全文获取类型
收费全文 | 395854篇 |
免费 | 80324篇 |
国内免费 | 104312篇 |
专业分类
测绘学 | 65547篇 |
大气科学 | 75319篇 |
地球物理 | 70346篇 |
地质学 | 204468篇 |
海洋学 | 67740篇 |
天文学 | 6887篇 |
综合类 | 40235篇 |
自然地理 | 49948篇 |
出版年
2024年 | 5085篇 |
2023年 | 16847篇 |
2022年 | 19780篇 |
2021年 | 21288篇 |
2020年 | 17757篇 |
2019年 | 22978篇 |
2018年 | 21976篇 |
2017年 | 15165篇 |
2016年 | 17064篇 |
2015年 | 20164篇 |
2014年 | 26768篇 |
2013年 | 24499篇 |
2012年 | 26365篇 |
2011年 | 26705篇 |
2010年 | 25871篇 |
2009年 | 25232篇 |
2008年 | 24607篇 |
2007年 | 22861篇 |
2006年 | 21895篇 |
2005年 | 18877篇 |
2004年 | 17706篇 |
2003年 | 16275篇 |
2002年 | 14726篇 |
2001年 | 13561篇 |
2000年 | 11244篇 |
1999年 | 8859篇 |
1998年 | 6308篇 |
1997年 | 5490篇 |
1996年 | 4983篇 |
1995年 | 4705篇 |
1994年 | 4466篇 |
1993年 | 4230篇 |
1992年 | 4192篇 |
1991年 | 3795篇 |
1990年 | 3598篇 |
1989年 | 3831篇 |
1988年 | 3984篇 |
1987年 | 3856篇 |
1986年 | 3292篇 |
1985年 | 3341篇 |
1984年 | 3006篇 |
1983年 | 2817篇 |
1982年 | 2343篇 |
1981年 | 1766篇 |
1980年 | 1302篇 |
1979年 | 832篇 |
1978年 | 413篇 |
1959年 | 307篇 |
1958年 | 334篇 |
1957年 | 317篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
781.
782.
783.
784.
本文利用地面实测重力资料和地形高程资料,采用普拉特-海福特(Pratt-ttayford)重力均衡理论模型,取1°×1°方格网,通过使用现成改正表格查取改正值与个别计算点用理论公式计算作校核的方法,计算了我国东北地区75个计算点的均衡重力异常值;并对局部第四系覆盖较厚地区作了第四系密度改正;在此基础上,构制了我国东北N39°—49°,E121°—131°大部分地区的均衡重力异常图;结合区域布格重力异常和区域空间重力异常特征以及莫霍界面的起伏特点作了对比分析和讨论 相似文献
785.
786.
We have used observations of the rare isotopes of HCN and HNC to determine the relative abundance of these two chemical isomers along the central ridge of the Orion molecular cloud. The abundance ratio [HCN]/[HNC] decreases by more than an order of magnitude from the relatively warm plateau and hot core sources toward the KL nebula to the colder, more quiescent clouds to the north and south. Even in the cooler regions, however, the ratio is an order of magnitude larger than that found in previous investigations of cold dark clouds. We determine the kinetic temperature in the regions we have studied from new observations of methylacetylene (CH3CCH), together with other recent estimates of the gas temperature near KL. The results suggest that the warmer portions of the cloud are dominated by different chemical pathways than those in the general interstellar cloud material. 相似文献
787.
Kumar K Taragi RC 《Population geography : a journal of the Association of Population Geographers of India》1986,8(1-2):58-63
There are great spatial variations in the distribution of the fast growing populations of India. About 70% of the 1981 population is concentrated in only 40% of the geographical area. Such a pattern of population concentration, with its spatio-temporal dimension, can be understood better if examined at the macro-regional level considering the physico-climatic and socio-economic diversity of the country. This is done by way of analyzing district level data for India as a whole and also for its various macro-regions. The study reveals that the Great Plains and Coastal Plains and Islands have a homogenous distribution and a higher population increase. The Deccan Plateau shows the most homogenous dispersion of population. The population is declining in the Northern Mountains and the Great Plains but accentuating in the Coastal Plains and Islands and the Deccan Plateau. The north and south of India are showing opposite trends in population concentration. 相似文献
788.
Trace and RE element geochemistry and genesis have been studied with respect to ferromanganese nodules from the sediments of the Pacific, Atlantic and Indian oceans. 相似文献
789.
A total of 21 elements, Si, Al, K, Na, Ca, Mg, Fe, Mn, Ti, P, Cu, Co, Ni, Zn, Pb, Cr, Rb, Sr Ba, Cd and Corg(organic carbon), was determined in the trough. Average contents of most elements were found to be higher in the trough bottom than in the western and eastern slopes. We call these elements the indicator elements because of their "transition" property. This is in keeping with the "intermediate" geographical position of the trough where the continental crust turns to oceanic crust.Though the authigenic fraction of some elements here increases, the detrital fraction .of most elements is still dominant, indicating that the influences of terrigenous and volcanic detrital materials on the trough sediments are still dominant.The abundant biogenous component in the trough is characterized by the increase of Ca, Sr and Corg contents. Ca existes mainly in the form of CaCo3 that is mostly found in various shells. Sr often easily replaces Ca within the shells.A belt-shaped distribution pattern of elements wa 相似文献
790.
"A modification of the population potential model is used to analyze the structure of the potential of population for cities of the Moscow region, to develop a relationship between population potential and city size, and to use this procedure to forecast future population change in Moscow Oblast cities." Cities in the region with a population of 50,000 or more at the censuses of 1959, 1970, and 1979 are included. 相似文献