But if the whole available mechanical power, laid in store in the coal mines, in addition to all the unimproved wind and water power, should seem to any one insufficient to work out this world's manifest destiny, the doctrine of the essential unity or conservation of force is not exhausted of consolation. All the coal of which we have spoken is but the result of the action of sun-light in past ages, decomposing carbonic acid in the vegetative process. The combustion of the carbon reproduces a force exactly equivalent to that of the sun-light which was absorbed or consumed in its vegetative separation. Supposing the whole estimated stock of coal in the world to be consumed at once, it would cover the entire globe with a stratum of carbonic acid about seventy-two feet deep. And if all the energy of sun-light which this globe receives or encounters in a year were to be devoted to its decomposition, according to Pouillet's estimate of the strength of sunshine,—and he probably knows, if any one does,— deducting all that would be wasted on rock or water, there would be enough to complete the task in a year or two. A marvellous growth of forest, that would be! But the coal is not to be burned up at once. When we get our steam-engines in motion to the amount of two or three thousand millions of horse-power, and are running off the coal at the rate of one tenth of one per cent per annum, the simple and inevitable consequence will be that the wood will be growing enough faster to keep good the general stock of fuel. Doubtless the forests are now limited in their growth and stunted from their ante-Saurian stature, not so much for want of soil, moisture, or sunshine as for want of carbonic acid in the air, to be decomposed by the foliage, the great deposition of coal in the primitive periods having exhausted the supply. Our present havoc of wood only changes the locality of wood-lots, and our present consumption of coal, rapid enough to exhaust the entire supply in about seventy-seven thousand years, is sure to increase the aggregate cordage of the forests. By the time we have brought our locomotive steam-cultivators to such perfection as to plough up and pulverize the great central deserts, we may see trees flourish where it would have been useless to plant the seed before we had converted so much of the earth's entrails into smoke.
There was a time, before we had harnessed the powers of Nature to found, forge, spin, weave, print, and drudge for us generally, that in every civilized country the strong-headed men used their strong-handed brethren as machines. Only he could be very knowing who owned many scribes, or he very rich who owned many hewers of wood and drawers of water. With our prodigious development of mechanical inventions, iron and coal, our mighty steam-driven machinery for making machines, the time for chattelizing men, or depending mainly on animal power of any sort for the production of wealth, has passed by. Abrogate the golden rule, if you will, and establish the creed of caste,—let the strongest of human races have full license to enslave the weakest, and let it have the pick of soil and staples,— still, if you do not abolish the ground rules of arithmetic, and the fact that a pound of carbon costs less than a pound of corn, and must cost less for at least a thousand years to come, chattelism of man will cease in another generation, and the next century will not dawn on a human slave. At present, a pound of carbon does not cost so much as a pound of corn in any part of the United States, and in no place visited by steam-transportation does it cost one fifth as much. We are already able to get as much work out of a pound of carbon as can be got from a pound of corn fed to the faithfullest slave in the world. Mr. Joule has shown us that there is really in a pound of carbon more than twice as much work as there is in a pound of corn. The human corn-consuming machine comes nearer getting the whole mechanical duty or equivalent out of his fuel than our present steam-engine does, but the former is all he ever will be, while the latter is an infant and growing.
We shall doubtless soon see engines that will get the work of two slaves out of the coal that just balances one slave's food in the scales. Our iron-boned, coal-eating slave, with the advantage of that peculiar and almost infinitely applicable mechanical element, the wheel, may be made to go anywhere and do any sort of work, and, as we have seen, he will do it for one tenth of the cost of any brute or human slave.
