Yet even the rarest gases must considerably obstruct and modify the vibratory waves, while liquids and solids, according to their density and structural arrangement of atoms, must do it far more. The luminiferous ether, in which all systems are immersed, kept hereabout in an incessant quiver through its complete and perhaps three-fold gamut of vibrations by the sun, strikes the aërial ocean of the earth about an average of five hundred million millions of blows per second, for each of the seven colors, or luminous notes, not to speak of the achromatic vibrations, whose effects are other than vision or visionary. The aërial ocean is such open-work, that these infinitesimal billows are not much, though somewhat, broken by it; but when they reach the terraqueous globe itself, they dash into foam which goes whirling and eddying down into solids and liquids, among their wild caverns of ultra-microscopic littleness, and this foam or whirl-storm of ethereal substance is heat, if we are not much mistaken. According to its intensity, it expands by its own mere motion all grosser material. The quantity of this ethereal foam, yeast, whirlwind, hubbub, or whatever else you please to call it, which is got up or given up by the combustion of three pounds of good bituminous coal, according to Mr. Joule's experiments, is more than equivalent to a day's labor of a powerful horse. With our best stationary steam-engines, at present, we get a day's horse-power from not less than twenty-four pounds of coal. At this rate, the whole supply of mineral coal in the world, as it may be roughly estimated, is equivalent only to the labor of one thousand millions of horses for fifteen hundred years. With the average performance of our present engines, it would support that amount of horse-power for only one thousand years. But could we obtain the full mechanical duty of the fuel by our engines, it would be equal to the work of a thousand millions of horses for sixteen thousand years, or of about fifteen times as many men for the same time. This would materially postpone the exhaustion of the coal, at which one so naturally shudders,--to say nothing of the saving of having to dig but one eighth as much of the mineral to produce the same effect. Hence some of the interest that attaches to this discovery of Mr. Joule, which has given a new impulse to the labor of inventors in pushing the steam-engine towards perfection.