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energy in our coal-fields, and to convert a fair proportion of the ninety per cent. now lost into useful work, by transformation of chemical energy more directly into mechanical power.

The science of thermo-dynamics teaches that heat and mechanical energy are only different phases of the same thing, the one being the motion of molecules, and the other that of masses or the production of work by overcoming resistances. In either case, the measure of the energy or the work is the product of a resistance into the space through which it is overcome, or the intensity of a pressure into the volume through which it is operative, as in the steam cylinder of an engine; or it is the product of the mass or masses in motion into the squares of their velocities. In the latter case, the energy is stored in the moving mass, or in the flying molecules, and can be, by proper methods, converted into useful work, as when the energy of steam drives the piston of the locomotive. Heat is molecular motion, and the pressure of steam or of gases is due to the effort of the confined particles, moving at enormous velocities, to break through the walls of the chamber in which they are confined. These molecules are exceedingly minute; but their high velocities give them the power of producing considerable pressures, and the pressure observed by the barometer or by the steam gauge is the aggregate effect of the unintermitting impacts of countless molecules on the area which receives and transmits that pressure. Approximate determinations made by physicists indicate that the molecules of hydrogen, for example, at common atmospheric temperature and pressure, are distant from one another, on the average, about totoo of a millimeter, or adoo of an inch ; while their diameters are not far from ato that amount. The smallest mass visible to the microscopist of to-day has a volume capable of inclosing a hundred millions of such molecules.

The higher the temperature of the mass, the greater the rapidity of motion ; and the greater the pressure and the closer the approximation of the particles one to another, the more frequent the impact of each upon the confining boundary ; both effects being observed when, as in the steam-boiler, or in a gasengine, heat is imported into the mass from the combustible in which it had been previously stored. It is known that the

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quantity of power, of energy, stored in any case is proportional to the quantity of matter, and to its temperature, measured from what is called the “absolute zero," a point far below the zero of the thermometer, and at which it is presumed heat-motion

From this it follows that, on the introduction of a mass of working fluid into any heat-engine at an observed temperature, and its rejection at any lower temperature, there being no wastes, and the change of temperature being due to the conversion of the heat-energy into mechanical energy, the proportion of the original store so converted must be measured by the range of temperature worked through, divided by the initial temperature, measured from the absolute zero, about 273° on the centigrade scale, or 460° on the Fahrenheit scale, below ordinary

The most efficient steam-engines work from about 350° Fabrenheit down to about 110° or 120°; or, on the absolute scale, from about 810° down to 580°. The highest possible efficiency is thus only sufficient to convert the proportion 8187580 = 0.28 into mechanical energy, and of this twenty-eight per cent. a quarter or a third is generally lost at the boiler, if we measure the original stock at the furnace; not less than ten per cent. is wasted by friction and resistances of the engine itself; and from one-tenth to one-quarter of what remains is wasted by conduction and radiation inside and outside the cylinder, in consequence of the fact that we are compelled to make our machinery of metal, a good conductor and radiator of heat. So that only about 0.28-0.07—0.02-0.05=0.14 per cent. of the potential energy stored in the coal, during the carboniferous era, by the chemical forces awakened by the heat of the sun in those days when there was light, but when the lights in the firmament were still obscured by the mists and fogs of a world all tropical, is, with the best efforts of the greatest engineers since Watt, utilized for the benefit of mankind.

With gas-engines the case is as yet no better. The range of temperature available and the initial temperature of the working fluid are greater than with the steam-engine; but, in order to prevent injury to the machine by the beat of the burning gases, it is found necessary to inclose the cylinder in a water jacket to keep it cool, and about one-half of all the heat produced is lost by this apparatus. The friction of the machine is also greater, in proportion to work done, than in the steam-engine, and the machine, though displacing the steam-engine in some cases, in small powers, cannot be looked upon as a seriously dangerous rival as yet. But we can see that the steam-engine cannot be expected to be rapidly, nor perhaps greatly, improved hereafter; it is near its very best, apparently; while we can see possibilities in the gas-engine which indicate that we may yet find in it a much more effective machine than it is to-day. Engines of this class tested by the writer have given efficiencies as high as seventeen per cent., notwithstanding the waste by the jacket; could that waste be evaded, the gas-engine, now superior in thermo-dynamic efficiency to the steam-engine, would become a machine of more than double efficiency. The serious loss in friction to which it is subject must continue an obstacle until a way is found to work with higher pressures. The immediate outlook indicates, apparently, that the work of the world must continue, for a long time at least, to be done by the steam-engine, if its only possible rival is the gas-engine; but that the latter will displace the former, for small powers, because of its greater convenience of operation, its freedom from the risk incurred where a steam-boiler is used, and its compactness. Its improvement to such an extent as to permit its application far more extensively than at present, seems possible, and this may place it more nearly beside the steam-engine, as an economical motor, than to-day. On the other hand, the conversion of the ordinary steam-engine into a “superheated-steam engine” (steam-gas engine) would open the same possibilities in the direction of its improvement; and it is not impossible, though it seems now very improbable, that it is in that direction, combining high temperatures with high pressures, that the steam-engine is to make its next great advance.

