One day George Stephenson and a friend stood watching a train drawn by one of his locomotives.

“What moves that train?” asked Stephenson.

“The engine,” replied his friend.

“And what moves the engine?”

“The steam.”

“And what produces the steam?”

“Coal.”

“And what produces coal?”

This last query nonplussed his friend, and Stephenson himself replied, “The sun.”

The “bottled sunshine” that drove the locomotive was stored up millions of years ago in the dense forests then covering the face of the globe. Every day vegetation was built by the sunbeams, and in the course of ages this growth was crushed into fossil form by the pressure of high-piled rock and dÉbris. To-day we cast “black diamonds” into our grates and furnaces, to call out the warmth and power that is a legacy from a period long prior to the advent of fire-loving man, often forgetful of its real source.

We see the influence of the sun more directly in the motions of wind and water. Had not the sun’s action deposited snow and rain on the uplands of the world, there would be no roaring waterfall, no rushing torrent, no smooth-flowing stream. But for the sun heating the atmosphere unequally, there would not be that rushing of cool air to replace hot which we know as wind.

We press Sol into our service when we burn fuel; our wind-mills and water-mills make him our slave. Of late years many prophets have arisen to warn us that we must not be too lavish of our coal; that the time is not so far distant, reckoning by centuries, when the coal-seams of the world will be worked out and leave our descendants destitute of what plays so important a part in modern life. Now, though waste is unpardonable, and the care for posterity praiseworthy, there really seems to be no good reason why we should alarm ourselves about the welfare of the people of the far future. Even if coal fails, the winds and the rivers will be there, and the huge unharnessed energy of the tides, and the sun himself is ready to answer appeals for help, if rightly shaped. He does not demand the prayers of Persian fire-worshippers, but rather the scientific gathering of his good gifts.

Place your hand on a roof lying square to the summer sun, and you will find it too hot for the touch. Concentrate a beam of sunshine through a small burning-glass. How fierce is the small glowing focal spot that makes us draw our hands suddenly away! Suppose now a large glass many feet across bending several square yards of sun rays to a point, and at that point a boiler. The boiler would develop steam, and the steam might be led into cylinders and forced to drudge for us.

Do many of us realise the enormous energy of a hot summer’s day? The heat falling in the tropics on a single square foot of the earth’s surface has been estimated as the equivalent of one-third of a horse-power. The force of Niagara itself would on this basis be matched by the sunshine streaming on to a square mile or so. A steamship might be propelled by the heat that scorches its decks.

For many centuries inventors have tried to utilise this huge waste power. We all know how, according to the story, Archimedes burnt up the Roman ships besieging his native town, Syracuse, by concentrating on them the sun heat cast from hundreds of mirrors. This story is less probable than interesting as a proof that the ancients were aware of the sun’s power. The first genuine solar machine was the work of Ericsson, the builder of the Monitor. He focused sun heat on a boiler, which gave the equivalent of one horse-power for every hundred square feet of mirrors employed. This was not what engineers would call a “high efficiency,” a great deal of heat being wasted, but it led the way to further improvements.

In America, especially in the dry, arid regions, where fuel is scarce and the sun shines pitilessly day after day, all the year round, sun-catchers of various types have been erected and worked successfully. Dr. William Calver, of Washington, has built in the barren wastes of Arizona huge frames of mirrors, travelling on circular rails, so that they may be brought to face the sun at all hours between sunrise and sunset. Dr. Calver employs no less than 1600 mirrors. As each of these mirrors develops 10-15 degrees of heat it is obvious, after an appeal to simple arithmetic, that the united efforts of these reflectors should produce the tremendous temperature 16,000-24,000 degrees, which, expressed comparatively, means the paltry 90 degrees in the shade beneath which we grow restive multiplied hundreds of times. Hitherto the greatest known heat had been that of the arc of the electric lamp, in which the incandescent particles between pole and pole attain 6000 degrees Fahrenheit.

The combined effect of the burning mirrors is irresistible. They can, we are told, in a few moments reduce Russian iron to the consistency of warmed wax, though it mocks the heat of many blast-furnaces. They will bake bricks twenty times as rapidly as any kiln, and the bricks produced are not the friable blocks which a mason chips easily with his trowel, but bodies so hard as to scratch case-hardened steel.

There are at work in California sun-motors of another design. The reader must imagine a huge conical lamp-shade turned over on to its smaller end, its inner surface lined with nearly 1800 mirrors 2 feet long and 3 inches broad, the whole supported on a light iron framework, and he will have a good idea of the apparatus used on the Pasadena ostrich farm. The machine is arranged in meridian, that is, at right angles to the path of the sun, which it follows all day long by the agency of clockwork. In the focus of the mirrors is a boiler, 13 feet 6 inches long, coated with black, heat-absorbing substances. This boiler holds over 100 gallons of water, and being fed automatically will raise steam untended all the day through. The steam is led by pipes to an engine working a pump, capable of delivering 1400 gallons per minute.

The cheapness of the apparatus in proportion to its utility is so marked that, in regions where sunshine is almost perpetual, the solar motor will in time become as common as are windmills and factory chimneys elsewhere. If the heat falling on a few square yards of mirror lifts nearly 100,000 gallons of water an hour, there is indeed hope for the Sahara, the Persian Desert, Arabia, Mongolia, Mexico, Australia. That is to say, if the water under the earth be in these parts as plentiful as the sunshine above it. The effect of water on the most unpromising soil is marvellous. Already in Algeria the French have reclaimed thousands of square miles by scientific irrigation. In Australia huge artesian wells have made habitable for man and beast millions of acres that were before desert.

It is only a just retribution that the sun should be harnessed and compelled to draw water for tracts to which he has so long denied it. The sun-motor is only just entering on its useful career, and at present we can but dream of the great effects it may have on future civilisation. Yet its principle is so simple, so scientific, and so obvious, that it is easy to imagine it at no far distant date a dangerous rival to King Coal himself. To quarry coal from the bowels of the earth and transform it into heat, is to traverse two sides of a triangle, the third being to use the sunshine of the passing hour.