全文获取类型
收费全文 | 115034篇 |
免费 | 18825篇 |
国内免费 | 42543篇 |
专业分类
测绘学 | 4418篇 |
大气科学 | 26596篇 |
地球物理 | 27892篇 |
地质学 | 62352篇 |
海洋学 | 23394篇 |
天文学 | 21149篇 |
综合类 | 4118篇 |
自然地理 | 6483篇 |
出版年
2022年 | 839篇 |
2021年 | 1431篇 |
2020年 | 2764篇 |
2019年 | 6271篇 |
2018年 | 7737篇 |
2017年 | 7329篇 |
2016年 | 7722篇 |
2015年 | 5902篇 |
2014年 | 6472篇 |
2013年 | 9217篇 |
2012年 | 6909篇 |
2011年 | 7211篇 |
2010年 | 7151篇 |
2009年 | 7278篇 |
2008年 | 6124篇 |
2007年 | 6144篇 |
2006年 | 5512篇 |
2005年 | 4684篇 |
2004年 | 4973篇 |
2003年 | 4617篇 |
2002年 | 4298篇 |
2001年 | 3826篇 |
2000年 | 3341篇 |
1999年 | 3132篇 |
1998年 | 3205篇 |
1997年 | 3282篇 |
1996年 | 2642篇 |
1995年 | 2522篇 |
1994年 | 2378篇 |
1993年 | 2172篇 |
1992年 | 1982篇 |
1991年 | 1704篇 |
1990年 | 1795篇 |
1989年 | 1516篇 |
1988年 | 1387篇 |
1987年 | 1443篇 |
1986年 | 1217篇 |
1985年 | 1452篇 |
1984年 | 1540篇 |
1983年 | 1406篇 |
1982年 | 1408篇 |
1981年 | 1240篇 |
1980年 | 1117篇 |
1979年 | 1091篇 |
1978年 | 1019篇 |
1977年 | 900篇 |
1976年 | 851篇 |
1975年 | 769篇 |
1974年 | 839篇 |
1973年 | 854篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
901.
902.
903.
The historical development of analytical aerial triangulation block adjustment is mentioned by way of introduction. The advantages of separating the plan and height adjustments are given and the observation equations of a typical adjustment section are analysed to show that this convenient practice is justified in normal circumstances. The principles of a direct solution of the normal equations suitable for a moderate-sized computer, are explained and developed in detail for the height adjustment. The advantages of tie-strips in the height adjustment are demonstrated and the difficulties which then arise are discussed. An approximate method which overcomes these difficulties is developed. Details of the computer programmes used are given in Appendix B. 相似文献
904.
Barite has been found for the first time in the Swiss Jura (Hirni, Passwang). Small, white crystals of barite with yellow ankerite grow on calcite in different chambers of a Bajocian ammonite. Chemical data (by microprobe analysis) and lattice parameters (refined by leastsquares from powder patterns) are given for the two minerals. There is a distinct correlation between the cell dimensions and the chemical composition for both of them. The genesis is discussed. 相似文献
905.
This paper examines the effects of heavy metals on microorganisms in the aqueous environment; the mechanisms by which metals may exert toxic effects on microbes and the factors affecting microbial response to metals; the ways in which microbial activity may alter the metal balance of an environment and the modifications produced in microbes by heavy metal ions; the effects of the toxic copper ion on the growth, respiration, magnesium content, cytochrome synthesis and osmotic sensitivity of some organisms studied in the laboratory; and the feasibility of the participation of microbes in geochemical processes considering the demonstrable resistance to toxic metals by some bacteria and the fact that natural environments may contain high levels of metals rendered less toxic by binding to natural chelating compounds.
Zusammenfassung In dieser Arbeit werden verschiedene Wirkungen von Schwermetallen auf Mikroorganismen in wäßrigem Milieu untersucht. Es wird gezeigt, daß Schwermetalle toxische Wirkungen auf Mikroorganismen ausüben können und welche Faktoren die Reaktionen von Mikroben gegenüber Metallen bestimmen. Weiter wird beschrieben, wie die Mikrobenaktivität das Metallgleichgewicht in einem gegebenen Milieu verändern und beeinflussen kann und wie andererseits Änderungen in den Mikroben selbst durch Schwermetall-Ionen hervorgerufen werden. Dabei wird besonders die Wirkung des toxischen Kupfer-Ions auf das Wachstum, die Atmung, den Magnesiumgehalt, die Cytochromsynthese und die osmotische Empfindlichkeit einiger Mikroorganismen dargestellt. Die Anwesenheit von Mikroben bei geochemischen Prozessen wird unter Berücksichtigung der Resistenz einiger Bakterien gegenüber toxischen Metall-Ionen diskutiert. Dabei spielt die Tatsache eine große Rolle, daß durch Chelatbildung die Schwermetall-Ionen in der natürlichen Umwelt der Mikroorganismen ihre toxische Wirkung verlieren.相似文献
906.
In an earlier paper, values of exospheric density were obtained from the orbit of Echo 2 for the years 1964–1965. The results indicated a semi-annual variation in density by a factor of between 2 and 3, considerably larger than predicted by existing atmospheric models.
These studies have now been extended to the beginning of 1967, using both Echo 2 and Calsphere 1, to show how the density is responding to increasing solar activity. Variations in density during 1964 have been analysed in more detail. The long-term variation associated with the solar cycle and the short-term variations associated with magnetic and solar disturbances agree with the variations expected on the basis of current models. The semi-annual variation is persisting to higher levels of solar activity, and although its amplitude is diminishing the factor of variation was still 1.6 in 1966. 相似文献
907.
M. A. Anastassiades L. N. Carapiperis N. K. Kariambas P. G. Paraskevopoulos 《Pure and Applied Geophysics》1962,52(1):143-152
Summary In this study the different forms of the electromagnetic field strength recordings at the Thera-Crete microwave line of link are examined, in comparison with the corresponding weather situations prevailing over that area. According to the amplitude of fluctuations, the different forms of recordings have been classified in three basic typesA, B, C and the annual frequency of the appearance of these types has been observed. Finally, the influence of weather situations on radio wave propagation was examined and it has been found that in most cases a close relationship exists between weather situations and the different fading forms of the electromagnetic field so that a prediction of the fading form be possible by means of the weather forecast.This study was sponsored by a research grant from NATO's Scientific Committee. 相似文献
908.
Ohne Zusammenfassung 相似文献
909.
910.