Later, the switch from wood to coal made it easy to obtain high temperatures and energy densities which allowed progress in the technology of refining metallic resources. This, in turn, led to the invention and improvement of the steam engine, which propelled us from the age of sailing ships at the mercy of the wind to the age of steam-powered vessels. With the advent of petroleum fuels, we learned how to rise above the weather and soar through the stratosphere. Over the centuries, mankind has mastered technologies that have allowed us to tap various sources of energy. Until the middle of this century, however, we could not access energy sources not originating from the sun.
Wood is termed a primary product, created by plants using solar energy.
Coal and petroleum are secondary products, the fruit of time and subterranean pressure. Only nuclear power is a source of energy that is completely independent of the sun. The recent excess expectation that some people hold for solar energy and wind power seems strange when seen from this viewpoint. However, the idea that they are potential mainstays of our energy supply is nothing short of puzzling. Solar and wind power stations throughout the world will have relatively low levels of use because natural sources of energy are so changeable, which is precisely why mankind is always searching for new and better sources of energy.