That which was utterly unknown to the most splendid civilizations of the past is in our time the chief power of civilization, daily engaged in making that history of a new era that is yet to be written in words. It has been demonstrated long since that men's lives are to be influenced not by theory, or belief, or argument and reason, so much as by that course of daily life which is not attempted to be governed by argument and reason, but by great physical facts like steam, electricity and machinery in their present applications.

The greatest of these facts of the present civilization are expressed in the phrase, Steam and Steel. The theme is stupendous. Only the most prominent of its facts can be given in small space, and those only in outline. The subject is also old, yet to every boy it must be told again, and the most ordinary intelligence must have some desire to know the secrets, if such they are, of that which is unquestionably the greatest force that ever yielded to the audacity of humanity. It is now of little avail to know that all the records that men revere, all the great epics of the world, were written in the absence of the characteristic forces of modern life. A thousand generations had lived and died, an immense volume of history had been enacted, the heroes of all the ages, and almost those of our own time, had fulfilled their destinies and passed away, before it came about that a mere physical fact should fill a larger place in our lives than all examples, and that the evanescent vapor which we call steam should change daily, and effectively, the courses and modes of human action, and erect life upon another plane.

It may seem not a little absurd to inquire now "what is steam?" Everybody knows the answer. The non-technical reader knows that it is that vapor which, for instance, pervades the kitchen, which issues from every cooking vessel and waste-pipe, and is always white and visible, and moist and warm. We may best understand an answer to the question, perhaps, by remembering that steam is one of the three natural conditions of water: ice, fluid water, and steam. One or the other of these conditions always exists, and always under two others: pressure and heat. When the air around water reaches the temperature of thirty-two degrees by the scale of Fahrenheit, or ° or zero by the Centigrade scale, and is exposed to this temperature for a time, it becomes ice. At two hundred and twelve degrees Fahrenheit it becomes steam. Between these two temperatures it is water. But the change to steam which is so rapid and visible at the temperature above mentioned is taking place slowly all the time when water, in any situation, is exposed to the air. As the temperature rises the change becomes more rapid. The steam-making of the arts is merely that of all nature, hastened artificially and intentionally. The element of pressure, mentioned above, enters into the proposition because water boils at a lower temperature, with less heat, when the weight of the atmosphere is less than normal, as it is at great elevations, and on days when, as we now express it, there is a low barometer. Long before any cook could explain the fact it was known that the water boiling quickly was a sign of storm. It has often been found by camping-parties on mountains that in an attempt to boil potatoes in a pot the water would all "boil away," and leave the vegetables uncooked. The heat required to evaporate it at the elevation was less than that required to cook in boiling water. It is one of the instances where the problems of nature intrude themselves prominently into the affairs of common life without previous notice.

This universal evaporation, under varying circumstances, is probably the most important agency in nature, and the most continuous and potent. There was only so much water to begin with. There will never be any less or any more. The saltness of the sea never varies, because the loss by evaporation and the new supply through condensation of the steam--rain--necessarily remain balanced by law forever. The surface of our world is water in the proportion of three to one. The extent of nature's steam-making, silent, and mostly invisible, is immeasurable and remains an undetermined quantity. The three forms of water combine and work together as though through intentional partnership, and have, thus combined, already changed the entire land surface of the world from what it was to what it is, and working ceaselessly through endless cycles will change it yet more. The exhalations that are steam become the water in a rock-cleft. It changes to ice with a force almost beyond measurement in the orderly arrangement of its crystals in compliance with an immutable law for such arrangement, and rends the rock. The process goes on. There is no high mountain in any land where water will not freeze. The water of rain and snow carries away the powdered remains from year to year, and from age to age. The comminuted ruins of mountains have made the plains and filled up and choked the mouth of the Mississippi. The soil that once lay hundreds of miles away has made the delta of every river that flows into the sea. The endless and resistless process goes on without ceasing, a force that is never expended, and but once interrupted within the knowledge of men, then covered a large area of the world with a sea of ice that buried for ages every living thing.

