全文获取类型
收费全文 | 75960篇 |
免费 | 1327篇 |
国内免费 | 912篇 |
专业分类
测绘学 | 1956篇 |
大气科学 | 5700篇 |
地球物理 | 14875篇 |
地质学 | 27551篇 |
海洋学 | 6835篇 |
天文学 | 17134篇 |
综合类 | 176篇 |
自然地理 | 3972篇 |
出版年
2021年 | 717篇 |
2020年 | 824篇 |
2019年 | 949篇 |
2018年 | 1949篇 |
2017年 | 1866篇 |
2016年 | 2256篇 |
2015年 | 1251篇 |
2014年 | 2128篇 |
2013年 | 3903篇 |
2012年 | 2351篇 |
2011年 | 3043篇 |
2010年 | 2730篇 |
2009年 | 3508篇 |
2008年 | 3079篇 |
2007年 | 3028篇 |
2006年 | 2969篇 |
2005年 | 2114篇 |
2004年 | 2119篇 |
2003年 | 2023篇 |
2002年 | 1989篇 |
2001年 | 1722篇 |
2000年 | 1745篇 |
1999年 | 1466篇 |
1998年 | 1449篇 |
1997年 | 1430篇 |
1996年 | 1208篇 |
1995年 | 1161篇 |
1994年 | 1129篇 |
1993年 | 921篇 |
1992年 | 926篇 |
1991年 | 854篇 |
1990年 | 959篇 |
1989年 | 816篇 |
1988年 | 793篇 |
1987年 | 941篇 |
1986年 | 812篇 |
1985年 | 994篇 |
1984年 | 1071篇 |
1983年 | 1047篇 |
1982年 | 992篇 |
1981年 | 872篇 |
1980年 | 872篇 |
1979年 | 787篇 |
1978年 | 804篇 |
1977年 | 732篇 |
1976年 | 663篇 |
1975年 | 681篇 |
1974年 | 691篇 |
1973年 | 679篇 |
1972年 | 447篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
An estimate for the anisotropy of the turbulent viscositys is given in a convective layer heated from below and rotating around a vertical axis. In the case of two-dimensional convection,
there is a stationary regime withs⊇2 regardless of the rotation. In the case of three-dimensional convection in a slowly rotating layer (with the Taylor number
equal to 1600), nonstationary turbulent regimes take place withs⊇1.6 forR=2.5×104 (R is the Rayleigh number) ands⊇1.2 forR=104.
The parameters plays an, important role in the theory of differential rotation of the convective solar or stellar envelopes. So far, it
has been evaluated empirically or semi-empirically. Some prospects in the development of the theory of differential rotation
are discussed here in terms of the moment theory of hydrodynamic fields. The relation between this strict approach and an
anisotropic viscosity approximation is considered. 相似文献
212.
Vertical measurements of , and N2O concentrations, and uptake, and oxidation rates were measured at 5 sites in western Cook Strait, New Zealand, between 31 March and 3 April 1983. Nitrate increased with depth at all stations reaching a maximum of 10.5 μg-atom at the most strongly stratified station whereas was relatively constant with depth at all stations (~0.1 μg-atom ). The highest rates of oxidation generally occurred in the near surface waters and decreased with depth. N2O levels were near saturation with respect to the air above the sea surface and showed no obvious changes during 24 h incubation. oxidation by nitrifying bacteria may account for about 30% of the total utilization (i.e. bacterial+agal) and, assuming oxidation through to , may supply about 40% of the algal requirements of in the study area. These results suggest that bacterial nitrification is of potential importance to the nitrogen dynamics of the western Cook Strait, particularly with respect to the nitrogen demands of the phytoplankton. 相似文献
213.
214.
215.
In this paper, the currently accepted correlation of the Early Pleistocene Ludhamian stage of England with the Tiglian‐A sub‐stage of the Netherlands is challenged. Recent investigations of Early Pleistocene marine North Sea deposits from a borehole near Noordwijk (the Netherlands) yielded evidence from molluscs, dinoflagellate cysts and sporomorphs for an alternation of warm‐temperate and arctic intervals within the Praetiglian and Tiglian stages. Marine equivalents of the terrestrial‐based pollen sub‐stages Tiglian A and B have been recognised in the upper part of the sequence. A Praetiglian age can be assigned to the lower part of the sequence on the basis of mollusc analysis. Within the Praetiglian, an alternation of warm and cold phases has been recognised from both the dinoflagellate cyst and molluscan records. Three cold phases within the Praetiglian are tentatively correlated with marine isotope stages (MIS) 96–100. The molluscan assemblages provide evidence for climate forcing of the sea level: highest sea levels are reached in the warm‐temperate intervals. Within the Praetiglian, an interval with an acme zone of the dinoflagellate cyst Impagidinium multiplexum, is correlated with the Ludhamian and tentatively linked to MIS 97 and/or MIS 96. The cold molluscan assemblages from the Noordwijk borehole include an acme zone of Megayoldia thraciaeformis, the first and only occurrence of this North Pacific bivalve in the North Sea Basin. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
216.
Erosion of man-made, forestry drainage channels occurs when the plough cuts through the surface peat layer into the underlying erodible mineral soil. A procedure is developed, based on hydraulic considerations, which will allow the drainage engineer to design stable drainage networks in upland forestry plantations. An example design chart is given for an erodible sandy loam type soil. 相似文献
217.
Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ18O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the 16O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted 16O‐rich presolar dust that exchanged oxygen with 16O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with 16O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in 16O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated 16O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ~10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed. 相似文献
218.
219.
It was found recently that fast electrons travel through the plasma of the solar corona in the form of beam-plasma structure
(BPS), which consists of electrons and Langmuir waves. In this paper the influence of scattering BPS Langmuir waves off plasma
ions (l+i=l+i) on BPS velocity is studied. We show that the maximum BPS velocity equals 0.35c, which is close to the velocity of Type III bursts sources. 相似文献
220.
P. G. Jonker M. van der Klis C. Kouveliotou M. Méndez W. H. G. Lewin T. Belloni 《Monthly notices of the Royal Astronomical Society》2003,346(2):684-688
We have observed the eclipsing low-mass X-ray binary MS 1603.6+2600 with Chandra for 7 ks. The X-ray spectrum is well fit with a single absorbed power law with an index of ∼2. We find a clear sinusoidal modulation in the X-ray light curve with a period of 1.7 ± 0.2 h , consistent with the period of 1.85 h found before. However, no (partial) eclipses were found. We argue that if the X-ray flare observed in earlier X-ray observations was a type I X-ray burst, then the source can only be an accretion disc corona source at a distance of ∼11–24 kpc (implying a height above the Galactic disc of ∼8–17 kpc). It has also been proposed in the literature that MS 1603.76+2600 is a dipper at ∼75 kpc. We argue that, in this dipper scenario, the observed optical properties of MS 1603.6+2600 are difficult to reconcile with the optical properties one would expect on the basis of comparisons with other high-inclination, low-mass X-ray binaries, unless the X-ray flare was not a type I X-ray burst. In that case, the source can be a nearby soft X-ray transient accreting at a quiescent rate, as was proposed by Hakala et al., or a high-inclination source at ∼15–20 kpc. 相似文献