全文获取类型
收费全文 | 109015篇 |
免费 | 2002篇 |
国内免费 | 1694篇 |
专业分类
测绘学 | 3543篇 |
大气科学 | 9045篇 |
地球物理 | 22506篇 |
地质学 | 39450篇 |
海洋学 | 8292篇 |
天文学 | 20467篇 |
综合类 | 2316篇 |
自然地理 | 7092篇 |
出版年
2020年 | 596篇 |
2019年 | 631篇 |
2018年 | 5723篇 |
2017年 | 5019篇 |
2016年 | 4259篇 |
2015年 | 1531篇 |
2014年 | 1975篇 |
2013年 | 4266篇 |
2012年 | 2917篇 |
2011年 | 5388篇 |
2010年 | 4382篇 |
2009年 | 5781篇 |
2008年 | 5054篇 |
2007年 | 5098篇 |
2006年 | 3162篇 |
2005年 | 2896篇 |
2004年 | 3145篇 |
2003年 | 3005篇 |
2002年 | 2770篇 |
2001年 | 2296篇 |
2000年 | 2298篇 |
1999年 | 1946篇 |
1998年 | 1930篇 |
1997年 | 1882篇 |
1996年 | 1657篇 |
1995年 | 1549篇 |
1994年 | 1414篇 |
1993年 | 1302篇 |
1992年 | 1199篇 |
1991年 | 1078篇 |
1990年 | 1251篇 |
1989年 | 1075篇 |
1988年 | 1011篇 |
1987年 | 1210篇 |
1986年 | 1080篇 |
1985年 | 1326篇 |
1984年 | 1565篇 |
1983年 | 1499篇 |
1982年 | 1357篇 |
1981年 | 1325篇 |
1980年 | 1166篇 |
1979年 | 1104篇 |
1978年 | 1190篇 |
1977年 | 1074篇 |
1976年 | 1002篇 |
1975年 | 947篇 |
1974年 | 1004篇 |
1973年 | 991篇 |
1972年 | 622篇 |
1971年 | 555篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A numerical analysis of cyclotron instabilities is carried out by computing the dispersion relation for a three component cold plasma-beam system. Rates of growth and damping for various values of the stream density are calculated from the dispersion relation. The rates of growth and damping increase monotonically as the number density of the proton stream increases. It is found that the frequencies at the rates of maximum growth and the damping decrease slightly to lower frequencies and a sharp peak at these frequencies becomes blunt. The minimum e-folding times of an ion cyclotron wave for (a) σs = 10−4, σi = 10−2 and (b) σs = 10−1, σi = 10−2 are about 3·84 and 0·16 sec respectively in the vicinity of the equatorial plane at 6 Re, where σs and σi are the ratios of the beam density Ns and the helium ion (H6+) density Ni to the total positive ions in the plasma-beam system. 相似文献
992.