The common idea of the steam that we make by boiling water is that it is all water, composed of that and nothing else, and this conception is gathered from apparent fact. Yet it is not entirely true. Steam is an invisible vapor in every boiler, and does not become what we know by sight as steam until it has become partly cooled. As actual steam uncooled, it is a gas, obeying all the laws of the permanent gases. The creature of temperature and pressure, it changes from this gaseous form when their conditions are removed, and in the change becomes visible to us. Its elasticity, its power of yielding to compression, are enormous, and it gives back this elasticity of compression with almost inconceivable readiness and swiftness. To the eye, in watching the gliding and noiseless movements of one of the great modern engines, the power of which one has only a vague and inadequate conception seems not only inexplicable, but gentle. The ponderous iron pieces seem to weigh nothing. There is a feeling that one might hinder the movement as he would that of a watch. There is an inability to realize the fact that one of the mightiest forces of nature is there embodied in an easy, gliding, noiseless impulse. Yet it is one that would push aside massy tons of dead weight, that would almost unimpeded crush a hole through the enclosing wall, that whirls upon the rails the drivers of a locomotive weighing sixty tons as though there were no weight above them, no bite upon the rails. There is an enormous concentration of force somewhere; of a force which perhaps no man can fairly estimate; and it is under the thin shell we call a boiler. Were it not elastic it could not be so imprisoned, and when it rebels, when this thin shell is torn like paper, there is a havoc by which we may at last inadequately measure the power of steam.

We have in modern times applied the word "engine" almost exclusively to the machine which is moved by the pressure of steam. Yet we might go further, since one of the first examples of a pressure engine, older than the steam machine by nearly four hundred years, is the gun. Reduced to its principle this is an engine whose operation depends upon the expansion of gas in a cylinder, the piston being a projectile. The same principle applies in all the machines we know as "engines." An air-engine works through the expansion of air in a cylinder by heat. A gas-engine, now of common use, by the expansion, which is explosion, caused by burning a mixture of coal-gas and air, and the steam-engine, the universal power generator of modern life, works by the expansion of the vapor of water as it is generated by heat. Steam may be considered a species of gradual explosion applied to the uses of industry. It often becomes a real one, complying with all the conditions, and as destructive as dynamite.

It cannot be certainly known how long men have experimented with the expansive force of steam. The first feeble attempt to purloin the power of the geyser was probably by Hero, of Alexandria, about a hundred and thirty years before Christ. His machine was also the first known illustration of what is now called the "turbine" principle; the principle of reaction in mechanics. [1] He made a closed vessel from whose opposite sides radiated two hollow arms with holes in their sides, the holes being on opposite sides of the tubes from each other. This vessel he mounted on an upright spindle, and put water in it and heated the water. The steam issuing from the holes in the arms drove them backward. The principle of the action of Hero's machine has been accepted for two thousand years, though never in a steam-engine. It exists under all circumstances similar to his. In water, in the turbine wheel, it has been made most efficacious. The power applied now for the harnessing of Niagara for the purpose of sending electric currents hundreds of miles is the turbine wheel.

1. This principle is often a puzzle to students. There is an old story of the man who put a bellows in his boat to make wind against the sail, and the wind did not affect the sail, but the boat went backward in an opposite direction from the nozzle of the bellows. There is probably no better illustration of reaction than the "kick" of a gun, which most persons know about. The recoil of a six-pound field piece is usually from six to twelve feet. It can be understood by supposing a gun to be loaded with powder and an iron rod longer than the barrel to be left on the charge. If the outer end of this rod were then placed against a tree, and the gun were fired, it is manifest that the gun would become the projectile, and be fired off of the rod backward or burst. In ordinary cases the air in the bore, and immediately outside of the muzzle, acts comparatively, and in a measure, as the supposed rod against the tree would. It gives way, and is elastic, but not as quickly as the force of the explosion acts, and the gun is pushed backwards. It is the turbine principle, running into hundreds of uses in mechanics.

THE SUPPOSED HERO ENGINE

Hero appears to the popular imagination as the greatest inventor of the past. Every school boy knows him. Archimedes, the Greek, was the greater, and a hundred and fifty years the earlier, and was the author of the significance of the word "Eureka," as we use it now. But Hero was the pioneer in steam. He made the first steam-engine, and is immortal through a toy.

