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The equations of motion governing the evolution of a collisionless gravitating system of particles in an expanding universe can be cast in a form which is almost independent of the cosmological density parameter, Ω, and the cosmological constant, Λ. The new equations are expressed in terms of a time variable τ=ln D , where D is the linear rate of growth of density fluctuations. The dependence on the density parameter is proportional to ε=Ω−0.2 −1 times the difference between the peculiar velocity (with respect to τ) of particles and the gravity field (minus the gradient of the potential); or, before shell-crossing, times the sum of the density contrast and the velocity divergence. In a one-dimensional collapse or expansion, the equations are fully independent of Ω and Λ before shell crossing. In the general case, the effect of this weak Ω dependence is to enhance the rate of evolution of density perturbations in dense regions. In a flat universe with Λ7ne;0, this enhancement is less pronounced than in an open universe with Λ=0 and the same Ω. Using the spherical collapse model, we find that the increase of the rms density fluctuations in a low-Ω universe relative to that in a flat universe with the same linear normalization is ∼0.01ε(Ω)〈δ3 〉, where δ is the density field in the flat universe. The equations predict that the smooth average velocity field scales like Ω0.6 , while the local velocity dispersion (rms value) scales, approximately, like Ω0.5 . High-resolution N -body simulations confirm these results and show that density fields, when smoothed on scales slightly larger than clusters, are insensitive to the cosmological model. Haloes in an open model simulation are more concentrated than haloes of the same M /Ω in a flat model simulation. 相似文献
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Ohne Zusammenfassung 相似文献
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Franz Nusser 《Ocean Dynamics》1953,6(2):77-80
Zusammenfassung Der Winter 1951/52 war ein eisarmer Winter. Es traten drei Kälteperioden auf, von denen allein die beiden ersten Eis brachten. Im Nordseebereich wurden nur die Zuflüsse und Teile der inneren Wattengebiete, im Ostseebereich die Schlei, Kieler Förde und Trave für kurze Zeit mit leichten Eisarten bedeckt.
Ice conditions on the German coasts between the rivers Ems and Trave during the winter 1951/52
Summary Ice formation in winter 1951/52 was insignificant. There were three cold periods of which only the first two gave rise to formation of ice. Within the region of the North Sea only the rivers and some parts of the inner tidal flats and within the region of the Baltic the Schlei, the Kiel Fjord and the river Trave were covered with thin ice for a short time.
Rapport sur les conditions des glaces sur les côtes allemandes entre l'Ems et le Trave pendant l'hiver 1951/52
Résumé La formation de glaces pendant l'hiver 1951/52 était insignifiante. Il y avait trois périodes d'air froid dont seulement les premières deux donnaient lieu à la formation de glaces. Sur la côte de la mer du Nord les affluents et quelques régions à l'intérieur des bas-fonds et sur la côte orientale du Schleswig-Holstein le Schlei, le fiord de Kiel et le Trave ne furent couverts que pour une courte durée de glaces minces.相似文献
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The Lyman α forest provides important constraints on the smoothness of the Universe on large scales. We calculate the flux distribution along the line of sight to quasars in a universe made of randomly distributed clumps, each of them with a Rayleigh–L'evy fractal structure with dimension D <2. We consider the probability distribution function of the normalized flux in the line of sight to quasars. We illustrate that the truncated clustering hierarchy model with D <2 shows far too many voids along the line of sight to quasars compared with the observed flux distribution and the distribution in a cold dark matter model. This supports the common view that on large scales the Universe is homogeneous, rather than fractal-like. 相似文献
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