共查询到20条相似文献，搜索用时 250 毫秒
1.
Lawton JH 《Science (New York, N.Y.)》1995,269(5222):328-331
Some of the classic experiments in ecology have involved real organisms interacting in the laboratory, that is, model systems. Ecologists are increasingly using model systems to investigate problems of global environmental change and questions about the assembly, persistence, and stability of complex communities. Model laboratory systems are a halfway house between mathematical models and the full complexity of the field, and they yield powerful insights into the dynamics of populations and ecosystems. 相似文献
2.
Alferness RC 《Science (New York, N.Y.)》1986,234(4778):825-829
Optical fiber is rapidly becoming the transmission medium of choice for new telecommunication systems. For a true lightwave network to evolve, however, optical control devices such as optical switches and multiplexer-demultiplexers will be essential. Optical guided-wave devices built with photolithographic fabrication techniques and electro-optic substrates are compact, low drive power devices that provide these functions. Research is particularly advanced on integrated-optic devices based on waveguides formed by titanium diffused into lithium niobate. Switch arrays, tunable filters, and high-speed modulators have been demonstrated and used in research systems experiments. 相似文献
3.
Maccoun RJ 《Science (New York, N.Y.)》1989,244(4908):1046-1050
Because trial juries deliberate in secrecy, legal debates about jury functioning have relied heavily on anecdote and speculation. In recent years, investigators have begun to challenge many common assumptions about jury behavior. An important tool in this effort has been the mock jury experiment, in which research participants are randomly assigned to alternative trial conditions and asked to reach a verdict in a simulated case. Researchers have used mock jury experiments to test hypotheses about causal influences on jury behavior and to develop theoretical models of the jury deliberation process. 相似文献
4.
Simon HA 《Science (New York, N.Y.)》1974,183(4124):482-488
I have explored some of the interactions between research on higher mental processes over the past decade or two and laboratory experiments on simpler cognitive processes. I have shown that, by viewing experimentation in a parameter-estimating paradigm instead of a hypothesis-testing paradigm, one can obtain much more information from experiments-information that, combined with contemporary theoretical models of the cognitive processes, has implications for human performance on tasks quite different from those of the original experiments. The work of identifying and measuring the basic parameters of the human information processing system has just begun, but already important information has been gained. The psychological reality of the chunk has been fairly well demonstrated, and the chunk capacity of short-term memory has been shown to be in the range of five to seven. Fixation of information in longterm memory has been shown to take about 5 or 10 seconds per chunk. Some other "magical numbers" have been estimated-for example, visual scanning speeds and times required for simple grammatical transformations-and no doubt others remain to be discovered. But even the two basic constants discussed in this article-short-term memory capacity and rate of fixation in long-term memory-organize, systematize, and explain a wide range of findings, about both simple tasks and more complex cognitive performances that have been reported in the psychological literature over the past 50 years or more. 相似文献
5.
解析了农业文化遗产的概念,认为农业文化遗产概念专注于人类赖以生存的农业系统、农业景观、传统农业知识体系、农业生物多样性等,指出其核心思想是"遗产不仅是关于过去的,更是关乎未来的"。在参考大量相关文献的基础上,认为目前关于农业文化遗产的研究主要集中在农业生物多样性保护、农业文化遗产系统的生态价值和生态机制、传统农业知识的保护、社区参与机制、农业文化遗产保护的相关政策法规以及农业文化遗产的动态保护探索方面。强调了农业文化遗产旅游是农业文化遗产动态保护的重要方式,分析了其不同于乡村旅游、农业旅游以及"农家乐"旅游等形式的特点,认为其本质是一种遗产旅游,其重要功能是确立地方的文化身份。研究表明,农业文化遗产作为一种旅游资源具有广阔的发展前景。 相似文献
6.
User participation is a critical ingredient for relevant technology development, whether in agriculture or industry. This has long been recognized in private sector R&D firms. In most public sector agricultural research organizations in developing countries, however, systematic involvement of farmers, especially poor farmers, in research has been weak. These farmers are rarely powerful or well organized enough to bring pressure to bear on government agencies to respond to their needs and priorities. Farmer-responsive research methods, such as on-farm research, farming systems research, and farmer participatory research, have been introduced into research organizations to compensate for the lack of mechanisms for bringing farmers' views into the formulation of research priorities and agendas. The impact of these approaches in achieving this objective, however, has been less than hoped for. Insufficient attention to the political and institutional dimensions of developing client-responsive research is a major reason for this lack of impact. To bring about permanent change, farmer-responsive research methods need to be reinforced by changes in the balance of power between research and its clients and in the constellation of decisionmakers responsible for formulating research agendas. Participatory planning methods applied at the level of research programs provide new opportunities for involving farmers in decision-making about program priorities and for systematically incorporating information about client's needs. Recent experiments with strengthening farmers' associations and linking them with research organizations suggest new opportunities for increasing farmers' ability to express demand, act as an external pressure group, and serve as viable partners with research organizations. 相似文献
7.
