全文获取类型
收费全文 | 79105篇 |
免费 | 1429篇 |
国内免费 | 1942篇 |
专业分类
测绘学 | 2627篇 |
大气科学 | 5705篇 |
地球物理 | 15572篇 |
地质学 | 31849篇 |
海洋学 | 6162篇 |
天文学 | 13916篇 |
综合类 | 2386篇 |
自然地理 | 4259篇 |
出版年
2022年 | 493篇 |
2021年 | 789篇 |
2020年 | 761篇 |
2019年 | 854篇 |
2018年 | 6227篇 |
2017年 | 5422篇 |
2016年 | 4197篇 |
2015年 | 1224篇 |
2014年 | 1716篇 |
2013年 | 2951篇 |
2012年 | 2993篇 |
2011年 | 5219篇 |
2010年 | 4278篇 |
2009年 | 5166篇 |
2008年 | 4314篇 |
2007年 | 4932篇 |
2006年 | 2624篇 |
2005年 | 1875篇 |
2004年 | 1975篇 |
2003年 | 1862篇 |
2002年 | 1698篇 |
2001年 | 1324篇 |
2000年 | 1216篇 |
1999年 | 955篇 |
1998年 | 1004篇 |
1997年 | 952篇 |
1996年 | 803篇 |
1995年 | 783篇 |
1994年 | 682篇 |
1993年 | 575篇 |
1992年 | 606篇 |
1991年 | 543篇 |
1990年 | 610篇 |
1989年 | 538篇 |
1988年 | 482篇 |
1987年 | 599篇 |
1986年 | 488篇 |
1985年 | 603篇 |
1984年 | 694篇 |
1983年 | 621篇 |
1982年 | 618篇 |
1981年 | 574篇 |
1980年 | 570篇 |
1979年 | 511篇 |
1978年 | 482篇 |
1977年 | 470篇 |
1976年 | 428篇 |
1975年 | 416篇 |
1974年 | 423篇 |
1973年 | 483篇 |
排序方式: 共有10000条查询结果,搜索用时 484 毫秒
171.
172.
Data from the Pulkovo spectrophotometric data base on the absolute quasimonochromatic fluxes from oCet in the 320–1080 nm range are used to determine the physical parameters of this star in different phases of its light curve. The continuum emission layer is found to expand between the phases of the cycle corresponding to the rising and falling branches of the light curve. The average expansion velocity is 32 km/s. By the time the star’s brightness has fallen by roughly three magnitudes, its radius has increased by almost a factor of three. Over this same time the temperature of the layer has fallen from 3000 K to 2200 K. For this expansion velocity, the calculated mass rate loss is ⊙ M /year.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 175–189 (May 2005). 相似文献
173.
174.
Magnetic stratigraphy observed in ferromanganese crust 总被引:1,自引:0,他引:1
Summary. A sample of ferromanganese crust deposit (SCHW-1D) was cut into two sets of 1 mm slices for palaeomagnetic study. Alternating field and thermal demagnetization, and isothermal remanent magnetization analyses were performed on each thin slice. The results reveal the presence of a stable magnetism and both normal and reversed polarity intervals in the specimens. The observed polarity intervals cannot be confidently correlated with the geomagnetic polarity time-scale of the last 10 Myr due to the polarity overlap inherent in the sampling technique. But the results confirm the slow accretion rate of ferromanganese deposit determined by 10 Be method and suggest potential use of ferromanganese deposits in palaeomagnetic and tectonic studies. 相似文献
175.
176.
Surface waters of Alsea Bay, an unpolluted estuary on the Oregon coast, were analysed for nitrous oxide, nitrate and nitrite on a weekly or biweekly basis during the summer of 1979. The estuary was found to be a variable source of N2O to the atmosphere. Large and rapid increases in the concentrations of N2O, NO3?, and NO2? occurred at the beginning of the sampling period and are attributed to the influx of nutrient-rich upwelling water into the estuary with the tide. The subsequent decline in concentrations of nitrate, nitrite and nitrous oxide over the remainder of the summer is attributed to a decrease in upwelling intensity, a decline in nitrification rates and to assimilatory nitrate reduction. Measurements of nitrous oxide at six stations along the Alsea River were also made in September and October before and after the onset of the rainy season. Samples taken after flood conditions were established were systematically 50% higher than pre-flood samples. The data suggest that soil runoff results in elevated concentrations of N2O in rivers. 相似文献
177.
178.
Summary. Exact spherical harmonic expansions are given for calculating the gravitational and magnetic fields associated with certain uniform solids of revolution. The figures are those made by rotating a conic section about one of its principal axes. The coefficients in the expansions can be computed accurately and efficiently and this approach leads to a very satisfactory method for calculating the fields of geological bodies with approximate circular symmetry about a vertical axis. A complete theory of convergence is given for the expansions. Somewhat unexpectedly, the sphere of convergence is determined by the location of a number of equivalent point or line sources that lie within the body or on its edges. 相似文献
179.
We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar
Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures
are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute
umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW
initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency
in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the
properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW
propagation velocity in different sunspots vary from 12″ to 30″ and from 28 to 60 – 70 km s−1 correspondingly. 相似文献