From time immemorial people knew the two motive forces of flowing water and of streaming air, or wind. When the steam-engines came into use, the latter had gradually superseded the former, partly because rapidly flowing or falling water is not always procurable, partly also because the supply of water, as well as its power, depends on the quantities of rain falling in the higher districts. The latter inconvenience, the variability of power, made itself still more strongly felt in the application of the wind. The most absolute quietness in the air may be followed by tempests so dangerous that the skipper is obliged to furl his sails, and the miller finds it necessary to stop his mill, in order to avoid the most disastrous consequences. Now, when the mill stops, it becomes a useless machine; for then the work of the men is stopped, and ultimately their wages. Much valuable time is lost, and time is known to be money. Add to this that a steam-engine may be worked unremittingly, so that the manufacturer can be sure to finish any given work in any stipulated time, and it must be clear enough why the powers of water and wind got to be superseded by steam-power, on account of the latter's superior regularity. "Meanwhile it is impossible to overlook the double fact that water and wind may be had for nothing, and that steam involves expense. Moreover, so immense is the quantity of vital or working power of the water falling down on the surface of our earth, and also of the atmospheric currents, that the locomotive power of all existing steam-engines is comparatively trifling by the side of them. One single great cataract has more working power than all the steam-engines of Europe together, and one single thunder-storm may produce such frightful destruction that it would be ridiculous to measure them by horse-power. "As, therefore, steam became more and more expensive, one naturally looked for means by which, without losing the regularity and stability of steam-power, one might turn to account the forces of wind and falling water. The question had really come to this--how to regularly distribute over a certain period of time a force or power so intensely variable. It seemed as if the working-power of water and wind had to be collected and saved up, so as to have a regular provision of such forces in case of need. In like manner Nature had saved her working-power when she caused the forests to grow, from whence resulted the coal layers. Art had already done the same in preparing gunpowder and other explosive matters. Why, then, could the experiment not be tried in analogous form, namely, by temporary imprisonment or detention of that vital power which appeared to be so inexhaustible?" That was the problem. With regard to its solution I could not well follow the details. All I could learn from Bacon was this, that the black cylinders on the waggon already referred to bore the name of Energeiathecs,