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 in its neighborhood the electrical and calorific ethers, and sets them into motions which may be communicated from one to the other, but which are communicated to ponderable matter, or result in mechanical action, only or chiefly by the impulse of the denser or calorific ether. When the sun shines on land and water, as we have already said, there is a violent ethereal commotion in the interstices of the superficial matter, which we will now suppose to be that of the calorific ether; and by virtue of this motion, together with whatever affinities this ether may be supposed to have for ponderable matter, we may account for evaporation, and the production of those vast aërial currents by which the evaporated water is diffused. In the production of aërial currents, heat is converted into force, and hence vapor is converted into watery globules mechanically suspended on clouds, which, by their friction, sweep the electrical ether into excessive condensation in the great Leyden-jar arrangement of the sky. Whatever it may be that gives relief to this condensation, the relief itself consists in motion, either translatory or vibratory, of the electrical ether or ethers. As this motion, if it be such, often takes place through gases, liquids, and solids, without any sensible mechanical effect, and at other times is contemporary with phenomena of intense heat, we may, till otherwise informed, suppose, that, whenever it produces a mechanical effect, it is by so impinging on the calorific ether as to produce the motion of heat, which is instantly thereafter converted into mechanical force. It is not so much the greatness of the amount of this mechanical force which gives it its peculiar destructiveness, as the inequality of its strain; not so much the quantity of matter projected, as the velocity of the blow. One may have his brains blown out by a bullet of air as well as one of lead, if the air only blows hard enough and to one point. Whatever its material, the edge of the thunder-axe is almost infinitely sharp, and its blow is as destructive as it is timeless. But it is always heat, not electrical discharge, which only sometimes causes heat, that strikes the blow.
Now in the case of a steam-boiler, when the water, having been reduced too low, is allowed suddenly to foam up on the overheated crown-sheet of the furnace, there must be just that sudden or instantaneous conversion of heat into force which may take place when the current of the electrical discharge passes through the gnarled fibres of an oak. The boiler and the oak are blown to shivers in equally quick time. The only difference seems to be, that in one case electricity stood immediately, in point of time, behind the heat, and in the other it stood away back beyond the crocodiles, playing its rôle more genially in the growth of the monster forests whose remains we are now digging from the bowels of the earth as coal. In the normal action of a steam-boiler, the steam-generating surfaces being all under water, however unequally the fire may act in different localities, the water, by its rapid circulation, if not by its heat-absorbing power, diffuses the heat and constantly equalizes the strain resulting from its conversion into mechanical force. The increase of pressure takes place gradually and evenly, and may easily be kept far within safe limits. It is quite otherwise when the conductivity of the boiler-plate is not aided and controlled by the distributiveness of the water, as it is not whenever the plate is in contact with the fire on one side without being also in contact with the water on the other. Everybody knows that boilers explode under such circumstances, but everybody does not know why.
A cylinder of plate-iron will withstand a gradually applied, evenly distributed, and constant pressure, one thousandth part of which, acting at one spot, as a blow, would rend its way through, or establish a crack. This slight rent, giving partial relief to the sudden but comparatively small force that causes it, would be nothing very serious in itself,—no more so than a rent produced by the hydraulic press,—if the whole force, equal, perhaps, to that of a thousand wild horses imprisoned within, did not take instant advantage of it to enlarge the breach and blow the whole structure to fragments, or, in other words, if it did not permit nearly the whole of the accumulated heat in the boiler to be at once converted into mechanical motion. For example, a boiler whose ordinary working pressure is one hundred pounds to the square inch, which may give an aggregate on the whole surface of five millions of pounds, would not give way, perhaps, if that pressure were gradually and evenly increased to thirty millions. But if the water is allowed to get so low that some part of the plate exposed to the fire is no longer covered with it, that part will directly become far hotter than the water or the mass of the steam,—dry steam having no more power to carry away the excess of heat than so much air. After that, when the water rises again, the first wave or wallop that strikes the overheated plate absorbs the excess of heat, and its conversion into steam of higher pressure than that already existing is so sudden that it may be regarded as instantaneous. It is to be remembered that for every pound of water raised one degree, or heat to that amount absorbed in generating steam, a force of seven hundred and seventy-two pounds is created. In this case a new or additional force is created, which, acting in all directions from one point, first takes effect on the line which joins that point with the nearest opposite point in the wall of the boiler. If it is not like smiting with the edge of a ponderous battle-axe, it is at least as dangerous as a cannon ball shot along that line. If the local heat so suddenly absorbed be but enough to raise ten pounds of water ten degrees, it is equivalent to the force acquired by seventy-seven thousand two hundred pounds falling through a foot, or of a cannon-ball of one hundred pounds flying at the rate of more than a mile per second. If by any miracle the boiler should stand this shock or series of shocks, the pressure becomes equalized, and the overheated plate having parted with its excess of heat, safety is restored. But if cohesion is anywhere overcome by the sudden blow, the wild horses stampede in all directions. The boiler, minus the water and boiler-head perhaps, goes through ceiling, roof, and brick walls, as if they were cobwebs, and, surrounded with fragments of men and things, is seen descending like a comet through the neighboring air.
