全文获取类型
收费全文 | 95286篇 |
免费 | 1050篇 |
国内免费 | 1398篇 |
专业分类
测绘学 | 2991篇 |
大气科学 | 6066篇 |
地球物理 | 18695篇 |
地质学 | 37058篇 |
海洋学 | 7417篇 |
天文学 | 18944篇 |
综合类 | 2316篇 |
自然地理 | 4247篇 |
出版年
2022年 | 603篇 |
2021年 | 924篇 |
2020年 | 1017篇 |
2019年 | 1079篇 |
2018年 | 6812篇 |
2017年 | 5939篇 |
2016年 | 4836篇 |
2015年 | 1343篇 |
2014年 | 2292篇 |
2013年 | 3764篇 |
2012年 | 3414篇 |
2011年 | 5688篇 |
2010年 | 4792篇 |
2009年 | 5776篇 |
2008年 | 4924篇 |
2007年 | 5497篇 |
2006年 | 3039篇 |
2005年 | 2145篇 |
2004年 | 2272篇 |
2003年 | 2184篇 |
2002年 | 1953篇 |
2001年 | 1656篇 |
2000年 | 1497篇 |
1999年 | 1160篇 |
1998年 | 1219篇 |
1997年 | 1190篇 |
1996年 | 970篇 |
1995年 | 986篇 |
1994年 | 889篇 |
1993年 | 770篇 |
1992年 | 734篇 |
1991年 | 742篇 |
1990年 | 838篇 |
1989年 | 723篇 |
1988年 | 661篇 |
1987年 | 794篇 |
1986年 | 636篇 |
1985年 | 862篇 |
1984年 | 941篇 |
1983年 | 883篇 |
1982年 | 805篇 |
1981年 | 827篇 |
1980年 | 724篇 |
1979年 | 643篇 |
1978年 | 679篇 |
1977年 | 600篇 |
1976年 | 568篇 |
1975年 | 571篇 |
1974年 | 540篇 |
1973年 | 606篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
73.
Observations of the brightness, color, and polarization of the fuor V1057 Cyg over more than 30 years are presented and briefly discussed. Variability of the linear polarization was found. 相似文献
74.
Today one of the main questions standing before geology of Venus is comprehension of tectonic style on surface of this planet. The observed ridge belts on the images of the radar surveyors of Venera-15 and -16 are related to one aspect of a solution of this problem. In general there are two views on the origin of the ridge belts on the planet. Some of the investigators consider them as a result of compression while others as extension. These discrepancies are connected with uncertainties of interpretations of the major structures of these belts, i.e., the ridges. This paper represents a review of evidence in favor of extension and compression observed within the ridge belts. Moreover it discusses the major speculative ideas and models of the ridge belts origin. The supporters of a ridge formation by extension suggest that these linear features are results of crust cracking and intruding of magma along these cracks so they propose that the ridge belts on the plains of Venus can represent analogies of the spreading zones on Earth. Other investigators assume that the ridge of belts represent structures of folding and or thrust faulting due to the tectonic compression environment so the ridge belts can be analogies of the orogenic belts and subduction zones on Earth. So the question of the ridge belts' origin remains controversial and for a solution, set one's hope on the Magellan mission.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena). 相似文献
75.
A new procedure to separate the instrumental and atmospheric components of stray light is presented. It is based on the dependence of the aureole's atmospheric component on the air mass and is applied to measurements taken with the Vacuum Newton Telescope (VNT) at the Observatorio del Teide (Tenerife). The resulting instrumental part is independent of the air mass.The variation of both components with wavelength is also studied. The instrumental component shows no dependence on wavelength, in contrast to the atmospheric one which is greater in the blue than in the red.It is concluded that observations with air masses larger than two will probably be strongly affected by stray light. 相似文献
76.
77.
An object that is partially insonified by a collimated sound beam may have a scattering cross section sometimes much larger than when the object is totally covered by the incident beam. We quantitatively study this partial insonification problem here, under the classical method of physical optics. The importance of this study stems from the fact that partial coverage of the target by the beam is the situation most likely to occur in many cases of practical importance. We consider several basic target shapes partially insonified by finite beams. These shapes include the spherical, the infinite and finite cylinder, the flat plate, and the capped sphere. High-frequency approximations of the resulting integrals, obtained by means of the saddle-point method, show the relative importance of the scattering centers located at the beam's specular reflection points, or at the edges of the spots that the beam shines on the scatterers. The physical-optics method is extended to obtain formulas for the bistatic cross sections of partially insonified objects. The results are numerically evaluated and graphically displayed in many pertinent instances and compared to the predictions of approaches, such as the Fresnel-zones method and the Geometrical Theory of Diffraction (GTD). The predictions of the physical-optics method have all the advantages and deficiencies of this method and, with very minor modifications, hold equally well for the partial illumination of objects by beams of electromagnetic radiation. 相似文献
78.
CHUDongsheng LIANGMeng SHIXin ZHANGLing 《中国海洋大学学报(英文版)》2004,3(1):106-110
1 Introduction Inrecentyearstherehasbeengrowingresearchin terestinthetwo dimensional (2 D )systemtheory .The 2 Dsystemsmayfindapplicationsinareassuchasmarineseismicdataprocessingandimageprocessing .Althoughmoreandmorevaluableresultshavebeengained ,mos… 相似文献
79.
G. A. Gurzadyan 《Astrophysics and Space Science》1992,193(1):105-107
The emission lines in the spectra of some planetary nebula experience variations with the time. These variations (due to evolution effects and by their nature) differ from the spectral changes which we usually observe in stars. It appears that the long-scale and systematic observations on the intensity behaviour of different emission lines of nebula may be used as an independent indicator to receive a principally new and quite unpredicted category of information related to the physics of nebulae and their nuclei. Particularly, the analysis of long standing observations carried out in relation to the forbidden lines 4363 [Oiii] andN
1+N
2 [Oiii] of double-ionized oxygen lead to a conception about the possible existence ofrelativistic electrons of moderate energy in planetary nebulae and the generation oftransition radiation as a result of electrodynamic interaction of these electrons with the dust particles in nebula (Gurzadyan, 1991). Just on the basis of the same observational material, the long-scale periodicity in the activity or variations of a star-like planetary nebula IC 4997 is discovered about which is the present note. 相似文献
80.
J. V. WALTHER 《Journal of Metamorphic Geology》1996,14(3):351-360
ABSTRACT With increasing temperature during prograde metamorphism reactions will occur first at the lithological contacts of mixed pelite and calcsilicate terranes. At these interfaces, a fluid of lower chemical potential of H2O and CO2 than that required to produce a fluid in either layer can be produced whether reaction is caused by fluid infiltration or is initially fluid absent. If the interface region does not allow fluid transport then as temperature increases, a fluid pressure greater than lithostatic can develop. At some degree of over-pressure relative to rock pressure, the fluid hydraulically fractures the rock and a gradient in fluid composition away from the contact can be produced. These phenomena occur at the compositional interfaces whenever univariant reactions in the differing layers cross on a temperature vs. mole fraction of CO2 diagram with slopes of opposite sign. The first occurrence of these reaction products at lithological contacts delineates an isograd that defines temperature as well as the mole fraction of CO2 at constant pressure in systems open to fluid transport. These isograds can be contrasted with fluid-producing isograds in closed systems. As an illustration of possible effects, the reactions quartz + clinozoisite + muscovite = anorthite + K-feldspar + H2O and phlogopite + quartz + calcite = tremolite + K-feldspar + H2O + CO2 at 4 kbar are analysed and equations for fluid production and transport are developed. 相似文献