same range of temperature. Since the time of Du Tremblywhose engines were finally rejected on the ground of being dangerous, one of his ships having actually been burned in the harbor of Bahia-plans similar in principle to those of Cartwright and of that inventor have been brought forward with marvelous frequency and regularity. The failure of the device of to-day is forgotten to-morrow, and a new "invention" of the same sort comes up the next day, and is pushed with the same enthusiasm and with the same rose-colored prospectuses; and the always credulous public rewards the new orator and poet with the same liberality as it did his predecessors; then, the failure following with due promptness, the promoters retire, well compensated for their temporary hallucination, and the stockholders take their disappointment philosophically, or otherwise, according to temperament. Still the crop of credulous capitalists never grows less. Ether-engines, bisulphide-of-carbon engines, ammonia-engines, and carbonic-acid-gas engines, cloud-engines, and chloroform-engines come and go with all the certainty, if not the regularity, of the seasons, and each lives its short life and disappears, only to be succeeded by another of the same tribe. The race of uneducated inventors never dies, and the smoothtalking promoters find their prey ever ready.

A few years ago, disappointed in the hope of being allowed to make a careful and crucial trial of the best of recent devices of this seductive class, for the benefit of a wealthy friend who was ready, if so advised, to "take a little stock," the writer had a very complete study made of the several vapors best adapted for such use, thus securing a correct scientific comparison of the possibilities, if not of the practicabilities, of the several presumptive rivals of steam as a motor. The work was very thoroughly done by Messrs. H. L. Gantt and D. H. Maury, and was subse quently published in "Van Nostrand's Magazine," in the latter part of 1884. Briefly, the results were as follows: Comparing alcohol, ether, carbon-disulphide (bisulphide of carbon), and chloroform, the most promising of the available fluids, with water and its vapor, steam, it was found that, as the well-known laws of thermo-dynamics indicate, all were of precisely equal efficiency, if worked in perfect engines within the same range of temperature,

with no waste occurring other than that which has already been described in the earlier part of this paper as unavoidable. If worked, in each case, through the range of pressure familiar to us as employed in the steam-engine, it was found that steam required the smallest engine to do a given amount of work. It was also the most economical, with the single exception of chloroform, which was a fraction of one per cent. better; quite too slight a difference to be considered, and probably much more than compensated by the greater waste by friction to be met with in the larger machine. Comparing them between the temperatures customary in the steam-engine as ordinarily operated, the results were essentially the same; and when studied as working fluids, with the same back-pressures in all cases, substantially the same results were again found. The final conclusions were, beyond doubt, that, the limit being taken as the controllable and admissible pressure, steam is the most efficient of all, and that all the apparent advantages claimed under exceptional conditions for either of the less familiar vapors may be secured in the steam-engine by that increase of initial pressure illustrated continually in the history of that motor from the days of Watt. "None of the non-aqueous vapors will ever successfully compete with steam."

But it will be asked, as it has often been asked before, "Are we not to expect something in this direction from the new materials and new forces coming into view, and continually being rendered more and more serviceable in so many ways by the engineer and the inventor? What is the outlook for the introduction of petroleum as a fuel? what for electricity as a motor? Is not the one likely to bring down the cost of the fuel? and is not the other certain, in some directions, at least, to supersede steam?" As to the first, no one can to-day tell. The extent of available deposits of petroleum, the relative demand, and the consequent resultant price, are matters of pure speculation. No geologist can tell us what to expect, and these questions are only determinable by waiting and by experience. We shall learn in time; but no one can predict the outcome of the marvelous developments of the oil and the gas deposits of our own and other countries. We naturally hope for much; we have already

realized much; but the geologists shake their heads, and it is certain that mineral oils cannot yet, under ordinary conditions, compete in cost with the coals as fuels for use in the boiler of the steam-engine.

