全文获取类型
收费全文 | 36410篇 |
免费 | 153篇 |
国内免费 | 277篇 |
专业分类
系统科学 | 588篇 |
丛书文集 | 831篇 |
教育与普及 | 116篇 |
理论与方法论 | 117篇 |
现状及发展 | 14451篇 |
研究方法 | 1306篇 |
综合类 | 18610篇 |
自然研究 | 821篇 |
出版年
2013年 | 401篇 |
2012年 | 677篇 |
2011年 | 1535篇 |
2010年 | 436篇 |
2009年 | 411篇 |
2008年 | 770篇 |
2007年 | 905篇 |
2006年 | 915篇 |
2005年 | 840篇 |
2004年 | 694篇 |
2003年 | 624篇 |
2002年 | 561篇 |
2001年 | 1034篇 |
2000年 | 1078篇 |
1999年 | 696篇 |
1992年 | 584篇 |
1991年 | 524篇 |
1990年 | 549篇 |
1989年 | 484篇 |
1988年 | 512篇 |
1987年 | 503篇 |
1986年 | 483篇 |
1985年 | 628篇 |
1984年 | 517篇 |
1983年 | 413篇 |
1982年 | 364篇 |
1981年 | 371篇 |
1980年 | 481篇 |
1979年 | 1037篇 |
1978年 | 825篇 |
1977年 | 821篇 |
1976年 | 670篇 |
1975年 | 770篇 |
1974年 | 995篇 |
1973年 | 873篇 |
1972年 | 870篇 |
1971年 | 1020篇 |
1970年 | 1312篇 |
1969年 | 1048篇 |
1968年 | 947篇 |
1967年 | 987篇 |
1966年 | 894篇 |
1965年 | 665篇 |
1959年 | 385篇 |
1958年 | 578篇 |
1957年 | 443篇 |
1956年 | 377篇 |
1955年 | 372篇 |
1954年 | 372篇 |
1948年 | 244篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
1.
This is a contribution towards a history and philosophy of modeling in its early stages in electromagnetism. In 1873, James Clerk Maxwell (1831–1879) hinted at the methodology of modeling at the end of his Treatise on Electricity and Magnetism. We focus on Maxwell's impact on physicists who immediately followed him, specifically Oliver Lodge (1851–1940) and George Francis FitzGerald (1851–1901). We begin with the role that the scientific concept of model played in the late nineteenth century, as assessed by Ludwig Boltzmann (1844–1906). We then discuss the role of hypothesis as a methodology, the appeal to (dynamical) illustration, and the way Maxwell applied model and working model in his studies of electromagnetism. We show that for Maxwell these key terms were kept distinct, but Lodge did not maintain these distinctions and, in this regard, FitzGerald followed Lodge. Notwithstanding Lodge's influence, Fitzgerald modified Maxwell's theory based on the mechanical model he designed, thereby implicitly taking the first step towards modeling. This methodology consists in drawing consequences from the (mechanical) model to the (electrodynamic) theory and modifying the latter in light of the functioning of the former. At the core of our argument is the thesis that it was a methodological novelty to move from the concept of model to the methodology of modeling. The introduction of modeling as a new methodology into physics in the late nineteenth century was a major event which deserves proper recognition. 相似文献
2.
R.S.K. Barnes 《Journal of Natural History》2018,52(1-2):87-113
Areas of the Knysna estuarine bay in the Western Cape are dominated by three endemic South African truncatelloid microgastropods, temporarily known as ‘Hydrobia’ knysnaensis (Krauss), ‘Assiminea’ capensis (Sowerby) and ‘Assiminea’ globulus Connolly. Although first described 80–170 years ago and present in abundance (up to 100,000 m?2), they remain surrounded by confusion and still await taxonomic assignment, largely because they appear most atypical members of their groups by virtue of anatomy and/or biogeography and/or habitat. This study contributes in-life perspectives to morphological and phylogenetic analyses known to be on-going. At Knysna, they are syntopic: at least two occurring in >85% and all three in >40% of individual 0.0026 m2 samples from their region of dominance. Nevertheless, they tend to greater abundance in divergent microhabitats; ‘A.’ globulus dominating higher tidal levels, and ‘A.’ capensis and ‘Hydrobia’ lower ones; the former especially unvegetated sediment, the latter, if anything, seagrass. Interspecific feeding interactions appear unlikely to be responsible for these patterns, other evidence suggesting that all are maintained below carrying capacity. Field biology of ‘H.’ knysnaensis generally appears equivalent to that of northern-hemisphere intertidal hydrobiids and that of ‘A.’ globulus is typically assimineid, albeit at atypically low shore height. Unlike assimineids, however, ‘A.’ capensis is truly aquatic. The success of these truncatelloids in unusual circumstances may be consequent on the absence from South Africa of other microgastropod groups that fill their niches elsewhere in the southern hemisphere. 相似文献
3.
4.
5.
Russell L. Burke Beverly Clendening Alexandra Kanonik 《Journal of Natural History》2018,52(25-26):1723-1732
Clutch size (CS) is a well-studied life history characteristic, and optimal egg size theory predicts that CS should correlate positively with reproductive investment. Turtles are good subjects for studies of reproductive strategies because few turtle species exhibit parental care; therefore quantifying their short-term reproductive investment is mostly limited to measuring egg size and number. Clutch size is usually reported as an average value for each turtle population, and where CS variation is noted, it is usually attributable to variation in adult body size. In two long-term studies of ecologically dissimilar species we detected a dramatic increase in CS in a common snapping turtle (Chelydra serpentina) population and in a diamondback terrapin (Malaclemys terrapin) population. It is unknown whether these changes are due to either variation in adult body size or resource availability; but the temporal patterns we observed have apparently never been reported previously. These trends remain unexplained and should be explored further. 相似文献
6.
刘伟霞 《科技情报开发与经济》2004,14(6):147-148
对矿井索车优化设计中的几个关键性问题进行了探讨,包括驱动轮和尾轮的设计、钢丝绳直径的选择计算、托绳轮间距的确定、绳卡的设计、张紧力和张紧行程的确定、互锁问题的解决等。 相似文献
7.
The shrink proofing on wool with treatment of protease named Argaenzyme STL was studied. The various pretreating auxiliaries, different parameters for protease treating process and the effect of stabilizer were discussed in detail. The varieties of some properties before and after protease treatment were also investigated. 相似文献
8.
西文期刊回溯建库中几个重要项目的著录 总被引:4,自引:0,他引:4
在实践的基础上,从期刊的出版日期、出版周期、卷、期、年、月及其他标识、ISSN、增刊、出版者以及期刊的历史发展等方面对西文期刊机读目录的标准化著录问题进行了探讨。 相似文献
9.
关于21世纪图书馆员继续教育问题的思考 总被引:4,自引:1,他引:3
罗雪晴 《科技情报开发与经济》2004,14(6):14-15
论述了21世纪对图书馆员进行继续教育的必要性,介绍了图书馆员继续教育的内容,针对我国图书馆员继续教育的状况,提出了应采取加大投入、进行针对性培训、运用激励机制等措施来加强对图书馆员的继续教育。 相似文献
10.