To get rid of this liability to have a Thor-hammer or thunderbolt generated in the stomach of a steam-engine, at any moment when the vigilance of the engineer happens to be at fault, something is going to be done. No safety-valve or fusible plug is adequate. The boiler cannot be all safety-valve. The trouble is, the hammer is not more likely to strike the first of its terrible series of blows on the valve than anywhere else. A safety-valve, in good order, is a sovereign precaution against the excess of an equally distributed strain, but it is not an adequate protection against a shock or unequal strain. The old-fashioned gaugecocks, which are by no means to be dispensed with, reveal the state of the water in the boiler to the watchful engineer about as surely as the stethoscope reveals to the doctor the condition of his patient's lungs. A surer and more convenient indication is the tubular glass gauge, on the fountain principle, which in its best form is both trustworthy and durable. No well-informed proprietor suffers his boiler to be without one; but it is not a cure for carelessness. It is only a window for the vigilant eye to look through, not the eye itself. Steam-boilers will have to be constructed so that when the subsidence of the water fails to check itself by enlarging the supply, it shall, before the point of danger is reached, infallibly check the combustion, let off the steam, and blow a whistle or ring a bell, which the proprietor may, if he pleases, regard as the official death-knell of the careless engineer. Human vigilance must not be superseded, but fortified,—as in the case of the watchman watched by the tell-tale clock. The steam-creature must be so constituted as to refuse to work itself down to the zone where alone unequal strains are possible; it must cry out in horror and strike work. Mechanically the solution of the problem is easy, and the enhancement in cost of construction will be nothing, compared to the risk of loss from these explosions. With this guard against the deficiency of water, steam-power will become the safest, as it is the most manageable, of all forces that have hitherto been subsidized by the civilized man.
But there is one more improvement worth mentioning. We do great injustice to our steam-slaves by the slovenly and unphilosophical way in which we feed them. We take no hints from animal economy or the laws of dietetics.
Our creature has no regular meals, especially if he is one of the fast kind; but a grimy nurse stands by, and, opening his mouth every few minutes, crams in a few spoonfuls of the black pudding. The natural consequence is more or less indigestion and inequality of strength. We have not yet taken full advantage of the laws of combustion, or adapted our apparatus to the peculiarities of the best and cheapest fuel. Nature manages more wisely in her machinery. Combustion, the union of fuel with oxygen, ceases for want of air as well as for want of fuel. In the case of fuels compounded of carbon and hydrogen, if the air be withheld when the mass is in rapid combustion, the heat will cause a portion of the fuel to pass off by distillation, unconsumed, and this portion will be lost. But from the best anthracite, which is nearly pure carbon concentrated, if oxygen be entirely excluded, not much can distil away with any degree of heat. The combustion of this fuel, therefore, admits of very easy and economical regulation, by simply regulating the supply of air. When the air is admitted at all, it should be admitted above as well as below the fuel, so that the carbonic oxyde that is generated in the mass may be burned, or converted into carbonic acid, over the top. Why, then, should not the iron horse, before leaving his stable, take a meal of anthracite sufficient to last him fifty or one hundred miles? Let him swallow a ton at once, if he need it. Before starting, let the temperature of the mass in the furnace be got up to the point where the combustion will go on with sufficient rapidity for the required speed by simply supplying air, which should also be fed as hot as possible. This done, the engineer throughout the trip will have perfect control of his force by means of the steam-blast and air-openings. There will be no smoke nuisance, the combustion being complete so far as it takes place at all. There will be no need of loading the furnace with firebrick to equalize the heat,—the mass of incandescent fuel serving that purpose; and no waste or inequality will occur from opening the door to throw in a cold collation.