But will not our artificial slave be more liable to insurrection? Everybody admits that he already accomplishes incalculable drudgery in the huge mill, on the ocean, and on the iron highway. But almost everybody looks upon him as a sleeping volcano, which must sooner or later flare up into irresistible wrath and do frightful mischief. Underwriters shake their prudent heads at him. Coroners' inquests, sitting solemnly over his frequent desolations, find only that some of his ways are past finding out. Can such a creature be domesticated so as to serve profitably and comfortably on by-roads as well as high-roads, on farms, in gardens, in kitchens, in mines, in private workshops, in all sorts of places where steady, uncomplaining toil is wanted? Can we ever trust him as we trust ourselves, or our humble friends, the horse and the ox? The law of the conservation of force, now so nearly developed, will perhaps throw some light on this inquiry. Boiler explosions have a sort of family resemblance to the freaks of lightning or the thunderbolt. Indeed, so striking is the similarity, that people have been prone to think, that, previously to an explosion, the steam in the boiler must have become in some inexplicable way charged with electricity like a thunder-cloud, and that the discharge must have occasioned the catastrophe. It is needless to say to those who understand a Leyden jar, that nothing of the sort takes place. The friction of the watery globules, carried along by the steam in blowing off, is found to disturb the electrical equilibrium, as any other friction does; but the circumstances in the case of a boiler are always so favorable to its restoration, that an electrical thunderbolt cannot possibly be raised there that would damage a gnat. Yet a boiler explosion may, after all, depend on the same immediate cause as the mechanical effect which is frequently noticed after an electrical discharge in a thunder-storm. Let us hypothetically analyze what takes place in a thunder-storm. For the sake of illustration, and nothing more, we will suppose the existence, throughout all otherwise void space, of three interflowing ethers, the atoms of each of which are, in regard to each other, repellant, negative, or the reverse of ponderable, and that these ethers differ in a series by vast intervals as to size and distance of atoms, that each neither repels nor attracts the other, that only the rarest is everywhere, and that the denser ones, while self-repellant, have affinities, more or less, which draw them from the interplanetary spaces towards the ponderable masses. Let the rarest of these ethers be that whose vibrations cause the phenomena of light,--the next denser that which, either by vibration or translatory motion, causes the electrical phenomena,-- and the most dense of the three that which by its motions, of whatever sort, causes the phenomena of heat. The solar impulse propagated through the luminiferous ether towards any mass encounters