The first practical device in which expansion was used seems to have been for the exploiting of an ecclesiastical trick intended to impress the populace. There is a saying by an antique wit that no two priests or augurs could ever meet and look at each other without a knowing wink of recognition. Hero is said to have been the author of this contrivance also. The temple doors would open by themselves when the fire burned on the altar, and would close again when that fire was extinguished, and the worshippers would think it a miracle. It is interesting because it contained the principle upon which was afterwards attempted to be made the first working low-pressure or atmospheric steam-engine. Yet it was not steam, but air, that was used. A hollow altar containing air was heated by the fire being kindled upon it. The air expanded and passed through a pipe into a vessel below containing water. It pressed the water out through another pipe into a bucket which, being thereby made heavier, pulled open the temple doors. When the fire went out again there was a partial vacuum in the vessel that had held the water at first, and the water was sucked back through the pipe out of the bucket. That became lighter again and allowed the doors to close with a counter-weight. All that was then necessary to convince the populace of the genuineness of the seeming miracle was to keep them from understanding it. The machinery was under the floor. There have been thousands of miracles since then performed by natural agencies, and there have passed many ages since Hero's machine during which not to understand a thing was to believe it to be supernatural.

THE TEMPLE-DOOR TRICK

From the time of Hero until the seventeenth century there is no record of any attempt being made to utilize steam-pressure for a practical purpose. The fact seems strange only because steam-power is so prominent a fact with ourselves. The ages that intervened were, as a whole, times of the densest superstition. The human mind was active, but it was entirely occupied with miracle and semi-miracle; in astrology, magic and alchemy; in trying to find the key to the supernatural. Every thinker, every educated man, every man who knew more than the rest, was bent upon finding this key for himself, so that he might use it for his own advantage. During all those ages there was no idea of the natural sciences. The key they lacked, and never found, that would have opened all, is the fact that in the realm of science and experiment there is no supernatural, and only eternal law; that cause produces its effect invariably. Even Kepler, the discoverer of the three great laws that stand as the foundation of the Copernican system of the universe, was in his investigations under the influence of astrological and cabalistic superstitions. Footnote: Kepler, a German, lived between 1571 and 1630. His life was full of vicissitudes, in the midst of which he performed an astonishing Even the science of amount of intellectual labor, with lasting results. He was the personal friend of Galileo and Tycho Brahe, and his life may be said to have been spent in finding the abstract intelligible reason for the actual disposition of the solar system, in which physical cause should take the place of arbitrary hypothesis. He did this.] medicine was, during those ages, a magical art, and the idea of cure by medicine, that drugs actually cure, is existent to this day as a remnant of the Middle Ages. A man's death-offense might be that he knew more than he could make others understand about the then secrets of nature. Yet he himself might believe more or less in magic. No one was untouched; all intellect was more or less enslaved.

And when experiments at last began to be made in the mechanisms by which steam might be utilized they were such as boys now make for amusement; such as throwing a steam-jet against the vanes of a paddle-wheel. Such was Branca's engine, made nine years after the landing of our forefathers at Plymouth, and thought worthy of a description and record. The next attempt was much more practical, but cannot be accurately assigned. It consisted of two chambers, from each of which alternately water was forced by steam, and which were filled again by cooling off and the forming of a vacuum where the steam had been. One chamber worked while the other cooled. It was an immense advance in the direction of utility.

About 1698, we begin to encounter the names that are familiar to us in connection with the history of the steam-engine. In that year Thomas Savery obtained a patent for raising water by steam. His was a modification of the idea described above. The boilers used would be of no value now, nevertheless the machine came into considerable use, and the world that learned so gradually became possessed with the idea that there was a utility in the pressure of steam. Savery's engine is said to have grown out of the accident of his throwing a flask containing a little wine on the fire at a tavern. Concluding immediately afterwards that he wanted it, he snatched it off of the fender and plunged it into a basin of water to cool it. The steam inside instantly condensing, the water rushed in and filled it as it cooled.

