美国高新技术现状

<正> (九)主要高技术领域的发展概况 (1) 软件工业 美国的软件工业起步于60年代中期,当时联邦政府开始对软件基础研究进行投入。目前,全世界软件工业产值约3000亿美元,到2003年,将以15％的速度增加,软件工业的发展大大地改     

美国归纳逻辑与人工智能研究概况

近年来,美国归纳逻辑研究的一个重要转变是从纯理论的研究转向应用研究,特别是归纳逻辑在人工智能和认知科学中的应用研究。美国哲学家和逻辑学家一方面注重从人工智能的研究中总结和概括有关的归纳逻辑问题;另一方面他们重视在归纳逻辑研究中借鉴人工智能中的逻辑方法。这些研究涉及的主要问题是:第一,关于贝叶斯主义与非贝叶斯主义的争论问题;第二,关于概率解释的问题;第三,关于因果性与概率的关系问题,他们还致力于确立机器学习的归纳逻辑基础,探究非单调推理在归纳逻辑中的应用。     

Residual currents in A.C. discharges.

A point by point determination is made of the residual current flowing during the reignition period of an a.c. arc between pure graphite electrodes in atmospheric air.The air pressure is varied from 0.14 to 5.4 atmospheres absolute and the gap varied from 1/4 to 5 mm. Alternating current of 6o cycles per second is supplied to the arc by a io Kv. power transformer and the r.m.s. discharge current is limited by ohmic resistance to values between o.o5 and 2 amperes. The residual current rises to a peak shortly after the voltage reverses and the current then decreases as the recovery voltage increases to its reignition value. The peak of the residual current increases directly with the r.m.s. current of the arc, and is essentially independent of both the length of the gap and the pressure of the gas. The reignition voltage is shown to be a function of the value of residual current existing just prior to ignition. The rate of decay of residual current decreases as gas pressure and gap length are increased.     

普通高等学校体育课程师资队伍建设的若干思考

采用文献资料法、调查访问法、逻辑分析法和归纳法,论述了我国普通高等学校体育课程师资队伍建设的现状,探讨了存在的问题,提出了对策与建议.     

枫杨中两性花的发现

This paper reports the bisexual structure of the flowers of Pterocarya stenoptera. The bisexual flowers are borne at the end of a leafy shoot of the current year in many-flowered terminal pendulous catkins. They have the same structure as the general female ones. Each flower grows in the axil of a bract,  with a pair of bracteoles and four small perianths. Each flower has two or three carpels in the centre of the flower,  and upon them there are two or three styles with stigmas on the inner face. They differ from the ge- neral female ones in that each of them contains 4-6 stamens,  forming a single whorl. The stamens alternates with,  or is opposite to,  the perianth elements. Sometimes they contain 8 (-10) stamens,  forming two whorls,  with 6 in the outer whorl and 2 (-4) in the in- ner whorl, and in this case the pistil in the bisexual flower of terminal catkins often becomes a rudiment.       It is interesting that we have also found bisexual flowers in another tree,  which are borne in lateral male catkins. They have the same structure as general male ones,  and the pistils are often represented by a rudiment.       Manning (1940) points out that some female flowers of Pterocarya stenoptera and P. fraxinifolia occasionally have stamens ( ? ) opposite the sepals. In P. stenoptera we have found that both the stamens and the stigmas of bisexual flowers are functional. They are capable of producing functional fruits. This is the same case as in Myrica Gale described by Davey and Gibson (1917). Rendle (1952) points out that in the male flowers of Platycarya the pistils often appeared as a rudiment. He considers,  however,  the male flowers derived from the bisexual flowers with an indefinite number of stamens. The rudimentary pistils of later- al male catkins in P. stenoptera we found are just the same as the ones found in Platycarya by Rendle.       The discovery of the bisexual flowers in P. stenoptera may prove that the unisexual flowers of the present-day Juglandaceae are derived from ancestors with bisexual flowers.This tends to support the hypothesis that Cycadicae is the possible ancestor of the angiosperms.     

美国当代高科技现状

<正> 风险投资在美国的高科技发展中发挥了十分重要的作用。自从1946年美国第一个风险投资公司成立后,风险投资发展很快。目前,美国有风险投资公司981家,其中一半在加州的硅谷地区。 1999年,美国的风险投资总额达356亿美元,其中加利福尼亚州占169亿美元,包括硅谷地区134亿美元。90％的风险投资投向与网络有关的领域,4006个公司得到风险投资的支持,其中96％是与技术有关的公司。下表显示美国、加州、硅谷地区和圣地亚哥地区1997年至2000年第二季度的风险投资情况。