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81.
William A. Koelsch 《Geographical review》2008,98(2):260-279
ABSTRACT. Thomas Jefferson (1743–1826) had a lifelong interest in geography. Except for his role in the Lewis and Clark Expedition and a few references to his Notes on the State of Virginia, however, geographers have taken a relatively slight interest in this aspect of his thought, despite his having sometimes been referred to as “one of the greatest American geographers.” This essay suggests that we need to reexamine Jefferson as a geographical thinker. Reviewing some of the more important literature thus far, it suggests where such topics may profitably be extended and points to some aspects of his geographical interests not yet incorporated into the geographical literature. 相似文献
83.
WILLIAM E. O'BRIEN 《Geographical review》2012,102(2):166-179
African Americans had access to only a small number of state parks in the Jim Crow South. Between the end of World War II and the U.S. Supreme Court's 1954 Brown v. Board of Education decision, however, state park officials oversaw a relative expansion in the construction of facilities available to southern blacks. Emphasizing developments leading up to that landmark court ruling, I note that this trend did not indicate waning support for segregation among whites. Rather, this relative expansion was part of a strategy to protect Jim Crow by demonstrating that the “separate‐but‐equal” principle was being successfully achieved by southern park agencies. This intensified construction was largely a reaction to increasingly successful legal action in federal courts by the naacp generally; and officials in the state agencies hoped—unsuccessfully—to avoid challenges to state park segregation. After the Brown decision, several border states integrated their park systems, but most agencies displayed the reactionary defiance that characterized most white Southerners as the civil rights movement grew. 相似文献
84.
Analytical and Numerical Study of the Mechanics of Rockbolt Reinforcement around Tunnels in Rock Masses 总被引:1,自引:0,他引:1
C. Carranza-Torres 《Rock Mechanics and Rock Engineering》2009,42(2):175-228
Summary This paper addresses the problem of quantifying the mechanical contribution of rockbolts installed systematically around tunnels
excavated in rock masses. The mechanical contribution referred to here is that of increased stress confinement and decreased
tunnel convergences as compared with corresponding stresses and displacements obtained for non-reinforced tunnels. The problem
is treated analytically first by presenting a closed-form solution for stress and displacement distributions around a circular
tunnel excavated in elastic material and reinforced by grouted or anchored rockbolts. The analytical solution assumes that
rockbolts are regularly spaced around the tunnel and that axi-symmetry conditions of geometry and loading apply. The results
obtained with the closed-form solution are shown to be equivalent to the results of the same problem solved with traditional
numerical methods. Based on the analytical and numerical results and by introducing dimensionless ratios that allow to quantify
the increase of radial stresses and the decrease of radial displacements in the reinforced region of the tunnel, the paper
shows that reinforcement can have a significant mechanical effect (i.e., increasing the confinement and decreasing the convergences)
in tunnels excavated in rock masses of poor to very poor quality. The paper analyzes then the mechanical contribution of rockbolt
reinforcement when the rock mass is assumed to behave elasto-plastically. For this case, it is shown that rockbolt reinforcement
can also have a critical effect in controlling the extent of the plastic failure zone and the convergences of the tunnel.
Correspondence: C. Carranza-Torres, Department of Civil Engineering, University of Minnesota, Duluth Campus, 1305 Ordean Court,
Duluth, USA 相似文献
85.
Xabier Pontevedra-Pombal Daniel Castro Martín Souto Isabel Fraga William H.Blake Maarten Blaauw José A.López-Sáez Sebastián Pérez-Díaz Marcos Valcárcel Eduardo García-Rodeja 《地学前缘(英文版)》2019,10(4):1521-1533
The northwest region of the Iberian Peninsula is home to a unique ecosystem of bogs,which are particularly sensitive to projected climate cha nge.In this context,the rate of carbon(C)accumulation in Chao de Veiga Mol.an intact raised bog,was analysed.Changes in the accumulation rate over the past10 millennia were determined in a peat core of 847 cm in depth,with a high mean rate of peat growth(11 yr cm^-1,0.09 cm yr^-1).An age-depth model was generated from 22 14C dates and fallout radionuclides.Chronological,stratigraphical and physico-chemical data confirmed the existence of a single cycle of peat formation throughout the Holocene and the formation of ombrotrophic peat 9500 years ago.The total mean C content was 50.2%,and over 10 millennia 583 kg C m^-2 accumulated at a mean rate of 35.3 g C m^-2 yr^-1,with a long-term(apparent)rate of carbon accumulation in the catotelm of59.9 g C m^-2 yr^-1.These values are much higher than reported for other Iberian peatlands and are amongst the highest documented for peatlands in the northern hemisphere.The dynamics of C accumulation and other measured parameters reveals important variations throughout the Holocene.They could be associated with the main climatic events described in the northern hemisphere and are highly consistent with models established for northern latitudes.The Chao de Veiga Mol raised bog is unique and of great potential value for carrying out high resolution palaeoenvironmental studies,especially in relation to regional and Iocal modulations in southern Europe. 相似文献
86.
