全文获取类型
收费全文 | 3717篇 |
免费 | 261篇 |
国内免费 | 468篇 |
专业分类
测绘学 | 184篇 |
大气科学 | 71篇 |
地球物理 | 461篇 |
地质学 | 323篇 |
海洋学 | 1548篇 |
天文学 | 26篇 |
综合类 | 282篇 |
自然地理 | 1551篇 |
出版年
2024年 | 18篇 |
2023年 | 73篇 |
2022年 | 201篇 |
2021年 | 208篇 |
2020年 | 207篇 |
2019年 | 201篇 |
2018年 | 164篇 |
2017年 | 209篇 |
2016年 | 207篇 |
2015年 | 184篇 |
2014年 | 200篇 |
2013年 | 290篇 |
2012年 | 188篇 |
2011年 | 202篇 |
2010年 | 156篇 |
2009年 | 152篇 |
2008年 | 167篇 |
2007年 | 177篇 |
2006年 | 162篇 |
2005年 | 149篇 |
2004年 | 108篇 |
2003年 | 116篇 |
2002年 | 94篇 |
2001年 | 68篇 |
2000年 | 79篇 |
1999年 | 76篇 |
1998年 | 47篇 |
1997年 | 48篇 |
1996年 | 35篇 |
1995年 | 32篇 |
1994年 | 37篇 |
1993年 | 38篇 |
1992年 | 23篇 |
1991年 | 18篇 |
1990年 | 10篇 |
1989年 | 15篇 |
1988年 | 14篇 |
1987年 | 12篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 14篇 |
1981年 | 10篇 |
1980年 | 1篇 |
排序方式: 共有4446条查询结果,搜索用时 78 毫秒
31.
1 .Introduction Since the most significant feature of a movingloadisits mobility,the interaction betweenthe ve-hicle and bridge is very complicated,which can be classified as a coupled vibration problem.There-fore ,much attention has been paid to the dyna… 相似文献
32.
33.
Miriam Gherardi Margherita Sciscioli Elena Lepore Giuseppina Todisco & Adriana Giangrande 《Marine Ecology》2007,28(2):306-314
Different aspects of reproductive biology of the polychaete Timarete filigera (Delle Chiaje) were studied during a period of 2 years (from September 2002 to August 2003 and from September 2004 to August 2005). Timarete filigera specimens were collected monthly in the port of Bari (South Adriatic Sea, Mediterranean Sea). For population analysis, regenerating and entire specimens were separately analyzed, the regenerating individuals were assigned to different categories related to the region of the body from which they derived. Regenerating individuals were present throughout the year. There were three to four times more individuals deriving from the middle part than the anterior and posterior regenerating portions. From the high percentage of regenerating individuals observed in the T. filigera population, it can be supposed that this species reproduces mainly asexually by schizogenesis. However, histological analysis revealed the presence of female gametes in a few individuals. In contrast, males were never collected, probably due to the limited number of reproductive individuals collected. The abundance has shown wide fluctuations during the year, with a decrease from Winter to Spring. The analysis of the size‐frequency histograms has shown a growing phase of whole specimens from Spring to Winter. The life cycle of the investigated species is discussed with the hypothesis that strategies could be linked to the colonized environments, agamic reproduction coupled with sexual reproduction assures the presence and expansion of the population after the colonization by a limited number of larvae. 相似文献
34.
35.
Jrg A. Ott 《Marine Ecology》1980,1(1):47-64
Abstract. In situ investigations of growth and production in a stand of Posidonia oceanica (L.) DELILE at a depth of 4 m at Ischia (Gulf of Naples) were carried out over two growing seasons. Posidonia starts to grow in August and an average bundle produces ten leaves in increasing time intervals until May. Growth curves for the leaves are given. Maximum leaf standing crop is in May with 1300 g dry weight per m-2, leaf area index at this time reaches 22 m2 m-2. Leaf net productivity is highest in March with 12 g dry weight per m2 per day. Annual leaf production is estimated as 3110 g dry weight per m2, “underground” production as 115 g dry weight per m2. About half the leaf production is exported from the system. Adaptive strategies of the growth and production pattern are discussed. 相似文献
36.
37.
Relationships Between Community Structure of the Intertidal Macroinfauna and Sandy Beach Characteristics Along the Chilean Coast 总被引:2,自引:0,他引:2
Eduardo Jaramillo Heraldo Contreras Cristian Duarte & Pedro Quijón 《Marine Ecology》2001,22(4):323-342
Abstract. Eight sandy beaches were seasonally sampled along the coast of Chile, from ca. 21 to 42° S (about 3000 km) to study the relationship between community structure of the intertidal macroinfauna and beach characteristics. Sediment samples (0.1 m2 , 30 cm deep) were collected (July – September 1998 and December 1998 – January 1999) with plastic cylinders at 15 equally spaced levels along three replicated transects extending from above the drift line to the swash zone. The sediment was sieved through a 1 mm mesh and the organisms collected stored in 5 % formalin. To define beach types, Dean's parameter (Ω) was calculated from wave heights and periods, and fall velocity of sand particles from the swash zone. Crustaceans (mainly peracarids) were the most diverse group with 14 species, followed by polychaetes with 5 species. The talitrid amphipod Orchestoidea tuberculata , the cirolanid isopods Excirolana braziliensis and E. hirsuticauda and the anomuran decapod Emerita analoga were the most widely distributed and common species. Regression analyses between species richness, abundance and biomass of the whole macroinfauna versus sediment characteristics, beach face slopes and morphodynamic beach states showed no significant relationships. Thus, macroinfaunal community characteristics did not increase linearly from lower intermediate to higher intermediate or dissipative beach states as had been found before in Chile or in other coasts. A comparative analysis with data from sandy beaches of other world regions showed that the number of species inhabiting Chilean sandy beaches was generally lower, whereas total population abundances were generally higher compared with values reported elsewhere. 相似文献
38.
1 .IntroductionRecentlygreatinteresthasbeenshowninthedevelopmentofverylargefloatingstructuressuchasMegaFloatofJapan (Isobe ,1 999)andMOBofUSA (Remmers ,1 999) .Owingtotheirextremelargesizeandgreatflexibility ,thecouplingbetweenthestructuraldeformationandfluidmotionissignifi cant.Thisisatypicalproblemofhydroelasticity .Efficientandaccurateestimationofthehydroelasticresponseofverylargefloatingstructuresinwavesisveryimportantfordesign .Manymethodshavebeenproposedinliteratureforthepredictiono… 相似文献
39.
40.