To produce power for commercial purposes, at least two things are absolutely essential. First, there must be uniformity in the character of the power produced; and, second, it must be available everywhere.
Water is the cheapest prime power, but its use is limited to streams or moving bodies of water. If derived from the air currents no dependence can be placed on the regularity of the energy.
Heat is the only universal power on the globe. The sun is the great source of energy. Each year it expends in heat a sufficient force to consume over sixty lumps of coal, each equal to the weight of the earth.
Of that vast amount the earth receives only a small part, but the portion which does come to it is equal to about one horse power acting continuously over every thirty square feet of the surface of our globe.
The great problem, in the minds of engineers, from the time the steam engine became a factor, was to find some means whereby that energy might be utilized, instead of getting it by way of burning a fuel.
One of the first methods proposed was to use a lens or a series of mirrors, by means of which the rays might be focused on some object, or materials, and thus produce the heat necessary for expansion, without the use of fuel.
Wonderful results have been produced by this method; but here, again, man meets with a great obstacle. The heat of the sun does not reach us uniformly in its intensity; clouds intervene and cut off the rays; the seasons modify the temperature; and the rotation of the globe constantly changes the direction of the beams which fall upon the lens.
The second method consists in using boxes covered with glass, the interior being blackened to absorb the heat, and by that means transmit the energy to water, or other substances adapted to produce the expansive force.
Devices of this character are so effective that temperatures much above the boiling point of water have been obtained. The system is, however, subject to the same drawbacks that are urged against the lens, namely, that the heat is irregular, and open to great variations.