Existing experimental evidence suggests that in pure Mg-forsterite with a fixed Mg/Si ratio, vacancies and interstitials on the Mg sublattice are the dominant intrinsic point defects, while O vacancies and conduction-band electrons are the primary oxygen-deficient defects. Orthopyroxene appears to dissolve readily in pure Mg-forsterite at high temperatures, the change in Mg/Si ratio being accommodated by Mg vacancies and Si interstitials; as a consequence, upper-mantle forsterite may be substantially non-stoichiometric. It is postulated that the amount of Fe that can be oxidized in the forsterite phase is directly related to the excess Si present, either incorporated in the structure, or available from another phase richer in SiO2 such as pyroxene. 相似文献
87.
88.
报道了2012年6月份桑沟湾藻华期间的浮游植物和纤毛虫群落结构,并对环境因子进行了初步分析。研究发现,藻华原因种为一种直径为2μm左右的小球型藻类,细胞形态和粒径均与近年来在中国秦皇岛近岸海域频繁引发水华的超微型浮游植物相似。藻华发生期间,调查区海水中藻华原因物种细胞丰度高达109个/L,高于2011年同期调查区该物种藻华期间的细胞丰度(108个/L)。除藻华原因种外,其他浮游植物共鉴定38种,隶属3门23属,以硅藻和甲藻为主,优势种为:具槽帕拉藻(Paralia sulcata)、裸甲藻(Gymnodinium sp.)、圆筛藻(Coscinodiscus sp.)、长菱形藻(Nitzschia longissima)和太平洋海链藻(Thalassiosira pacifica)等。纤毛虫共鉴定3属5种,以砂壳纤毛虫为主,优势种为百乐拟铃虫(Tintinnopsis beroidea)。细胞丰度分布方面,藻华原因种从湾内向湾外逐渐降低;硅藻细胞丰度在位于湾口处的6号站位最高,并向湾内和湾外递减;甲藻和纤毛虫细胞丰度均从湾内向湾外降低,这分别由优势种裸甲藻和百乐拟铃虫的分布决定。分析发现,调查区藻华原因物种细胞丰度与海水温度和层化系数呈良好的正相关性,与盐度呈负相关性;与裸甲藻和百乐拟铃虫细胞丰度呈良好的正相关性,这两种异养微型浮游生物对藻华原因种的摄食能力值得探索。与历史资料的对比发现,调查区藻华期间浮游植物群落多样性下降,群落稳定性降低。 相似文献
89.
南京信息工程大学缘气象而生、因气象而兴,是一所具有显著行业特色的高校。大气科学是南京信息工程大学的学科根基和学科之魂。回顾大气科学60年的发展历程,历经几代人的砥砺奋进,坚持改革创新、特色发展,铸就了大气科学的辉煌成就。20世纪60年代,大气学科为服务国家气象事业急需而创立,改革开放之后,对接中国气象事业发展需要,构建完整的学科体系,并为中国和世界气象事业发展做出了重大贡献,有着“中国气象人才培养摇篮”之美誉。21世纪以来,学科建设进一步加快,2017年大气科学入选国家“双一流”建设学科,开启了世界一流学科建设的新征程。自此,南信大站在历史发展新方位,擘画大气科学发展新蓝图,打造大气学科新高地,推动大气学科建设进入高质量发展新阶段。 相似文献
90.