When two bodies of liquid merge, their interfaces must also rupture and rearrange into one. Virtually no information is available concerning the small-scale dynamics of this process. Molecular dynamics simulations of coalescence in systems of about 10,000 Lennard-Jones particles have been performed, arranged so as to mimic laboratory experiments on dense liquids. The coalescence event begins when molecules near the boundary of one liquid body thermally fluctuate into the range of attraction of the other, forming a string of mutually attracting molecules. These molecules gradually thicken into a tendril, which continues to thicken as the bodies smoothly combine in a zipper-like merger. 相似文献
8.
The conventional theory of metals is in crisis. In the past 15 years, there has been an unexpected sprouting of metallic states in low-dimensional systems, directly contradicting conventional wisdom. For example, bosons are thought to exist in one of two ground states: condensed in a superconductor or localized in an insulator. However, several experiments on thin metal-alloy films have observed that a metallic phase disrupts the direct transition between the superconductor and the insulator. We analyze the experiments on the insulator-superconductor transition and argue that the intervening metallic phase is bosonic. All relevant theoretical proposals for the Bose metal are discussed, particularly the recent idea that the metallic phase is glassy. The implications for the putative vortex-glass state in the copper oxide superconductors are examined. 相似文献
9.
There has been concern about the possibility of selection bias in cloud seeding experiments. Covariates and experimental design have been used to obtain an estimate of this bias. The results indicate that there was no selection bias in the Caribbean and Florida series of cloud seeding experiments. 相似文献
10.
Nieto P Pijper E Barredo D Laurent G Olsen RA Baerends EJ Kroes GJ Farías D 《Science (New York, N.Y.)》2006,312(5770):86-89
The Born-Oppenheimer approximation of uncoupled electronic and nuclear motion is a standard tool of the computational chemist. However, its validity for molecule-metal surface reactions, which are important to heterogeneous catalysis, has been questioned because of the possibility of electron-hole pair excitations. We have performed experiments and calculations on the scattering of molecular hydrogen from a catalytically relevant metal surface, obtaining absolute probabilities for changes in the molecule's velocity parallel to the representative Pt(111) surface. The comparison for in-plane and out-of-plane scattering and results for dissociative chemisorption in the same system show that for hydrogen-metal systems, reaction and diffractive scattering can be accurately described using the Born-Oppenheimer approximation. 相似文献
11.
Combine Harvester Control Using Real Time Kinematic GPS 总被引：3，自引：0，他引：3
An accurate vehicle guidance system is required for some precision agriculture operations. It can be used for high speed direct seeding on a farm tractor. Such a system also ensures neither overlapping nor missing areas during the harvest, even when visibility is poor. But, automatic path following in the field is a difficult problem. Many vehicle guidance systems have been studied. Some use two dimensional (2D) information, while others are based upon 3D information. Most of the sensors use output information relative to their environment without absolute reference of the path. As the new reference is based on the previous pass, one of the main problems encountered with the edge following systems is an increase in the amplitude of oscillations due to guidance errors in successive passes. Thus, human operators frequently and periodically sacrifice efficiency in one pass to straighten out the edge of the worked area. Another kind of sensor, centimeter accuracy Real-Time Kinematic based Global Positioning System (GPS) receivers, can be used. The advent of accurate systems, with a standard deviation from the mean of about 10 mm, allows the design and implementation of absolute vehicle guidance systems. A few approaches to GPS-based control systems also include attitude measurement sensors such as fiber optic gyroscope. The preliminary work presented in this paper was aimed at validating the use of a GPS receiver in a vehicle guidance system, without any orientation sensor. We have designed a controller to perform a line-following task. Real-time experiments have been carried out on a combine harvester. 相似文献
12.
通过试验对土壤速效磷测定过程中存在的待测液脱色问题进行探讨,初步得出以下结果:(1)样品中加入0.25g含磷活性炭,对待测液的脱色效果较好,但对养分含量较低的土样测定结果有一定的影响,使样品的测定结果大约增加了1mg/kg。(2)在没有无磷活性炭的情况下,对于有颜色的待测液,可根据颜色深浅,适当减少待测液吸样量进行测定,对样品的测定结果没有影响。 相似文献
13.
14.