The hot-air engine has been brought out over and over again as the rival of steam and of the gas-engine; but the same facts and the same deductions apply here. In fact, a gas-engine is simply an air-engine in which the combustion of the fuel, in this case a gas, occurs in the engine itself, instead of in a separate furnace. This insures an opportunity to obtain a more complete utilization of the heat developed, and is so far an advantage. The best air-engines are found to be very economical of fuel for small powers, and they use a cheaper fuel than the gas-engines. It is impossible to-day to say what are likely to be, in the future, the relative positions of these two motors ; but the gradual reduction of the cost of gas is giving the gas-engine some advantage, and it seems to be coming into use more rapidly than the hot-air engine. Probably for a long time to come each will find a place and will do a work for which it is peculiarly well fitted. The outlook would seem to be most favorable to the gas-engine. The waste in the furnace of its companion engine is, and probably will continue to be, serious; while the thermo-dynamic efficiency of the gas-engine promises to become still higher in proportion to that of the other motors. It is the impression among scientific men that the possibilities, practically, are better for advance with the gas-engine than even with the steam-engine -certainly as regards small powers.

The modern history of the steam-engine has been also the history of continually repeated attempts to supersede it by other forms of motor, usually by other forms of heat-engine. These attempts have, as a rule, involved the use of some other fluid than water as the motive substance, and the transformation of heat into mechanical energy by what were supposed to be other processes than that illustrated in the operation of the steamengine. Before the establishment of the modern theory of thermo-dynamics, the nature of heat as energy not being understood and the laws of transformation of energy not being known, such efforts were countenanced and often earnestly encouraged by scientific men; but to-day no one having the slightest claim to scientific knowledge or authority looks for any great gain by substitution of one working substance for another. The number of ignorant, self-deceived, or swindling promoters of such schemes does not, however, appear to be diminishing. On the contrary, it is the experience of the writer that they are rather becoming more numerous than ever. The stock argument used by this class of self-styled inventors now is : "Scientific men have been deceived before; they may be deceived again, even in a matter in regard to which they are so positive as in this. They cannot certainly affirm the impossibility of further advance in the efficiency of motors, and it is not impossible that the claims made for this, the Keely, or other beclouded scheme, may be based upon natural law." For such advocates, the next step, the affirmation that a possibility is here identical with a reasonable probability, is easy, and many a really good business man, moved by no better evidence than this, has risked and lost thousands of dollars. One such doubtful “invention," in regard to which the writer was consulted, took several hundred thousands of dollars from the “wolves," and no one knows how much from the “lambs,” of Wall Street; and there is good reason to believe that another hardly less outrageous humbug has been for several years quietly fleecing sanguine but ignorant capitalists, and very possibly still more seriously. Its advocates have even claimed that they have received millions of dollars in exchange for their fabulous assurances of untold wealth to come. Experience shows that it is vastly easier for such pretenders to secure capital for their foolish or wicked schemes than for a really good invention to find liberal backing and reliable support, such as James Watt received from his partner Boulton, in introducing the steam-engine.

Such great promises and anticipations were not unfamiliar to engineers a century ago. As early as 1797, the Rev. Dr. Ed. ward Cartwright, one of the competitors of Watt in the introduction of the steam-engine, invented also a new engine, in which he proposed to use " ardent spirits or ether, or any other spirit more volatile than water," as the working substance, supposing that the greater volatility and the consequently higher pressures attainable at a given temperature, and the smaller amount of heat demanded for vaporization, would reduce in the same proportion the cost of producing the power. This old argument is still heard from all the promoters ” of similar schemes to-day. The fallacy lies in the facts that the greater the amount of heat which a fluid can take up in the process of vaporization, the greater the quantity of energy stored ; and the higher the pressure at a given temperature, the less the amount of work obtain. able with a given amount of expansion in the engine. The fluid of highest specific heat is most desirable as a motor fluid, and the

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