We now come to the beginning of the steam engine as we understand the term; the machine that involves the use of the cylinder and piston. These two features had been used in pumps long before, the atmospheric pump being one of the oldest of modern machines. The vacuum was known and utilized long before the cause of it was known. [2]

2.The discoverer was an Italian, Torricelli, about 1643. Gallileo, his tutor and friend, did not know why water would not rise in a tube more than thirty-three feet. No one knew of the weight of the atmosphere, so late as the early days of this republic. Many did not believe the theory long after that time. Torricelli, by his experiments, demonstrated the fact and invented the mercurial barometer, long known as the "Torricellian Tube." This last instrument led to another discovery; that the weight of the atmosphere varied from time to time in the same locality, and that storms and weather changes were indicated by a rising and falling of the column of mercury in the tube of the siphon-barometer. That which we call the "weather-bureau," organized by General Albert J. Myer, United States Army, in 1870, and growing out of the army signal service, of which he was chief, makes its "forecasts" by the use of the telegraph and the barometer. The "low pressure area" follows a path, which means a change of weather on that path. Notices by telegraph define the route, and the coming storm is not foretold, but foreknown; not prophesied, but ascertained. If we have been led from the crude pump of Gallileo's time directly to the weather bureau of the present with its invaluable signals to sailors and convenience to everybody, it is no more than is continually to be traced even to the beginning of the wonderful school of modern science.

But in the beginning it was not proposed to use steam in connection with the cylinder and piston which now really constitutes the steam-engine. Reverting again to the example of the gun, it was suggested to push a piston forward in a tube by the explosion of gunpowder behind it, or to repeat the Savery experiment with powder instead of steam. These ideas were those of about 1678-1685. The very earliest cylinder and piston engine was suggested by Denis Papin in 1690. These early inventors only went a portion of the way, and almost the entire idea of the steam-engine is of much later date. Mankind had then a singular gift of beginning at the wrong end. Every inventor now uses facts that seem to him to have been always known, and that are his by a kind of intuition. But they were all acquired by the tedious experience of a past that is distinguished by a few great names whose owners knew in their time perhaps one-tenth part as much as the modern inventor does, who is unconsciously using the facts learned by old experience. But the others began at the beginning.

EARLY NEWCOMEN PUMPING ENGINE. STEAM-COCK, COLD WATER COCK AND WASTE-SPIGOT ALL WORKED BY HAND

In 1711, almost a hundred years after the arrival at Jamestown and Plymouth of the fathers of our present civilization, the steam-engine that is called Newcomen's began to be used for the pumping of water out of mines. This engine, slightly modified, and especially by the boy who invented the automatic cut-off for the steam valves, was a most rude and clumsy machine measured by our ideas. There appears to have been scarcely a single feature of it that is now visible in a modern engine. The cylinder was always vertical. It had the upper end open, and was a round iron vessel in which a plunger moved up and down. Steam was let in below this plunger, and the walking-beam with which it was connected by a rod had that end of it raised. When raised the steam was cut off, and all that was then under the piston was condensed by a jet of cold water. The outside air-pressure then acted upon it and pushed it down again. In this down-stroke by air-pressure the work was done. The far end of the walking-beam was even counter-weighted to help the steam-pressure. The elastic force of compressed steam was not depended upon, was hardly even known, in this first working and practical engine of the world. Every engine of that time was an experimental structure by itself. The boiler, as we use it, was unknown. Often it was square, stayed and braced against pressure in a most complicated way. Yet the Newcomen engine held its place for about seventy-five years; a very long time in our conception, and in view of the vast possibilities that we now know were before the science. [3]

3. As late as 1880, the steam-engine illustrated and described in the "natural philosophy" text books was still the Newcomen, or Newcomen-Watt engine, and this while that engine was almost unknown in ordinary circumstances, and double-acting high-pressure engines were in operation everywhere. This last, without which not much could be done that is now done, was evidently for a long time after it came into use regarded as a dangerous and unphilosophical experiment, hardly scientific, and not destined to be permanently adopted.

In the year 1760, James Watt, who was by occupation what is now known as a model-maker, and who lived in Glasgow, was called upon to repair a model of a Newcomen engine belonging to the university. While thus engaged he was impressed with the great waste of steam, or of time and fuel, which is the same thing, involved in the alternate heating and cooling of Newcomen's cylinder. To him occurred the idea of keeping the cylinder as hot as the steam used in it. Watt was therefore the inventor of the first of those economies now regarded as absolute requirements in construction. He made the first "steam-jacket," and was, as well, the author of the idea of covering the cylinder with a coat of wood, or other non-conductor. He contrived a second chamber, outside of the cylinder, where the then indispensable condensation should take place. Then he gave this cylinder for the first time two heads, and let out the piston-rod through a hole in the upper head, with packing. He used steam on the upper side of the piston as well as the lower, and it will be seen that he came very near to making the modern engine.

