, or even reverse, although it is consistently equatorward and of increased strength after 02.00 L.T. The initial reversal of the zonal wind is consistent with changes in the wind expected as a result of convective electric fields penetrating to mid-latitudes indicating that these electric fields modify the mid-latitude wind pattern before effects due to auroral heating reach mid-latitudes. The reversal of the zonal wind back to eastward may also be the result of electric field effects. The large variability of the meridional wind, to the extent that it becomes poleward at times, indicates the importance of wind sources equatorward of the observatory. 相似文献
180.
P. K. Seidelmann 《Celestial Mechanics and Dynamical Astronomy》1985,37(3):199-207
The present fundamental reference system is defined by the FK5 Fundamental Reference Catalog and the associated IAU Resolutions of 1976 through 1982. The past is defined to be what preceded the present fundamental reference system and future is defined to be what ever will come after it. The resulting characteristics of the different fundamental reference systems are considered from the aspects of the observational data, the computational methods, and the underlying theories. Having developed an understanding of these operational fundamental reference systems, the desired properties of the ideal fundamental reference system are presented. Finally, some questions concerning the future fundamental reference system are presented. 相似文献