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J A Spring  D H Buss 《Nature》1977,270(5638):567-572
Alcoholic drinks were consumed in larger quantities in the eighteenth and nineteenth centuries than in the twentieth century, although there has been a recent increase from the historical low of 1930-60. Beer, spirits and wines once provided at least 2 MJ (nearly 500 kcal) per person per day compared with 0.67 MJ (160 kcal) in 1975, towards an average energy requirement of the total population little different from that needed now. Beer has always contributed most to the alcohol, energy and nutrient content of the diet, although its importance relative to spirits and wine has declined.  相似文献   

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J H Scholes 《Nature》1979,278(5705):620-624
The orderly layout of visual fibres in the optic nerves of cichlid fishes suggests that they are guided most of the way to the brain by contact with their neighbours. Before they reach the visual map in the tectum, however, their arrangement in the cross-section of the nerve is reorganised in a way which requires longer-range guidance.  相似文献   

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A Kennaway 《Nature》1992,355(6357):198-200
Britain urgently needs to return to traditional, but now abandoned, values in science. Scientific administrators and EC directorates should revise their policies in the light of Japan's experience.  相似文献   

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Thomas GA  Shraiman BI  Glodis PF  Stephens MJ 《Nature》2000,404(6775):262-264
An important scientific and technological goal in the field of optical communications is the achievement of the clarity limit in optical fibres--that is, ensuring that the SiO2 glass from which fibres are made is as transparent as possible. The clarity of the wavelength transmission window (and the width of that window) in existing fibres is already sufficient to form the basis of a world-wide optical communication system, yet it is still limited by contamination of the fibre by water. Here we measure the spatial distribution of water in the glass rods from which optical fibres are drawn and explain the distribution quantitatively with a mathematical model of diffusion in a medium with essentially perfect cylindrical symmetry. Our analysis shows that the water enters from the outside of the rod and diffuses into the molten, flowing glass much faster than is expected from extrapolation of low-temperature measurements. Our elucidation of the physics underlying the contamination process has already led to the fabrication of dry fibres, which have a clarified and broadened communications window. The improved operational range of wavelengths should yield applications for new lasers, optical amplifiers and detectors, and should substantially increase the information-carrying capacity of optical communications systems.  相似文献   

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P P Mitra  J B Stark 《Nature》2001,411(6841):1027-1030
The exponential growth in the rate at which information can be communicated through an optical fibre is a key element in the 'information revolution'. However, as for all exponential growth laws, physical limits must be considered. The nonlinear nature of the propagation of light in optical fibre has made these limits difficult to elucidate. Here we use a key simplification to investigate the theoretical limits to the information capacity of an optical fibre arising from these nonlinearities. The success of our approach lies in relating the nonlinear channel to a linear channel with multiplicative noise, for which we are able to obtain analytical results. In fundamental distinction to linear channels with additive noise, the capacity of a nonlinear channel does not grow indefinitely with increasing signal power, but has a maximal value. The ideas presented here may have broader implications for other nonlinear information channels, such as those involved in sensory transduction in neurobiology. These have been often examined using additive noise linear channel models but, as we show here, nonlinearities can change the picture qualitatively.  相似文献   

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Organochlorine pesticides in rainwater in the British Isles   总被引:1,自引:0,他引:1  
K R Tarrant  J O Tatton 《Nature》1968,219(5155):725-727
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花生麸中膳食纤维含量的测定   总被引:10,自引:0,他引:10  
采用Prosky法测定了花生麸中膳食纤维的含量,测定结果显示花生麸中水溶性膳食纤维含量为9.26%,水不溶性膳食纤维含量为39.58%。  相似文献   

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优化建筑施工教学提高学生专业素质   总被引:6,自引:1,他引:6  
针对教学改革,课时相应压缩的条件下,建筑施工教学面临的问题,提出从精简活化教学内容,改进教学方法,手段及提高教师本身素质入手,优化建筑施工课堂教学,适应新形势下学科建设的几点建议,以促进学生专业素质的形成和提高.  相似文献   

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推导了自聚焦光导纤维中光线传播的路径方程,阐明了自聚焦作用的物理实质。  相似文献   

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Significance of mitotic spindle fibre orientation in the neural tube   总被引:1,自引:0,他引:1  
A H Martin 《Nature》1967,216(5120):1133-1134
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Body weight, diet and home range area in primates.   总被引:10,自引:0,他引:10  
K Milton  M L May 《Nature》1976,259(5543):459-462
Primates show a strong positive relationship between body weight and home range area. Dietary habits also influence home range area. Folivorous primates occupy smaller home range areas for their body weight than do frugivores and omnivores. Primates generally require smaller home range area per individual than solitary terrestrial mammals, but primates living in social groups have much larger total home range than individual solitary mammals. This trend may necessitate higher expenditures of energy in food-gathering or modifications in movement patterns.  相似文献   

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