全文获取类型
收费全文 | 70576篇 |
免费 | 730篇 |
国内免费 | 547篇 |
专业分类
测绘学 | 1770篇 |
大气科学 | 4209篇 |
地球物理 | 12828篇 |
地质学 | 28339篇 |
海洋学 | 5942篇 |
天文学 | 15869篇 |
综合类 | 288篇 |
自然地理 | 2608篇 |
出版年
2022年 | 599篇 |
2021年 | 893篇 |
2020年 | 1013篇 |
2019年 | 1077篇 |
2018年 | 4169篇 |
2017年 | 3713篇 |
2016年 | 3312篇 |
2015年 | 1075篇 |
2014年 | 2048篇 |
2013年 | 3131篇 |
2012年 | 2891篇 |
2011年 | 4513篇 |
2010年 | 3923篇 |
2009年 | 4528篇 |
2008年 | 3838篇 |
2007年 | 4407篇 |
2006年 | 2580篇 |
2005年 | 1673篇 |
2004年 | 1601篇 |
2003年 | 1574篇 |
2002年 | 1469篇 |
2001年 | 1236篇 |
2000年 | 1105篇 |
1999年 | 803篇 |
1998年 | 844篇 |
1997年 | 836篇 |
1996年 | 666篇 |
1995年 | 653篇 |
1994年 | 655篇 |
1993年 | 527篇 |
1992年 | 522篇 |
1991年 | 530篇 |
1990年 | 602篇 |
1989年 | 455篇 |
1988年 | 443篇 |
1987年 | 497篇 |
1986年 | 374篇 |
1985年 | 569篇 |
1984年 | 547篇 |
1983年 | 506篇 |
1982年 | 519篇 |
1981年 | 437篇 |
1980年 | 481篇 |
1979年 | 375篇 |
1978年 | 425篇 |
1977年 | 341篇 |
1976年 | 327篇 |
1975年 | 336篇 |
1974年 | 314篇 |
1973年 | 341篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
871.
This article presents a comparative study of the breathing resistance and clearing characteristics of several types of commercially available snorkels in an attempt to device quantitative means that enable the divers to select the best snorkel and the designers to see the possible directions and modifications necessary for improving the performance of such important life support devices.Two test stands have therefore been built, the first to measure the breathing resistance and the second to monitor the clearing efficiency and speed of some of the commonly used snorkels.The tested snorkels varied from the simple J-type to the sophisticated wrap-around snorkels with convoluted flexible hoses and flared ends.The results obtained show that large diameter snorkels of the wrap-around design with sweeping barrels and flared ends (as the AMF Swimmaster snorkel number 2S55) offer the least breathing resistance. As for the clearing characteristics snorkels with small diameters and of the simple J-type (as the Spanish Aquasub snorkel: The Canarias) or with large diameters and sweeping barrels (as the Power Tuned snorkel of U.S. divers) possess the best clearing figure of merit. 相似文献
872.
873.
Measurements of 226Ra and 210Pb in barite samples separated from different depths of a piston core from the eastern equatorial Pacific are reported. 226Ra is enriched in the barite as would be expected from the chemistries of Ra and Ba; the ratio centers around 1.5 · 1014 atoms/mole. The results suggest that barite is concentrating Ra at a rate comparable to its decay rate, viz. 5.78 · 10?3 yr?1. The gross 210Pb depletion with respect to 226Ra is indicative of 222Rn loss due to the smallness of the barite crystals. 相似文献
874.
It is suggested that Bouvet Island is the surface manifestation of a mantle plume which has resulted in the creation of a chain of seamounts in the South Atlantic and a general shoaling of the region.The strike of two newly defined large fracture zones borth and south of the Bouvet Island pedestal have been utilized to determine a pole of rotation at 12.5°S, 12.5°W for the Africa-Antarctica motion. A pole at 75°S, 13°E has been calculated for the South America-Antarctica motion. At the triple junction the South America-Africa relative motion is 3.3 cm yr-1 (whole rate) at 075°. The Africa-Antarctica motion is 1.7 cm yr-1 at 065° and the South America-Antarctica motion is 1.6 cm yr-1 at 085. 相似文献
875.
