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排序方式: 共有383条查询结果,搜索用时 156 毫秒
1.
Some Bianchi Type IX string cosmological models with bulk viscous fluid for massive string is investigated. To get a determinate
solution, a supplementary condition a=b
n, between metric potentials, is used wherea and b are function of time alone. A particular solution for n=0 is also discussed. The behaviour of the model in presence and absence
of bulk viscosity, is discussed. The physical and implications of the model is also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
Geometric analysis of nested Riedel structures was used to identify and quantify strain localization processes within faulted Navajo sandstone. The analysis shows systematic deviation from the basic Riedel geometry complying with the Mohr–Coulomb criterion. Using cross-cutting relations amongst deformation bands within the Riedel structures, and comparing the orientations of the deformation bands to theoretical strain calculations, we identify two coupling deformation mechanisms involved in the early stages of shear-zone evolution, namely, granular flow and discrete faulting. Both mechanisms localize during strain accumulation, and the granular flow facilitates considerable change in the initial geometry of the Riedel structures. The analysis demonstrates a systematic sequence, by which new Riedel structures form after a constant amount of shear strain takes place in the sandstone. Analysis further indicates that granular flow is the major deformation mechanism during early stages of shear-zone evolution and discrete faulting is the dominant mechanism during later deformation stages. 相似文献
3.
Methane, a potential biosignature, has recently been detected in the martian atmosphere. This Note focuses on field investigations/operational simulations and laboratory studies which resulted in successful detection of methane within arid terrestrial soils, as distinct from the usual methanogen environment, but in at least partial analogy to martian conditions. 相似文献
4.
The influence of compressibility of media on both the statistical acceleration and the turbulent diffusion of cosmic-ray particles is investigated. The averaging over an ensemble of random velocity fields of the medium was performed in the kinetic equation. The kinetic coefficients, which are responsible for the particle acceleration, were obtained in the cases of weak and strong scattering due inhomogeneous magnetic fields. 相似文献
5.
Gregory P. Marchildon Suren Kulshreshtha Elaine Wheaton Dave Sauchyn 《Natural Hazards》2008,45(3):391-411
Agriculture in the southern Great Plains of Canada has been particularly vulnerable to prolonged episodes of drought. Using
climate data and a precipitation minus potential evapotranspiration index, the extent of the region’s exposure to drought
is examined. Between 1914 and 1917, the Dry Belt was particularly vulnerable to drought, whereas after 1928, a much larger
region known as the Palliser Triangle covering most of southern Alberta and Saskatchewan was much more exposed to drought.
These droughts provoked major institutional adaptation, in particular the establishment of the Special Areas Board by the
Government of Alberta, and the creation of the Prairie Farm Rehabilitation Administration by the Government of Canada. Both
organizations have proved to be relatively permanent public adaptations to the natural hazard of drought in the region. Moreover,
these earlier experiences with prolonged drought as well as institution-building may be of value in helping the residents
of the Palliser Triangle adapt to predicted climate changes in the future as well as anticipate some of the barriers to effective
institutional adaptation. 相似文献
6.
A theoretical evaluation of basic thermodynamic relationships reveals that variation of activity coefficients, ion pairing and electrical interactions must be considered when modelling ionic diffusion in seawater. The contributions of ion-pair formation and change in activity coefficient along the diffusion path were studied experimentally by conducting diffusion experiments in which solutions of KCl, NaCl, MgCl2, Li2SO4, K2SO4, Na2SO4 and MgSO4, at an ionic strength of 0.7, were allowed to diffuse into distilled water. The study reveals that the thermodynamic factor, required to correct for changes in the activity coefficient along the diffusion path, is significant for all the salts studied. Agreement between a simple diffusion model, which does not include ion pairing, and observed data was good for completely dissociated salts, but poor for salts which are known to form ion pairs at the concentration levels studied. The diffusion of MgSO4, 0.425 of which is associated at I = 0.7, was successfully modelled by assuming that the diffusion coefficient of the MgSO40 ion pair is different from the diffusion coefficient of the dissociated salt. The diffusion coefficient of this ion pair is estimated to be 1.9 × 10−5 cm2 s−1 at 30°C, as compared to 0.49 × 10−5 cm2 s−1 for the dissociated salt. It is suggested that the high mobility of this ion pair could cause magnesium enrichment in pore water of sulfate depleted sediments. 相似文献
7.
Long-term exposure of animals to sub-lethal doses of toxicants such as benzene (B) and dimethylnaphthalene (DMN) may result in subtle changes in their physiology and biochemistry. In crustaceans such changes include decreased rate of limb regeneration, extended time to molt and decreased growth increment at molt.1,2 These processes depend upon an adequate supply of stored nutrients in the tissues and appropriate release of neuroendocrine substances from the central nervous system.3,4 We are examining the effects of sub-lethal doses of B and DMN on osmotic and ionic regulation and on nutrient storage in the blue crab, Callinectes sapidus, and the green crab, Carcinus maenas. Disturbances in these physiological processes may reflect alterations in neuroendocrine functions brought about by the presence of toxicants. We find that exposure of C. maenas to B or DMN, followed by transfer to a dilute medium, results in impaired osmoregulatory capacity, while addition of these compounds to a dilute medium to which the animal is already acclimated does not affect their regulatory ability. Storage of carbohydrate in the midgut gland (hepatopancreas) is decreased by B or DMN, while the accessory storage tissues (gill, muscle) are not significantly affected. 相似文献
8.
9.
Archaeological investigations in Camels Back Cave, western Utah, recovered a series of small-mammal bone assemblages from stratified deposits dating between ca. 12,000 and 500 14C yr B.P. The cave's early Holocene fauna includes a number of species adapted to montane or mesic habitats containing grasses and/or sagebrush (e.g., Lepus townsendii, Marmota flaviventris, Reithrodontomys megalotis, and Brachylagus idahoensis) which suggest that the region was relatively cool and moist until after 8800 14C yr B.P. Between ca. 8600 and 8100 14C yr B.P. these mammals became locally extinct, taxonomic diversity declined, and there was an increase in species well-adapted to xeric, low-elevation habitats, including ground squirrels, Lepus californicus and Neotoma lepida. The early small-mammal record from Camels Back Cave is similar to the 11,300–6000 14C yr B.P. mammalian sequence from Homestead Cave, northwestern Utah, and provides corroborative data on Bonneville Basin paleoenvironments and mammalian responses to middle Holocene desertification. 相似文献
10.