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31.
Interrill erosion depends on soil detachment and sediment transport, which are affected by seal formation and runoff. The objective of this study was to investigate the effect of wetting rate (WR) on runoff and soil erosion in semi-arid Israeli soils varying in clay content and exchangeable sodium percentage (ESP). Six soils, ranging in clay content between 90 and 680 g kg−1 and ESP between 0.9 and 20, were packed in 0.2 m×0.4 m trays, wetted at 3 WRs (2, 8, or 64 mm h−1), and thereafter exposed to 60 mm of distilled water rain in a laboratory rainfall simulator. Under non-sodic conditions (ESP<2), highest runoff and erosion were obtained from loam (220 g kg−1 clay and 350 g kg−1 silt) which was ascribed to its high susceptibility to seal formation, runoff and detachability. Runoff and erosion increased with an increase in ESP and WR. The effect of WR on runoff and erosion was negligible in loamy sand and generally increased with an increase in clay content. In clay soils (>600 g kg−1 clay), WR played a greater role in determining runoff and erosion compared with raindrop impact. A linear type dependence existed between erosion and runoff for soils with ESP<5 or when slow WR was used. For high ESP soils, or when medium or fast WR were used, an exponential type relation described better the dependence of erosion on runoff. It is suggested that for sodic soils or for conditions favoring aggregate slaking, runoff level and its velocity were high enough to initiate rill erosion that supplemented raindrop detachment in markedly increasing erosion. 相似文献
32.
Levy EH 《Science (New York, N.Y.)》1972,178(4056):52-53
The assumption that the moon had an internal magnetic field produced in the same way as the geomagnetic field requires that the moon rotated faster than the angular velocity at which it would break up. This suggests that a lunar dynamo is not a tenable explanation for the magnetic remanence observed on the moon. 相似文献
33.
The Great Basin of the western United States has proven important for studies of Proterozoic and Paleozoic geology [2500 to 245 million years ago (Ma)] and has been central to the development of ideas about the mechanics of crustal shortening and extension. An understanding of the deformational history of this region during Mesozoic and Cenozoic time (245 Ma to the present) is required for palinspastic reconstruction of now isolated exposures of older geology in order to place these in an appropriate regional geographic context. Considerable advances in unraveling both the crustal shortening that took place during Mesozoic to early Cenozoic time (especially from about 150 to 50 Ma) and the extension of the past 37 million years have shown that earlier reconstructions need to be revised significantly. A new reconstruction is developed for rocks of middle Proterozoic to Early Cambrian age based on evidence that total shortening by generally east-vergent thrusts and folds was at least 104 to 135 kilometers and that the Great Basin as a whole accommodated approximately 250 kilometers of extension in the direction 287 degrees +/- 12 degrees between the Colorado Plateau and the Sierra Nevada. Extension is assumed to be equivalent at all latitudes because available paleomagnetic evidence suggests that the Sierra Nevada experienced little or no rotation with respect to the extension direction since the late Mesozoic. An estimate of the uncertainty in the amount of extension obtained from geological and paleomagnetic uncertainties increases northward from +/-56 kilometers at 36 degrees 30N to (-87)(+108) kilometers at 40 degrees N. On the basis of the reconstruction, the original width of the preserved part of the late Proterozoic and Early Cambrian basin was about 150 to 300 kilometers, about 60 percent of the present width, and the basin was oriented slightly more north-south with respect to present-day coordinates. 相似文献
34.
Bone marrow-derived lymphoid cells (B cells): functional depletion with cobra factor and fresh serum 总被引:2,自引:0,他引:2
Treatment of rat spleen cells with cobra factor and fresh rat serum provided a simple, rapid means of functionally eliminating complement receptor lymphocytes. Cells able to differentiate into plaque-forming cells in a syngeneic, irradiated host were diminished, but cells able to induce a graft-versus-host reaction were not diminished. There was no effect on plaque-forming cells from an immune spleen. 相似文献
35.