The distribution of the colour index is considered in the region bounded by 8–11°N and 13°30–18°30W based on the results of measurements made on board vessels of the Marine Hydrophysical Institute of the Ukrainian SSR Academy of Sciences (MHI) in 1977–1985. Mean values and statistical characteristics are calculated for the colour index variability over one-degree squares. A map of its multi-yearly average distribution is plotted.Translated by M. M. Trufanov. 相似文献
876.
A stochastic prognostic model of the atmospheric precipitation in the tropical area of the Atlantic Ocean is developed on the basis of a large data array. The data represent a series of monthly precipitation rates for 31 towns in the Republic of Guinea covering a period of 35–64 years as well as satellite data on the meridional displacement of the Intertropical Convergence Zone during 11 years. The model takes into account major regularities of the tropical precipitation: their impulsive periodic regime, trends, and the modulation of the stochastic component by the determined variability.Translated by Mikhai M. Trufanov. 相似文献
877.
The problem of monitoring of the mesoscale variability of hydrophysical fields in the shelf zone is considered. The usefulness of the data available in regional information centres is discussed. The possibility of monitoring the dynamics of the oceanic fields in the tropical Atlantic encompassing separate observation sites is demonstrated using the oceanographic databank of the scientific research centre (CERESCOR) in Conakry-Rogbane as an example. The results of assimilation of the hydrophysical survey data derived on the shelf in a telescopic model of the region are given.Translated by Mikhail M. Trufanov. 相似文献
878.
The Three Kings Ridge has been described as a remnant arc or an east-facing arc on the western side of the South Fiji Basin
under which symmetric South Fiji Basin magnetic anomaly lineations have been lost. We find evidence of neither arc-trench
morphology on the eastern flank of the ridge nor collision tectonism within the adjacent South Fiji Basin that might have
obliterated it. We believe there is room for the missing magnetic lineations within the South Fiji Basin and we recognize
the distinctive island-arc morphology characteristic of a west-facing arc on the western flank of the Three Kings Ridge. Our
interpretation requires that arc volcanism postdated formation of much of the South Fiji Basin and that oceanic lithosphere
from the adjacent Norfolk Basin must have been subducted beneath the west-facing Three Kings Arc. 相似文献
879.
Dorrik A. V. Stow 《Geo-Marine Letters》1983,3(2-4):217-222
The Brae oilfield reservoir in the North Sea comprises Upper Jurassic resedimented conglomerates and sandstones interbedded with organic-rich silstone and mudstone thin-bedded turbidites. The system represents a series of small overlapping fans that form a thick (300 m) slope-apron accumulation of sediments deposited in a narrow (<10 km wide) belt along an active fault zone. The complex lateral and vertical distribution of facies was due mainly to variable tectonic activity, and partly also to sediment supply and sea-level changes. 相似文献
880.
Two genetically different types of authigenic carbonate mounds are studied: those within an active hydrothermal field related to serpentinite protrusions in the zone of intersection of a transform fracture zone and the Mid-Atlantic Ridge, and those in an active field of methane seepings in the Dnieper canyon of the Black sea. The general geochemical conditions under which authigenic carbonate formation occurs in the two fields considered were found. They include the presence of reduced H2S, H2, and CH4 gases at the absence of free oxygen; the high alkalinity of the waters participating in the carbonate formation; the similarity of the textural and structural features of authigenic aragonite, which represents the initial mineral of the carbonate matter of the mounds; the paragenesis of aragonite with sulfide minerals; and the close relation of carbonate mounds with communities of sulfate-reducing and methane-oxidizing microorganisms. A new mechanism of formation of hydrothermal authigenic carbonates is suggested; it implies their microbial sulfate reduction over the hydrogen of the fluid in the subsurface zone (biosphere) of mixing between the hydrothermal solution and the adjacent seawater. 相似文献