首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   11篇
  免费   0篇
  国内免费   3篇
系统科学   2篇
现状及发展   2篇
综合类   9篇
自然研究   1篇
  2020年   2篇
  2018年   1篇
  2013年   1篇
  2011年   2篇
  2010年   1篇
  2008年   3篇
  2007年   1篇
  2005年   1篇
  2003年   1篇
  2002年   1篇
排序方式: 共有14条查询结果,搜索用时 390 毫秒
1.
On going trend of miniaturization in electronic rel at ed parts, which is an average of two times in every 5~7 years introduce grindin g challenges. In grinding process, the surface waviness control of thin parts is an ardent task due to its warpage, induced by the high specific grinding energy (2~10 J/mm 3). Therefore, coolant is often used to avoid thermal damage, obtai n better surface integrity and to prolong wheel life. However coolant, the incomp ressibility media introduce high forces at the gri...  相似文献   
2.
Nineteen exclosures on sagebrush steppe and shadscale rangelands, varying in age from 18 to 38 years, were sampled for plant species richness, plant composition, indicators of soil erosion, ground cover, vegetative cover, and herb-low shrub layer screening cover. Features within the exclosures were compared with adjacent sites of the same size that were open to grazing by livestock and wildlife. Species richness typically was slightly greater inside exclosures than in adjacent grazed sites (median = 2 more species inside exclosures), but the difference was not significant ( P = 0.16). Similarity of plant community composition between exclosures and adjacent grazed sites ranged from 45% to 82%. Evidences of soil movement, soil pedestals, and soil flow patterns were all more pronounced outside exclosures than inside ( P ≤ 0.02), even though many sites were on flat to mild slopes (median slope = 12%). Meta-analysis of the 19 exclosure sites indicated that grazing exclusion resulted in less bare ground cover compared with adjacent grazed sites ( P ≤ 0.05). The effect of grazing exclusion on visible soil surface cryptogams was significant ( P ≤ 0.05), with generally greater cover inside exclosures. Cryptogam cover differences between grazed sites and exclosures tended to increase with the number of years of grazing exclusion ( r = 0.64, P = 0.046). Pseudoroegneria spicata , a principal livestock forage, averaged greater basal cover inside exclosures than outside on 4 of 10 sites where it occurred, although no exclosure sites had greater P. spicata cover on grazed sites. Meta-analysis of the 10 sites indicated that grazing exclusion resulted in greater P. spicata cover compared with adjacent grazed areas ( P ≤ 0.05). Poa secunda , a short-growing grass that initiates growth early in the spring and is not important livestock forage, averaged greater basal cover outside exclosures on 5 of 15 sites where it occurred. Meta-analysis of the 15 sites indicated a significant treatment effect ( P ≤ 0.05), with greater Poa secunda basal cover outside exclosures. Grazing exclusion resulted in greater screening cover in the herb-low shrub layer (0-0.5 m height; P ≤ 0.05). These results indicate that despite improved livestock grazing management over the past half century, livestock grazing still can limit the potential of native plant communities in sagebrush steppe ecosystems, and that the health of semiarid ecosystems can improve with livestock exclusion in the absence of other disturbances. A few exclosure sites were similar for the measured parameters, suggesting that these sites were ecologically stable and that exclusion of livestock grazing was not sufficient to move succession toward more pristine conditions, at least within the time periods studied. Managed disturbance such as fire or mechanical brush treatments may be necessary to restore herb productivity on these ecologically stable sites.  相似文献   
3.
This article aims to reveal the moral and theological dimensions of William Whewell's philosophy of science. It suggests that, in addition to an internalist account of Whewell's method and epistemology, there is a need to view his philosophy of science (and knowledge) within the intellectual context constituted by the assumptions of natural theology. It argues that writers of natural theology saw man's ability to know the world as an indication of his special place in nature, and that epistemological theories were therefore invested with moral and theological significance. Whewell's work is interpreted as an attempt to dissociate natural science from Utilitarianism and empiricist philosophy: he sought to promote a philosophy of science which guaranteed the principles of natural theology and the values of Christianity. But the idealist epistemology which he proposed was criticized by both scientists and theologians. In 1853 (in his book Of the plurality of worlds), again within the framework of natural theology, Whewell attempted to justify this epistemology by affirming the metaphysics of a Christian Platonism. From this position, Whewell defended natural theology against the metaphysical scepticism of both Henry Mansel and the positivists.  相似文献   
4.