As to electricity, it is to-day the servant and auxiliary of steam. Every electric-lighting establishment is one in which the potential energy of fuel is released in the form of heat, is transformed into mechanical energy in the steam-engine, is again transformed and converted into electrical energy in the "dynamo;" and is sent out over the wires, to be ultimately reconverted, by the process of use, into heat-energy, and, in that form, distributed to the universe and lost in space. The electric current is not, in any sense or in any case, a "prime motor;" it is, like a train of gearing or a belt, always a means of transmission of energy from a point at which it is most convenient to produce it to a distant point at which it is to be utilized, either as light or as power, as the energy of motion of molecular vibration, or as the energy of masses doing mechanical work. Steam or water power is to-day invariably the real source of all displays of electrical energy. There is no reason, so far as can be perceived at present, for anticipating that it will ever become the prime mover in the sense in which that term is applied to steam or to heat motors, or to water power. Nevertheless, no one can safely assert that some coming inventor may not earn fame and fortune by conferring upon his fellows some new system of converting the energies of nature into electrical energy, and in such manner as to evade that tremendous waste which is apparently unavoida ble in our present methods of utilization of the grand source of artificial power, the energy of heat-motion. Were it possible to convert the vibrations of molecules, in the form of heat-energy, directly into that closely related form denominated electrical vibration, this problem, as is supposed, would be solved. We can reach a zero of electrical energy, but we cannot attain the absolute zero of heat. M. Brard, a French electrician and inventor, a few years since actually did make a beginning in this direction; and our own Edison has recently shown another way of working toward the same end. What will come of it, and what we may hope for in that direction, cannot be even

imagined; unless, indeed, we put faith in the general belief that we cannot be appreciably nearer the end of advance in such lines of improvement than we were a century ago, and that the open path conducts onward through eternity with never-ceasing progress.

This last is a direction, in the opinion of many scientific men, in which we may perhaps reasonably look for relief from the otherwise apparently inexorable operation of that law which compels us now to waste so tremendous a proportion of the heat-energy of our fuel. I have often taken occasion to remark that the world is awaiting the appearance of three inventors, greater than any who have gone before, and to whom it will accord honors and emoluments far exceeding all ever yet received by any of their predecessors. The first is he who will show us how, by the combustion of fuel, directly to produce the electric current; the second is the man who will teach us to reproduce the beautiful light of the glow-worm and the firefly, a light without heat, the production of which means the utilization of energy without a waste still more serious than the thermo-dynamic waste; while the third is the inventor who is to give us the first practically successful air-ship. The first two of these problems are set for the electrical engineer; and we may be pardoned excess of faith, should it prove to be such, when, contemplating the enormous gain to humanity which must come of such inventions, we look confidently for the genius who is to multiply the wealth of the world to an extent which will make even the boon conferred by the creators of the steam-engine and the telegraph appear secondary. When this inventor comes forward, and most probably not till then, it is very likely that we shall see steam superseded by a rival.

Finally, away far beyond this, we may foresee, in a future so distant that neither we nor our children are likely to see its dawn, a period when, our coal-beds practically exhausted, neither steam nor any one of its rivals dependent upon the production of heat-energy by combustion of coal being longer available to do service for mankind, and artificial heat no more obtainable for either warmth or power, the human race, driven into the tropics by the inexorable progress of nature and the uninter

mitted dissipation of energy, of which the steady cooling of the earth, of the sun, and of all the universe is the gauge, must, so far as can now be seen, depend solely for its heat-supply upon the diminished radiation of the sun. Then we can see but one way in which the forces of nature are to be made to do duty for the purposes of man. Power must be obtained by the direct conversion of the heat of the sun's rays, either into stored heatenergy, by some such plan as that already invented by Captain Ericsson, the "solar engine," or into electricity, by some effi cient form of thermo-electric battery, such as Professor Farmer utilized for lighting his house years ago, or very possibly by both systems. The rivals of steam will, by that date, probably have full sway; and humanity, with its marvelous aggregations of the fruits of inventive genius through thousands of years, guided by science and aided by facilities of which we to-day have no conception, despite all the obstructing natural changes, will have attained a state of enlightenment and civilization compared with which the days of "Vrill," imagined by Lord Bulwer, will appear rudest barbarism.

R. H. THURSTON.