Yet he did not make it. He was still unable to dispense with the condensing and vacuum and air-pressure ideas. Acting for the first time in the line of real efficiency, he failed to go far enough to attain it. He made a double-acting engine by the addition of many new parts; he even attained the point of applying his idea to the production of circular motion. But he merely doubled the Newcomen idea. His engine became the Newcomen-Watt. He had a condensing chamber at each end of the stroke and could therefore command a reciprocating movement. The walking-beam was retained, not for the purpose for which it is often used now, but because it was indispensable to his semi-atmospheric engine.

THE PERFECTED NEWCOMEN-WATT ENGINE

It may seem almost absurd that the universal crank-movement of an engine was ever the subject of a patent. Yet such was the case. A man named Pickard anticipated Watt, and the latter then applied to his engines the "sun-and-planet" movement, instead of the crank, until the patent on the latter expired. The steam-engine marks the beginning of a long series of troubles in the claims of patentees.

In 1782 came Watt's last steam invention, an engine that used steam expansively. This was an immense stride. He was also at the same time the inventor of the "throttle," or choke valve, by which he regulated the supply of steam to the piston. It seems a strange thing that up to this time, about 1767, an engine in actual use was started by getting up steam enough to make it go, and waiting for it to begin, and stopped by putting out the fire.

Then he invented the "governor," a contrivance that has scarcely changed in form, and not at all in action, since it was first used, and is one of the few instances of a machine perfect in the beginning. Two balls hang on two rods on each side of an upright shaft, to which the rods are hinged. The shaft is rotated by the engine, and the faster it turns the more the two balls stand out from it. The slower it turns the more they hang down toward it. Any one can illustrate this by whirling in his hands a half-open umbrella. There is a connection between the movement of these balls and the throttle; as they swing out more they close it, as they fall closer to the shaft they open it. The engine will therefore regulate its own speed with reference to the work it has to do from moment to moment.

THE GOVERNOR

Through all these changes the original idea remained of a vacuum at the end of every stroke, of indispensable assistance from atmospheric pressure, of a careful use of the direct expansive power of steam, and of the avoidance of the high pressures and the actual power of which steam is now known to be safely capable. [4] Then an almost unknown American came upon the scene. In English hands the story at once passes from this point to the experiments of Trevethick and George Stevenson with steam as applied to railway locomotion. But as Watt left it and Trevethick found it, the steam engine could never have been applied to locomotion. It was slow, ponderous, complicated and scientific, worked at low pressures, and Watt and his contemporaries would have run away in affright from the innovation that came in between them and the first attempts of the pioneers of the locomotive. This innovation was that of Evans, the American, of whom further presently.

4. In a reputable school "philosophy" printed in 1880, thus: "In some engines" (describing the modern high-pressure engine, universal in most land service) "the apparatus for condensing steam alternately above and below the piston is dispensed with, and the steam, after it has moved the piston from one end of the cylinder to the other, is allowed to escape, by the opening of a valve, directly into the air. To accomplish this it is evident that the steam must have an elastic force greater than the pressure of the air, or it could not expand and drive out the waste steam on the other side of the piston, in opposition to the pressure of the air." According to this teaching, which the young student is expected to understand and to entirely believe, a pressure of steam of, say eighty to a hundred and twenty pounds to the inch on one side of the piston is accompanied by an absolute vacuum there, which permits the pressure of the outside air to exert itself against the opposite side of the piston through the open port at the other end of the cylinder. That is, a state of things which would exist if the steam behind the piston were suddenly condensed, exists anyway. If it be true the facts should be more generally known; if not, most of the school "philosophies" need reviewing.

The first steam-engine ever built in the United States was probably of the Watt pattern, in 1773. In 1776, the year of beginning for ourselves, there were only two engines of any kind in the colonies; one at Passaic, N. J., the other at Philadelphia. We were full of the idea of the independence we had won soon afterwards, but in material respects we had all before us.