This paper focuses on a new optimization problem, which is called “The Multiple Container Packing Problem (MCPP)” and proposes a new evolutionary approach for it. The proposed evolutionary approach uses “Adaptive Link Adjustment Evolutionary Algorithm (ALA-EA)” as a basic framework and it incorporates a heuristic local improvement approach into ALA-EA. The first step of the local search algorithm is to raise empty space through the exchange among the packed items and then to improve the fitness value through packing unpacked items into the raised empty space. The second step is to exchange the packed items and the unpacked items one another toward improving the fitness value. The proposed algorithm is compared to the previous evolutionary approaches at the benchmark instances (with the same container capacity) and the modified benchmark instances (with different container capacity) and that the algorithm is proved to be superior to the previous evolutionary approaches in the solution quality.  相似文献   
5.
This paper focuses on a new optimization problem, which is called "The Multiple Container Packing Problem (MCPP)" and proposes a new evolutionary approach for it. The proposed evolutionary approach uses "Adaptive Link Adjustment Evolutionary Algorithm (ALA-EA)" as a basic framework and it incorporates a heuristic local improvement approach into ALA-EA. The first step of the local search algorithm is to raise empty space through the exchange among the packed items and then to improve the fitness value through packing unpacked items into the raised empty space. The second step is to exchange the packed items and the unpacked items one another toward improving the fitness value. The proposed algorithm is compared to the previous evolutionary approaches at the benchmark instances (with the same container capacity) and the modified benchmark instances (with different container capacity) and that the algorithm is proved to be superior to the previous evolutionary approaches in the solution quality.  相似文献   
6.
7.
Yeo P 《Nature》2007,446(7138):948
  相似文献   
8.
Among the elements of the modern scientific ethos, as identified by R.K. Merton and others, is the commitment of individual effort to a long-term inquiry that may not bring substantial results in a lifetime. The challenge this presents was encapsulated in the aphorism of the ancient Greek physician, Hippocrates of Kos: vita brevis, ars longa (life is short, art is long). This article explores how this complaint was answered in the early modern period by Francis Bacon’s call for the inauguration of the sciences over several generations, thereby imagining a succession of lives added together over time. However, Bacon also explored another response to Hippocrates: the devotion of a ‘whole life’, whether brief or long, to science. The endorsement of long-term inquiry in combination with intensive lifetime involvement was embraced by some leading Fellows of the Royal Society, such as Robert Boyle and Robert Hooke. The problem for individuals, however, was to find satisfaction in science despite concerns, in some fields, that current observations and experiments would not yield material able to be extended by future investigations.  相似文献   
9.
Hydrogenosomes are organelles that produce ATP and hydrogen, and are found in various unrelated eukaryotes, such as anaerobic flagellates, chytridiomycete fungi and ciliates. Although all of these organelles generate hydrogen, the hydrogenosomes from these organisms are structurally and metabolically quite different, just like mitochondria where large differences also exist. These differences have led to a continuing debate about the evolutionary origin of hydrogenosomes. Here we show that the hydrogenosomes of the anaerobic ciliate Nyctotherus ovalis, which thrives in the hindgut of cockroaches, have retained a rudimentary genome encoding components of a mitochondrial electron transport chain. Phylogenetic analyses reveal that those proteins cluster with their homologues from aerobic ciliates. In addition, several nucleus-encoded components of the mitochondrial proteome, such as pyruvate dehydrogenase and complex II, were identified. The N. ovalis hydrogenosome is sensitive to inhibitors of mitochondrial complex I and produces succinate as a major metabolic end product--biochemical traits typical of anaerobic mitochondria. The production of hydrogen, together with the presence of a genome encoding respiratory chain components, and biochemical features characteristic of anaerobic mitochondria, identify the N. ovalis organelle as a missing link between mitochondria and hydrogenosomes.  相似文献   
10.
Systemic Practice and Action Research - Recently, demand-side innovation policies for securing new driving-force for economic growth centered on developed countries, especially on the importance of...  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号