首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   264755篇
  免费   5192篇
  国内免费   3363篇
测绘学   6977篇
大气科学   19522篇
地球物理   55322篇
地质学   92793篇
海洋学   22178篇
天文学   57378篇
综合类   1040篇
自然地理   18100篇
  2021年   2255篇
  2020年   2594篇
  2019年   2857篇
  2018年   4412篇
  2017年   4149篇
  2016年   6181篇
  2015年   4219篇
  2014年   6906篇
  2013年   14184篇
  2012年   6843篇
  2011年   8622篇
  2010年   7730篇
  2009年   10242篇
  2008年   8931篇
  2007年   8620篇
  2006年   9702篇
  2005年   7909篇
  2004年   7769篇
  2003年   7259篇
  2002年   6803篇
  2001年   6016篇
  2000年   5961篇
  1999年   5219篇
  1998年   5235篇
  1997年   5030篇
  1996年   4679篇
  1995年   4431篇
  1994年   4104篇
  1993年   3854篇
  1992年   3636篇
  1991年   3591篇
  1990年   3765篇
  1989年   3521篇
  1988年   3305篇
  1987年   3848篇
  1986年   3407篇
  1985年   4224篇
  1984年   4737篇
  1983年   4406篇
  1982年   4312篇
  1981年   3928篇
  1980年   3637篇
  1979年   3508篇
  1978年   3488篇
  1977年   3280篇
  1976年   3044篇
  1975年   2960篇
  1974年   2914篇
  1973年   3077篇
  1972年   2025篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
471.
Wintertime oxygen and pH profiles across the marginal ice zone of the central and southeastern Bering Sea shelf are analyzed and compared with summer data. During the winter, at water depths shallower than 75 m, the water column is homogeneous and near freezing. Between the 75- and 200-m isobaths the structure is essentially two-layered, a cool and fresh upper layer overlying a warmer, more saline bottom layer. The oxygen concentration in the surface mixed layer is higher than the summer values, but the degree of saturation is lower because of the lower temperature in winter. The oxygen degree of saturation in the bottom mixed layer on the shelf in winter are higher than in the surface water in winter and the bottom water in summer.In summer the oxygen and carbon dioxide data show extreme variability governed primarily by biological processes. Winter oxygen and pH data, however, do not scatter as much as the summer data and indicate conservative mixing of several sub-surface water masses. The surface water is undersaturated in both oxygen and carbon dioxide and seems to absorb oxygen, but little carbon dioxide, from the atmosphere.Two stations were occupied in the Aleutian Basin. The homogeneous surface layer has the same oxygen and pH values as in the minimum temperature layer observed in the summer by other investigators at the same location. The result substantiates the hypothesis of early investigators that the summer minimum temperature layer is the remnant local winter water. All winter surface waters sampled are undersaturated with respect to oxygen, suggesting that the input of oxygen through the air-sea exchange does not keep up with the rate of upwelling and cooling, which reduces the degree of oxygen saturation. Surface carbon dioxide is also undersaturated because of cooling. The maximum temperature layer at these two Aleutian Basin stations is warmer, fresher, and contains more oxygen, but less carbon dioxide, than in the summer, suggesting advective input of some nonlocal seawater.  相似文献   
472.
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   
473.
New bulk chemical analyses are given of Aubres, Bishopville, Bustee, Khor Temiki, Norton County, Peña Blanca Spring and Shallowater, Selective attack by dry chlorine (350°C) on magnetic and non-magnetic fractions was used to determine the distribution of some normally lithophile elements (Al, Ca, Cr, K, Mg, Mn, Na, P and Ti) between silicate and sulphide groups of minerals.  相似文献   
474.
475.
476.
Dynamic spectra of Jupiter's decametric emission often display narrow-band features, referred to as events of type N (Carr et al., 1983). The average bandwidth of these emissions is in the vicinity of 200 kHz, their durations are typically in the decasecond range, and their f-t slopes are small and random. Although the N-bursts can be described as narrow-band L-bursts, it seems that they are realted to S-bursts in their area of occurrence in the Io-B region, the durations of the emission envelopes, and their bandwidths. Possible implications are discussed.  相似文献   
477.
The RS CVn-type eclipsing binary star MM Her has been observed in two colours,B andV, in 1979, 1980, and 1983. Several minima times were obtained during the observations and new light elements calculated. The light curves of the system obtained in blue and yellow lights show a significant wave-like distortion which migrates towards the decreasing orbital phases. Its migration period was estimated to be about 3.5 yr. The amplitudes of the wave-like distortion inB andV appear to change each year. The primary minimum of the system is a total eclipse with a duration of 0d.08.  相似文献   
478.
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   
479.
480.
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   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号