In 1787, Oliver Evans introduced improvements in grain mills, and was generally efficient as one of the beginners in the field of American invention. Soon afterwards he is known to have made a steam-engine which was the first high-pressure double-acting engine ever made. The engine that used steam at each end of the cylinder with a vacuum and a condenser, was in this first instance, so far as any record can be found, supplanted by the engine of to-day. The reason of the delay it is difficult to account for on any other grounds than lack of boldness, for unquestionably the early experimenters knew that such an engine could be made. They were afraid of the power they had evoked. Such a machine may have seemed to them a willful toying with disaster. Their efforts were bent during many years toward rendering a treacherous giant useful, yet entirely harmless. Their boilers, greatly improved over those I have mentioned, never were such as were afterwards made to suit the high pressures required by the audacity of Hopkins. This audacity was the mother of the locomotive, and of that engine which almost from that date has been used for nearly every purpose of our modern life that requires power. The American innovation may have passed unnoticed at the time, but intentionally or otherwise it was imitated as a preliminary to all modern engines. Nearly a century passed between the making of the first practical engine and that one which now stands as the type of many thousands. But now every little saw-mill in the American woods could have, and finally did have, its little cheap, unscientific, powerful and non-vacuum engine, set up and worked without experience, and maintained in working order by an unskilled laborer. A thousand uses for steam grew out of this experiment of a Yankee who knew no better than to tempt fate with a high-pressure and speed and recklessness that has now become almost universal.

There was with Watt and his contemporaries apparently a fondness for cost and complications. Most likely the finished Watt engine was a handsome and stately machine, imposing in its deliberate movements. There is apparently nothing simpler than the placing of the head of the piston-rod between two guide-pieces to keep it in line and give it bearing. Yet we have only to turn back a few years and see the elaborate and beautiful geometrical diagram contrived by Watt to produce the same simple effect, and known as a "parallel motion." It kept its place until the walking-beam was cast away, and the American horizontal engine came into almost universal use.

The object of this chapter so far has been to present an idea of beginnings; of the evolution of the universal and indispensable machine of civilization. The steam-engine has given a new impetus to industry, and in a sense an added meaning to life. It has made possible most that was ever dreamed of material greatness. It has altered the destiny of this nation, and other nations, made greatness out of crude beginnings, wealth out of poverty, prosperity upon thousands of square miles of uninhabitable wilderness. It was the chiefest instrumentality in the widening of civilization, the bringing together of alien peoples, the dissemination of ideas. Electricity may carry the idea; steam carries the man with the idea. The crude misconceptions of old times existed naturally before its time, and have largely vanished since it came. Marco Polo and Mandeville and their kind are no longer possibilities. Applied to transportation, locomotion alone, its effects have been revolutionary. Applied to common life in its minute ramifications these effects could not have been believed or foretold, and are incredible. The thought might be followed indefinitely, and it is almost impossible to compare the world as we know it with the world of our immediate ancestors. Only by means of contrasts, startling in their details, can we arrive at an adequate estimate, even as a moral farce, of the power of steam as embodied in the modern engine in a thousand forms.

Perhaps it might be well to attempt to convey, for the benefit of the youngest reader, an idea of the actual working of the machine we call a steam-engine. There are hundreds of forms, and yet they are all alike in essentials. To know the principle of one is to know that of all. There is probably not an engine in the world in effective common use--the odd and unusual rotary and other forms never having been practical engines--that is not constructed upon the plan of the cylinder and piston. These two parts make the engine. If they are understood only differences in construction and detail remain.

Imagine a short tube into which you have inserted a pellet, or wad of any kind, so that it fits tolerably, yet moves easily back and forth in the bore of the tube. If this pellet or wad is at one end of the tube you may, by inserting that end in your mouth and putting air-pressure upon it, make it slide to the other end. You do not touch it with anything; you may push it back and forth with your breath as many times as you wish, not by blowing against it, so to speak, but by producing an actual air-pressure upon it which is confined by the sides of the tube and cannot go elsewhere. The only pressure necessary is enough to move the pellet.

Now, if you push this little pellet one way by the air-pressure from your mouth, and then, instead of reversing the tube in the mouth and pushing it back again in the same way, reverse the process and suck the air out from behind it, it comes back by the pressure of the outside atmosphere. This was the way the first steam engines worked. Their only purpose was to get the piston lifted, and air-pressure did all the actual work.

If you turn the tube, and put an air-pressure first at one end and then at the other, and pay no attention to vacuum or atmospheric pressure, you will have the principle of the later modern, almost universal, high-pressure, double-acting steam-engine.

