全文获取类型
收费全文 | 87199篇 |
免费 | 1375篇 |
国内免费 | 656篇 |
专业分类
测绘学 | 2472篇 |
大气科学 | 6713篇 |
地球物理 | 17624篇 |
地质学 | 29603篇 |
海洋学 | 7299篇 |
天文学 | 20097篇 |
综合类 | 214篇 |
自然地理 | 5208篇 |
出版年
2021年 | 695篇 |
2020年 | 766篇 |
2019年 | 834篇 |
2018年 | 1927篇 |
2017年 | 1847篇 |
2016年 | 2356篇 |
2015年 | 1490篇 |
2014年 | 2330篇 |
2013年 | 4489篇 |
2012年 | 2435篇 |
2011年 | 3127篇 |
2010年 | 2913篇 |
2009年 | 3814篇 |
2008年 | 3358篇 |
2007年 | 3325篇 |
2006年 | 3123篇 |
2005年 | 2611篇 |
2004年 | 2593篇 |
2003年 | 2414篇 |
2002年 | 2387篇 |
2001年 | 2116篇 |
2000年 | 1978篇 |
1999年 | 1794篇 |
1998年 | 1728篇 |
1997年 | 1746篇 |
1996年 | 1431篇 |
1995年 | 1397篇 |
1994年 | 1322篇 |
1993年 | 1184篇 |
1992年 | 1093篇 |
1991年 | 1081篇 |
1990年 | 1165篇 |
1989年 | 1070篇 |
1988年 | 987篇 |
1987年 | 1160篇 |
1986年 | 983篇 |
1985年 | 1257篇 |
1984年 | 1484篇 |
1983年 | 1360篇 |
1982年 | 1316篇 |
1981年 | 1238篇 |
1980年 | 1073篇 |
1979年 | 1045篇 |
1978年 | 1048篇 |
1977年 | 947篇 |
1976年 | 872篇 |
1975年 | 825篇 |
1974年 | 863篇 |
1973年 | 895篇 |
1972年 | 566篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
Timing and amount of solar radiation were examined as factors influencing the distribution of seven perennial plants on a small mountain located in the Chihuahuan Desert. Average direct beam solar radiation fluxes at differing times throughout the day and year were estimated with computer calculations. Principal components analysis was used to reduce the number of solar radiation parameters and include the maximum available information with a manageable number of variables. The remaining solar radiation parameters were compared to plant distributions using redundancy analysis and generalized additive models. Unimodal, bimodal, and monotonic responses were all found depending upon the species and solar radiation parameter. Niche separation at this location depends upon the timing as well as the amount of solar radiation. 相似文献
192.
193.
Authigenic iron and titanium oxides occur in three main textural varieties in the St. Bees Sandstone, a Triassic red bed succession in Cumbria. These are: (a) overgrowths of haematite, titaniferous haematite, anatase, and rutile which generally occur on detrital host grains of similar compositions. The overgrowths may occur as syntaxial rim cements or rhombohedral and prismatic projections and are always in optical continuity, (b) discrete crystals of anatase (including octahedrite) and haematite 10-40 μm in size occur in pore spaces, and (c) fine grained ( < 1 μm) pigmentary haematite occurs as grain coatings around detrital grains and in the interstitial matrix. The euhedral nature and pore-filling habit clearly indicate the authigenic origin of these iron- and titanium oxides. All three textural varieties are believed to have been precipitated from ground waters containing dissolved ferrous and titanium ions. These ions were derived from the intrastratal solution of detrital iron and titanium bearing grains including ferromagnesian silicates and iron-titanium oxides. The precipitation of iron oxides from migrating ground waters in the manner described here goes some way to explaining how thick, uniformly red successions can be formed. 相似文献
194.
M. R. E. Proctor 《Astronomische Nachrichten》1977,298(1):19-25
The kinematic α-effect dynamo problem is investigated in the case of an exterior perfect conductor. It is shown that certain approximate symmetries discovered in the numerical analysis of ROBERTS (1972) are exact for this case. As an illustration, an exact solution is given in a cylindrical geometry, where the equations can be written in terms of one variable. The implications for the earth's dynamo are discussed. 相似文献
195.
In the present paper revised distances of optical objects from radio sources of the 5C2 survey are given. A statistical investigation of the data is given for blue objects and galaxies by the statistical method of the “first neighbour”. The identification rate on blue plates for both the blue objects and galaxies amounts to about 40% out of the total number of 26 identifications. For every blue object and galaxy which are proposed as an identification the statistical reliability is given. 相似文献
196.
