coal; (c) Solar radiation; and (d) Hydroelectric power. 

 Petroleum, although being found in large quantities still, 

 cannot last for very long as a primary energy source, and 

 coal is already being worked out in several countries. A 

 considerable further development of hydroelectric genera- 

 tion is possible in countries with even only medium-sized 

 rivers, and this power source is essentially permanent once 

 its use is started. The direct use of solar light and heat to 

 produce electricity is now approaching a real possibility 

 through the use of silicon solar cells: if these can be made 

 in enormous numbers, and at a low cost (something like 

 100,000 cells being needed to produce one kilowatt of elec- 

 tricity), and assembled over significant areas in sunny 

 regions of the Earth (about 2 acres of surface are needed 

 to generate one megawatt of power), then solar energy 

 will become a dependable permanent source. But the main 

 energy source of the future must be that derived from 

 uranium, thorium, and possibly water, in reactors. Enough 

 good uranium and thorium ores are available for some 

 time yet, (and further ores will most probably be discov- 

 ered using airborne scintillometers), but the long-term 

 nuclear fission reactors will have to be operated on uranium 

 extracted from granites, in which there is an inexhaustible 

 amount. Alternatively, if the construction of a hydrogen- 

 fueled nuclear fusion reactor is successfully accomplished, 

 of which there appears to be some hope now, then the oceans 

 will provide an inexhaustible supply of heavy hydrogen. In 

 either case, we have enough nuclear fuel to last for sev- 

 eral thousand million years — longer than the Earth will 

 remain habitable as the Sun slowly rises in temperature. 



The means for the effective distribution of materials 

 and manpower over the Earth are essential for a general 

 world-wide economy. Minerals and food have to be moved 



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