Similarity of synthetic peptide from human tumor to parathyroid hormone in vivo and in vitro 总被引:18,自引:0,他引:18
N Horiuchi M P Caulfield J E Fisher M E Goldman R L McKee J E Reagan J J Levy R F Nutt S B Rodan T L Schofield 《Science (New York, N.Y.)》1987,238(4833):1566-1568
One mechanism considered responsible for the hypercalcemia that frequently accompanies malignancy is secretion by the tumor of a circulating factor that alters calcium metabolism. The structure of a tumor-secreted peptide was recently determined and found to be partially homologous to parathyroid hormone (PTH). The amino-terminal 1-34 region of the factor was synthesized and evaluated biologically. In vivo it produced hypercalcemia, acted on bone and kidney, and stimulated 1,25-dihydroxy-vitamin D3 formation. In vitro it interacted with PTH receptors and, in some systems, was more potent than PTH. These studies support a long-standing hypothesis regarding pathogenesis of malignancy-associated hypercalcemia. 相似文献
36.
Observed diffraction pattern and proposed models of liquid water 总被引:1,自引:0,他引:1
37.
Kochendoerfer GG Chen SY Mao F Cressman S Traviglia S Shao H Hunter CL Low DW Cagle EN Carnevali M Gueriguian V Keogh PJ Porter H Stratton SM Wiedeke MC Wilken J Tang J Levy JJ Miranda LP Crnogorac MM Kalbag S Botti P Schindler-Horvat J Savatski L Adamson JW Kung A Kent SB Bradburne JA 《Science (New York, N.Y.)》2003,299(5608):884-887
We report the design and total chemical synthesis of "synthetic erythropoiesis protein" (SEP), a 51-kilodalton protein-polymer construct consisting of a 166-amino-acid polypeptide chain and two covalently attached, branched, and monodisperse polymer moieties that are negatively charged. The ability to control the chemistry allowed us to synthesize a macromolecule of precisely defined covalent structure. SEP was homogeneous as shown by high-resolution analytical techniques, with a mass of 50,825 +/-10 daltons by electrospray mass spectrometry, and with a pI of 5.0. In cell and animal assays for erythropoiesis, SEP displayed potent biological activity and had significantly prolonged duration of action in vivo. These chemical methods are a powerful tool in the rational design of protein constructs with potential therapeutic applications. 相似文献
38.
Agron PA Begun GM Levy HA Mason AA Jones CG Smith DF 《Science (New York, N.Y.)》1963,139(3557):842-844
Xenon reacts with fluorine to form XeF(2) which can be isolated before it reacts with fluorine to form XeF(4). The linear configuration of XeF(2) with the 2.00-A bond length and the vibrational force constants support the assignment of 10 electrons to the valence shell of xenon. Similar arguments support the assignment of 12 and 14 valence electrons respectively to xenon in XeF(4) and XeF(6). 相似文献
39.
Very long threadlike micelles observed in aqueous solutions of some surfactants have attracted much attention because of the peculiar rheological properties of these systems. Molecular dynamics simulations have suggested that branched threadlike micelles should exist in concentrated solutions of dimeric surfactants. Here experimental evidence, obtained from transmission electron microscopy at cryogenic temperature, is presented of branched threadlike micelles in aqueous solutions of a triquaternary ammonium (trimeric) surfactant made up of three amphiphilic moieties connected at the level of the head-groups by two propanediyl spacers. 相似文献
40.
Pease PJ Levy O Cost GJ Gore J Ptacin JL Sherratt D Bustamante C Cozzarelli NR 《Science (New York, N.Y.)》2005,307(5709):586-590
DNA translocases are molecular motors that move rapidly along DNA using adenosine triphosphate as the source of energy. We directly observed the movement of purified FtsK, an Escherichia coli translocase, on single DNA molecules. The protein moves at 5 kilobases per second and against forces up to 60 piconewtons, and locally reverses direction without dissociation. On three natural substrates, independent of its initial binding position, FtsK efficiently translocates over long distances to the terminal region of the E. coli chromosome, as it does in vivo. Our results imply that FtsK is a bidirectional motor that changes direction in response to short, asymmetric directing DNA sequences. 相似文献