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   8865篇
  免费   318篇
  9183篇
  2020年   126篇
  2019年   170篇
  2018年   181篇
  2017年   233篇
  2016年   301篇
  2015年   189篇
  2014年   176篇
  2013年   2264篇
  2012年   221篇
  2011年   266篇
  2010年   252篇
  2009年   213篇
  2008年   205篇
  2007年   176篇
  2006年   166篇
  2005年   121篇
  2004年   116篇
  2003年   106篇
  2002年   117篇
  2001年   129篇
  2000年   141篇
  1999年   174篇
  1998年   116篇
  1997年   80篇
  1996年   89篇
  1995年   98篇
  1994年   95篇
  1993年   95篇
  1992年   97篇
  1991年   75篇
  1990年   76篇
  1989年   81篇
  1988年   86篇
  1987年   97篇
  1986年   86篇
  1985年   124篇
  1984年   122篇
  1983年   130篇
  1982年   100篇
  1981年   105篇
  1980年   107篇
  1979年   103篇
  1978年   94篇
  1977年   109篇
  1976年   86篇
  1975年   95篇
  1974年   83篇
  1973年   79篇
  1972年   62篇
  1971年   65篇
排序方式: 共有9183条查询结果,搜索用时 15 毫秒
41.
42.
Gas liquid chromatography was used to analyse residues adhering to the inside of excavated potsherds from dated South African sites. The results suggested marine animal fat, possibly seal, had been cooked in the vessels. Fat of modern seals was baked and also analysed as a control sample. This gave similar results. The importance of seals in the prehistoric diet of the Southwestern Cape is supported by a large number of seal bones in the associated faunal material.  相似文献   
43.
44.
During the 7970s the structure of local government in Scotland was transformed from a traditional system based on counties and burghs to a nested spatial hierarchy of regions and districts. This reform represents the last stage of an evolutionary process, uniting the formerly separate rural and urban hierarchies of local government and administration. The city region was taken to be the guiding principle for reform, and the new regions were identified mainly on the basis of town hinterlands. Throughout all reforms there has been a tendency to concentrate local government functions at higher levels of authority.
Pendant les années 1970 la structure du gouvernement urbain et rural en Ecosse fut transformeé d'un système traditionnel de comptés et bourgs en une hiérarchie spatiale de régions et de districts. Une réforme récente qui unifie pour la première fois les échelons ruraux et urbains des administrations et des gouvernements locaux, représente la phase ultime d'une évolution vers cette hiérarchie. Cette réforme a été guidée par l'utilisation de la zone d'influence urbaine comme unité de base, et les nouvelles régions ont été identifiées surtout sur la base des aires tributaires des villes. Au cours des réformes il y avait une tendance è concentrer les fonctions du gouvernement local è des niveaux supérieurs d'autorité.  相似文献   
45.
46.
A chronic feature of Caribbean agriculture is the extent to which potential arable land lies idle. Some of this region's densely populated islands have more than one-third of their farmland in such a condition. Yet their populations have relied increasingly upon imported food for sustenance. During the 1970s this paradoxical situation was exemplified by Grenada, where possibly as much as 45 per cent of its farmland lay abandoned and imported food accounted for one-third of the national import bill. This paper first surveys the underlying factors accounting for idle land in this nation and then outlines the programs pursued by the People's Revolutionary Government (PRG) to reduce waste of this basic resource and, thereby, to effect a more stable economy.
Dans la scène agraire caraibe on trouve constamment des terres dormantes qui pourraient ětre cultivées. Quelques les, des plus fortement peuplées de la région, ont plus du tiers de leurs terres ambles dans une telle condition. Toutefois, leurs habitants se fient de plus en plus sur la nourriture importée pour se soutenir. Cette situation paradoxale s'illustre par la Grenade, des années soixante-dix, où jusqu'a 45 pour-cent du terroir aurait été abandonné, en méme que la nourriture importée se payait du tiers du budget disponible pour défrayer toutes les importations requises dans ľle. Cette communication souligne ďabord les facteurs qui ont Créé cette situation en Grenade, et présente ensuite les grandes lignes des programmes adoptés, par le gouvernement révolutionnaire du peuple, pour couiger cette situation et pour diriger ľle vers une économie plus stable qu'au paravant.  相似文献   
47.
48.
49.
Nearshore water circulation and bar patterns at Coledale, New South Wales, were surveyed daily over thirty-six consecutive days, from 3 November to 8 December 1978, to identify sequences of changes that might occur in response to spring-to-spring tide cycles of fourteen days, changing barometric and wind conditions associated with the passage of anticyclonic weather systems across the coast, and day to day variation in the nearshore wave regime. During the survey the bar patterns tended to vary from transverse bars, through a composite arrangement incorporating longshore and transverse bars, to a longshore bar pattern as the tide changed from its spring to neap tide phase. However, this sequence was too brief to unequivocally establish variations in nearshore bar pattern that might be attributed to the fourteen day, spring-to-spring tide cycle. Nevertheless, morphologic changes occurring in response to wave regime fluctuation at Coledale were broadly in accord with studies from elsewhere by Dolan et al. (1979, 1982), and Clarke and Eliot (1982, 1983) in that bars and rips were systematically distributed along the beach. At Coledale, bars tended to relocate approximately 0.4 and 0.75 of the distance along the beach from its southern end. The preferred bar spacing closely matches hypothetical standing waves trapped between the headlands of the Coledale embayment. Temporally, the nearshore morphology is highly responsive to wave regime changes such that, as primary breaker heights decrease and remain below Hmax=1.0m, the bar pattern changes from longshore through mixed to transverse bar patterns within three to five days. On one occasion the bar pattern switched from a longshore to a transverse bar pattern within twenty-four hours.  相似文献   
50.
The Multiregional Variable Input-Output (MRVIO) model is introduced to measure the development impact of a transportation system. The MRVIO model is a theoretically flexible, computationally simple, and cost-responsive model. Under the MRVIO model, regional input-output coefficients, trade coefficients, and trade flows become cost responsive and easy to compute. The MRVIO model is employed to measure the development impact of the Arkansas waterway during the period of 1974 to 1978. In the study, the U.S. economy is disaggregated into 3 regions and 35 industrial sectors. The MRVIO model estimates the economic conditions of these 3 regions with and without the waterway in terms of industrial output, income, employment, and trade flows.  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