Plates of the Tautenburg Schmidt can be calibrated with the aid of a beam-splitting prisma only if Schraffierkassette plates are used. The effects of image structure, scintillation and intermittence on in-focus images are different for the main component and the prismatic companion. Therefore, such plates cannot be used to set up a photometric scale. 相似文献
197.
P. Notni 《Astronomische Nachrichten》1976,297(3):147-154
Optical quasar candidate counts in the far reaching radio surveys B2 and 5C are consistent either with a luminosity function containing a high percentage of low luminosity objects and a cut off in quasar density or, more probably, with a normal number of quasars at high redshifts and a less steep luminosity function. The absence of high redshifted objects in currently available samples is to be expected of q0 ≈︂ o and if some of the few quasars observed at z>2.2 are exceptionally bright intrinsically and not typical for the bulk. 相似文献
198.
D. F. Dickinson G. Kojoyan C. R. Purton R. A. Sramek H. M. Tovmassian 《Astronomische Nachrichten》1976,297(6):283-286
Radio spectra of 16 Markarian galaxies are studied between 2.7 and 15.5 GHz. The Seyfert-type galaxies have power-law spectra with an average spectral index of -0.82, except for MRK 348, which is an active galaxy with a complex spectrum. The galaxies with featureless optical spectra have rather flat radio spectra, similar to BL Lac-type objects. 相似文献
199.
G. Pal 《Astronomische Nachrichten》1976,297(6):311-315
In a gravitationally bound and stable cluster of galaxies the amount and distribution of matter determine both the velocity dispersion of the members and the type of evolution of the system. The use of the first of these physical connections — the application of virial theorem — led to the idea of missing mass in clusters, that of the second one seems to support this idea by an independent “rediscovery” and “redistribution” of missing mass. On the basis of this “evolutionary approach” to the missing mass problem — that is free from the uncertainties of measuring and interpreting red shifts — evidences are obtained for the existence of large amounts of discretely distributed dark circumgalactic or intragalactic matter in rich clusters of galaxies. 相似文献
200.
Juan Pablo Corella Adel El Amrani Javier Sigró Mario Morellón Eugenio Rico Blas Lorenzo Valero-Garcés 《Journal of Paleolimnology》2011,46(3):469-485
We present a high-resolution, multiproxy reconstruction of the depositional history of Lake Arreo, northern Spain, for the
last 60 years. We conducted sedimentological, geochemical and diatom analyses in short cores and made a detailed comparison
with regional instrumental climate data (1952–2007), limnological monitoring of the lake (1992–2008) and recent land use changes
that affect the lake catchment. Chronology is based on “floating” discontinuous varve counts and 137Cs and 14C dates. Four periods were identified in the Lake Arreo recent history: (1) prior to 1963, varved facies intercalated with
fine turbidite deposits, and diatom assemblages dominated by Cyclotella taxa indicate predominantly meromictic conditions, (2) from 1964 to 1978, permanent anoxia persisted in bottom waters, as shown
by similar facies and diatom assemblages as before, though detrital layers were coarser, (3) from 1979 to 1994, sediment delivery
to the lake increased and laminated, clastic facies were deposited, and (4) from 1995 to 2008, dominance of massive facies
and an increase in Fragilaria tenera and Achnanthes minutissima reflect relatively lower lake levels, less frequent bottom anoxia with more frequent water column mixing, similar to modern
conditions. The period 1952–1979 was a time of meromixis and varved facies deposition, and was characterized by higher rainfall
and less intense agricultural pressure in the watershed. There were two short humid periods (1992–1993 and 1996–1998) when
monitoring data show more anoxic weeks per year and relatively higher lake levels. Increased cultivation of small landholdings
in 1963, and particularly after 1979, caused a large increase in sediment delivery to the lake. The inferred lake evolution
is in agreement with monitoring data that suggest a transition from dominantly meromictic conditions prior to 1993–1994 to
a predominantly monomictic pattern of circulation since then, particularly after 2000. The synergistic effects of intensive
water extraction for irrigation and lower rainfall since 1979, and particularly since 1994, brought the long period of meromictic
conditions in Lake Arreo to an end. Water balance and sediment delivery to the lake are dominant factors that control the
limnological and mixing conditions in Lake Arreo and they must be considered in management and restoration plans. 相似文献