收费全文 | 662篇 |
免费 | 18篇 |
国内免费 | 1篇 |
测绘学 | 3篇 |
大气科学 | 58篇 |
地球物理 | 188篇 |
地质学 | 293篇 |
海洋学 | 36篇 |
天文学 | 89篇 |
自然地理 | 14篇 |
2021年 | 7篇 |
2018年 | 10篇 |
2017年 | 8篇 |
2016年 | 13篇 |
2015年 | 19篇 |
2014年 | 10篇 |
2013年 | 20篇 |
2012年 | 11篇 |
2011年 | 22篇 |
2010年 | 16篇 |
2009年 | 21篇 |
2008年 | 18篇 |
2007年 | 13篇 |
2006年 | 14篇 |
2005年 | 18篇 |
2004年 | 13篇 |
2003年 | 15篇 |
2002年 | 22篇 |
2001年 | 14篇 |
2000年 | 11篇 |
1999年 | 13篇 |
1998年 | 9篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1994年 | 12篇 |
1991年 | 9篇 |
1990年 | 10篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 11篇 |
1979年 | 7篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 7篇 |
1975年 | 8篇 |
1974年 | 6篇 |
1973年 | 9篇 |
1972年 | 7篇 |
1971年 | 6篇 |
1968年 | 7篇 |
1958年 | 5篇 |
1955年 | 5篇 |
1954年 | 10篇 |
1952年 | 7篇 |
1951年 | 7篇 |
1950年 | 6篇 |
1949年 | 11篇 |
1948年 | 8篇 |
Pyrite is the main phase of inorganic reduced sulphur in the sediments from the Tyro and the Bannock Basins, and it has about the same average level (125 υmoles per gramme dry weight) in the cores recovered from the two areas. However, the distribution of pyrite in the top 100 cm of the two cores differs significantly. In the Bannock Basin a sharp increase is observed with depth, whereas in the Tyro Basin there is a small decrease with depth.
The total amount of reduced inorganic sulphur is less than the total amount of sulphur in the sediments. This indicates that there must be additional sulphur-bearing phases. One of these phases may be gypsum, and indeed, gypsum crystals have been observed in the Bannock Basin.
In neither basin is there a significant correlation between reduced sulphur and organic carbon. The pyrite that occurs in these basins may have been formed syngenetically at the interface of the anoxic brine and oxic seawater. Diagenetic pyrite may have been formed within the sediments of the basins. AVS and total zerovalent sulphur are still observed at depth. We therefore suggest that this may be due to the incomplete transformation of AVS and zerovalent sulphur into pyrite. 相似文献
The virtual heights of the ionospheric F2-layer depends on the solar activity characteristically with a double-period for one sunspots-cycle. Only at the Peruvian station Huancayo appears a single period with great heights at small solar activity and with deep-seated boundary of F2-layer at strong solar activity. *** DIRECT SUPPORT *** A2919023 00002相似文献
Sphero-conics and spheroids in navigation
Summary The sphero conic section has little nautical significance. Its geometrical nature has been thoroughly discussed long ago. On the other hand, its analogue on the spheroid, viz. the Loran line, is interesting and important.
La section sphéro-conique et le sphéro?de
Résumé La section sphéro-conique est à peu près sans importance dans le champ nautique. Son caractère géométrique a été profondément discuté il y a long temps. Cependant, son analogue, c'est à dire la courbe Loran sur le sphéro?de, est aussi intéressant qu'il est important.相似文献
Summary It is examined which changes of the vertical extension and of the density of rectangular prisms are allowed without changing their gravity effects beyond certain limits. Forms of mass respectively of geological layers which differ from each other only in geometrical dimensions and in density but have practically the same gravity effect are called gravimetrically equivalent. — A synthetical example is treated in which the dimensions are such that the rectangular prism can be regarded as isostatic compensation for a pattern mountain range of the extension of the Alps. — It is concluded that the examination of the gravity anomalies cannot furnish a clear conception of the extension of the disturbing geological layer below the mountain range, even if its form is known. — In an appendix the necessary formulas for calculating the gravimetrical equivalence are given.相似文献
Summary The ion-production of a monochromatic radiation in an atmosphere with variable temperature in various heights is considered. A stratification ofF-layer intoF 1-andF 2-layer appears by a heat zone between these layers. But always the temperature of theF 2-layer is lower as this ofF 1-layer. Thus some difficulties ofF 2-layer can be removed.相似文献
Experiences made with the ocean bottom sampler for ships under way
Summary The bottom sampler for ships under way as described by L. J. Worzel was reconstructed by order of the German Hydrographic Institute and tried on a research cruise in the North Sea. It was found that the samples, especially those composed of ooze with an admixture of fine granulated sand and of fine granulated sand with an admixture of ooze, risked to be washed away by the flow of water when the instrument was hauled in. In order to keep the cores from slipping away a funnel shaped nozzle was fitted into the sampling tube and as a result of this greater samples were retained. Besides, a cup was added to the instrument determined to be filled with the samples and to facilitate the withdrawal of the bottom samples. The instrument is described in detail and a short statement of the results obtained is made.
Expériences faites de l'échantillonneur permettant de prélever des sédiments du fond de la mer du bord de navires en marche
Résumé Cet appareil qui a été décrit par J. L. Worzel et qui permet de prélever des échantillons du fond de la mer sans arrêter le navire a été réconstruit par ordre de l'Institut Hydrographique Allemand et mis à l'épreuve au cours d'une croisière en mer du Nord. On a dû constater que les carottes contenant de la vase avec une admixtion de sables à fin grain ainsi que celles se composant des sables à fin grain avec une admixtion de vase sont souvent enlevées par le courant d'eau entrant dans le tube au moment de la récuperation de l'appareil. En montant une tuyère en forme d'entonnoir afin de prévenir la perte des carottes on a obtenu des carottes plus grandes que celles prises sans tuyère. En outre, une espèce de gobelet fut introduit dans le tube de l'appareil destiné à recevoir les carottes et à faciliter leur enlèvement. La construction de l'appareil y est précisée et les résultats en sont brièvement exposés.相似文献