Brain barrier tissues: end organs for atriopeptins 总被引：8，自引：0，他引：8
Little is known about the pathophysiology of cerebral edema and other disturbances of water balance that involve the barrier tissues at the interface of blood and brain. The present experiments show that these barrier tissues contain receptors and second messenger systems for atriopeptins, recently identified cardiac peptides involved in peripheral water regulation. They also show that atriopeptins can alter the rate of cerebrospinal fluid production. Because the blood-brain and blood-cerebrospinal fluid barriers are involved in normal water movements in the central nervous system, these studies suggest that brain barrier tissues may be important end organs for the atriopeptins and that atriopeptins could have therapeutic application to disorders of water balance in the central nervous system. An isolated, purified population of atriopeptin receptor cells, obtained from choroid epithelium, was used in these experiments. This cell population may provide a valuable model system for investigating the intracellular biochemical mechanisms through which atriopeptins exert their actions. 相似文献
15.
【目的】筛选适合陕南茶区种植的无性系茶树良种,确定主推品种,加快陕南茶园无性系良种化进程,促进陕西茶产业的发展。【方法】以引进的17个国内适制绿茶的茶树无性系良种为材料,根据引种苗成活率和抗病虫害情况进行初选。之后以福鼎大白为对照,比较初选品种的物候期、产量和制茶品质等指标,筛选出综合性状表现较好的品种。【结果】初选出龙井长叶、舒茶早、乌牛早、平阳特早、翠峰、福鼎大白(对照)等6个表现较好的茶树品种。物候期上,平阳特早发芽最早,其次是龙井长叶和翠峰,其发芽期比福鼎大白早10～15 d。从发芽密度、百芽质量、春梢长度和着叶数量、扦插成苗率、出圃率和移栽成活率等指标综合来看,龙井长叶、平阳特早和翠峰3个品种表现较好。在产量上,翠峰、龙井长叶和平阳特早均高于其余品种,经方差分析其产量极显著高于对照福鼎大白。龙井长叶、平阳特早、翠峰的制茶品质较优,以香气显花香为特点。【结论】龙井长叶、平阳特早和翠峰的综合性状表现明显优于其他品种,可作为陕南茶区当前主推的无性系茶树良种。 相似文献
16.
P Diaconis 《Science (New York, N.Y.)》1978,201(4351):131-136
In search of repeatable ESP experiments, modern investigators are using more complex targets, richer and freer responses, feedback, and more naturalistic conditions. This makes tractable statistical models less applicable. Moreover, controls often are so loose that no valid statistical analysis is possible. Some common problems are multiple end points, subject cheating, and unconscious sensory cueing. Unfortunately, such problems are hard to recognize from published records of the experiments in which they occur; rather, these problems are often uncovered by reports of independent skilled observers who were present during the experiment. This suggests that magicians and psychologists be regularly used as observers. New statistical ideas have been developed for some of the new experiments. For example, many modern ESP studies provide subjects with feedback--partial information about previous guesses--to reward the subjects for correct guesses in hope of inducing ESP learning. Some feedback experiments can be analyzed with the use of skill-scoring, a statistical procedure that depends on the information available and the way the guessing subject uses this information. 相似文献
17.
常用坐标系统换算的数学模型及程序设计 总被引：1，自引：0，他引：1
为了解决ＧＰＳ在导航及测量应用中不同坐标系统之间的换算关系，作者研究了各系统之间的几何关系，着重讨论了建立转换数学模型的有关问题及计算机通用程序编写的基本思路。文中附有几种常用坐标系统之间的转换参数，以给出一个坐标转换的参考模型。 相似文献
18.
对芦笋在冷藏及焯烫、冷冻、制罐、清炒等不同加工条件下的感官品质变化与细胞结构变化进行了研究。通过实验可知在各种加工及冷藏条件下,芦笋的感官质量与组织结构中的表皮、薄壁组织、导管等均会受到影响,而且与加热时间的长短、温度的高低及是否受冻有密切的关系。 相似文献
19.
本文简要介绍了新型硬质合金麻花钴的结构特点,对新型硬质合金麻花钴钻削时的轴向力、切削变形、加工效率、孔的加工精度和表面粗糙度、断屑性能等进行了试验,并与普通麻花钻进行了比较.试验结果表明,新型硬质合金麻花钴的切削性能明显优于普通麻花钻,是一种性能优秀的新钻型. 相似文献
20.
Knobler CM 《Science (New York, N.Y.)》1990,249(4971):870-874
Monolayers formed at the interface between air and water can be seen with fluorescence microscopy. This allows the phase behavior of these monolayers to be determined by direct observation and opens up the possibility of following the kinetics of phase transformations in two-dimensional systems. Some unexpected morphologies have been discovered that provide information about the nature of monolayer phases and have connections to pattern formation in other systems. 相似文献