The petrology of the East Otago Volcanic Province (late Miocene),including the Dunedin Complex, is dominated by undersaturatedsodic lavas and shallow intmsives whose compositions range fromalkali basalt, basanite, and nephelinite to trachyte and phonolite.A range of relatively potassic volcanic rocks is also developedin the Province. The degree of undersaturation and ratios suchas (FeO+Fe2O3):MgO and K2O:Na2O vary widely in rocks with comparabledifferentiation indices. Chemical and mineralogical data onwhole rock-glass pairs and variants of shallow intrusives inthe Province, assessed in conjunction with similar data fromother volcanic provinces, demonstrate the production of trachyticliquids from alkali basaltic parents and of phonolitic liquidsfrom basanitic parents. Fractionation trends in both sodic andpotassic series, defined by variation in differentiation indexand normative nepheline, indicate that the degree of undersaturationof the derivative salic liquids is dependent on that of theparent; the slope of the fractionation trend lines is influencedby additional factors, including Po2. The unequivocal productionof phonolite from trachyte in the East Otago Province has notbeen demonstrated. The mafic variants of theralites from Waihola and a basanite-pegmatoidassociation from Omimi are similar in composition, but the mostsalic differentiates from these differentiation sequences displaypronounced differences in their KO:Na2O ratios, tending towardsleuco-theralitic (cf. lugarite) and malignitic compositionsrespectively. A multicomponent spectrum of fractionation lineages is suggestedfor individual differentiated bodies and for lava series. Sodiclineages include: (I) alkali basalthawaiitemugearitebenmoreitetrachyte;(2) a more undersaturated series from basanite through nephelinehawaiite, nepheline mugearite, and nepheline benmoreite, tophonolite; (3) a nephelinite series, more restricted in compositionalrange. The relatively high Fe: Mg ratios characterizing themugearitic variants are sometimes exhibited by members of themore potassic series in which, however, Fe:Mg ratios tend tobe characteristically lower. This feature is correlated withthe frequent occurrence of kaersutitic amphibole or its resorptionproducts, indicative of intratelluric fractionation under relativelyhydrous conditions and probably relatively high Po2. Certainnepheline trachyandesites from East Otago are described in detail.Trachy-basalt-trachyandesite-tristanite-trachyte and sanidinebasanite-nepheline trachyandesite-nepheline tristanite-phonolitelineages are also proposed, and the nomenclature of these moreseries discussed. 相似文献
993.
H. G. Dawe 《The Photogrammetric Record》1969,6(34):335-347
This paper describes the planning and implementation of a very extensive aerial survey contract undertaken by Hunting Surveys on behalf of the Hong Kong Government. From photography carried out during January and February 1963, photogrammetric plotting at 1 : 600 and 1 : 1200 proceeded steadily over a period of 5 years to the point where all but the highest land in the Colony had been surveyed at one or other of these two large scales. The problems arising from the rather unique terrain conditions and the urban and industrial development of Hong Kong are dealt with mainly from the photographic and photogrammetric aspect but reference is made to the contribution of the Crown Lands and Survey Office of the Public Works Department of Hong Kong, who provided all ground control and undertook field verification. 相似文献
994.
995.
J. W. Norman 《The Photogrammetric Record》1968,6(32):133-149
The geologist's use of air photographs and the characteristics studied by photogeologists are briefly described. The influence of photographic factors such as scale, season, time of day, film type, processing, overlap and dimensional accuracy are considered from the geologist's viewpoint. 相似文献
996.
Photogrammetric methods using sequential aerial photography can provide the geomorphologist with comparative measurements of rapidly changing glacial landforms. Examples are given of eskers, kame and kettle areas, an ice-dammed lake and coastal features in south-east Iceland. 相似文献
997.
998.
999.
Frequency distributions of true interfacial angles in high-grade,regional metamorphic rocks from Broken Hill indicate that interfacialtension varies with relative crystallographic orientation incommon minerals. In some single-phase aggregates (e.g. quartz,feldspar, garnet, and calcite) the grains are equant and polygonal,anisotropy of interfacial tension having only a small effecton the appearance of such aggregates. This is also true of two-phaseaggregates of these phases, but their appearance is complicatedby the presence of inclusions, the relative proportions of ihephases, and the characteristic dihedral angle formed by a grainof one phase in contact with two grains of the other. In phases for which interfacial tension is more orientation-dependent,crystal forms exert some control on the appearance of the microstructure.For example, in hornblende and pyroxene aggregates, some interfacesare parallel to planes of the form {110}, although most areirrational. In still more anisotropic phases, low-energy forms(e.g. {001} of biotite and {110} of sillimanite) are very stable,giving rise to a predominance of planar, rational boundariesover curved, adjustment boundaries. Anisotropy of interfacial tension is expressed in the shapesof inclusions. However, even where strongly anisotropic mineralsare involved, inclusions with partly planar boundaries occuralongside inclusions with completely curved boundaries. 相似文献
1000.