But now you must imagine that the tube is fixed immovably, and that the air-pressure is constant in a pipe leading to the tube, and yet must be admitted first to one end of the tube and then to the other alternately, in order to push the pellet back and forth in it. It seems simple. Perhaps the young reader can find a way to do it, but it required about a hundred years for ingenious men to find out how to do precisely the same thing automatically. It involves the steam-chest and the slide-valve, and all other kinds of steam valves that have been invented, including the Corliss cut-off, and all others that are akin to it in object and action.

But now imagine the tube closed at each end to begin with, and the little moving pellet, or plunger, on the inside. To get the air into both ends of the tube alternately, and to use its pressure on each side of the pellet, we will suppose that the air-pipe is forked, and that one end of each fork is inserted into the side of the tube near the end, like the figure below, and imagine also that you have put a finger over each end of the tube.

Fig. 1

We are now getting the air-pressure through the pipe in both ends of the tube alike, and do not move the pellet either way. To make it move we must do something more, and open one end of the tube, and close that fork of the air-pipe, and thus get all the pressure on one side of the pellet. Remove one finger from the end of the tube, and pinch the fork of the air-tube that is on that side. The pellet will now move toward that end of the tube which is open. Reverse the process, and it can be pushed back again with air-pressure to the other end, and so on indefinitely.

Let us improve the process. We will close each end of the tube permanently, and insert four cocks in the tube and forked pipe.

We have here two tubes inserted at each end of the large tube, and in each of these is a cock. We have each cock connected by a rod to the lever set on a pin in the middle of the tube. We must have these cocks so arranged that when the lever is moved (say) to the right, A. is opened and B. is closed, and D. is opened and C. is closed. Now if the air-pressure is constant through the forked air-tube, and the cock E. is open, if the top of the lever is moved to the right, the pellet will be pushed to the left in the large tube. If the lever is moved to the left, and the two cocks that were open are closed, and the two that were closed are opened again, the pellet will be sent back to the other end of the tube. This movement of the pellet in the tube will occur as often as the lever is moved and there is any air-pressure in the forked tube. There is a supply-cock, opened and an escape-cock closed, and an escape-cock opened and a supply-cock closed, at each end of the tube, every time the lever is moved.

Fig. 2

We are using air instead of steam, and the movement of these four cocks all at the same time, and the result of moving them, is precisely that of the slide-valve of a steam-engine. The diagrams of this slide-valve would be difficult to understand. The action of the cocks can be more readily understood, and the result, and even much of the action, is precisely the same.

But to make the arrangement entirely efficient we must go a little further into the construction of a steam-engine. The pellet in the tube has no connection with the outside, and we can get nothing from it. So we give it a stem, thus: and when we do so we change it into a piston and its rod. Where it passes through the stopper at the end of the tube it must pass air- (or steam-) tight. Then as we push the piston back and forth we have a movement that we can attach to machinery at the end of the rod, and get a result from. We also move the cocks, or valves, automatically by the movement of the rod.

Fig. 3

Turning now to Fig. 3 again let us imagine a connection made between the rod and the end of the lever in Fig. 2. Now put on the air (or steam) pressure, and when the piston has reached the right-hand end of the tube it automatically, by its connections, closes B. and opens A., and opens D. and closes C. The pellet will be pushed back in the tube and go to the other end of it, through the pressure coming against the piston through the part of the air tube where the cock D. is open. It reaches the left-hand end of the tube, and we must imagine that when it gets there it, in the same manner and by the proper connections, closes D., opens C., closes A. and opens B. If these mechanical movements are completed it must be plain that so long as the air (or steam) pressure is continued in the forked pipe the piston will automatically cut off its supply and open its escape at each alternate end, and move back and forth. Any boy can see how a backward and forward movement may be made to give motion to a crank. All other details in an engine are questions of convenience in construction, and not questions of principle or manner of action.

Of older readers, I might request the supposition that, in Fig. 2, only the valves A. and B. were automatically and invariably opened and closed by the action of the piston-rod of Fig. 3, and that C. and D. were controlled solely by the governor, before mentioned, which we will suppose to be located at E. Then the escape of the steam ahead of the piston must always come at the same time with reference to the stroke, but the supply will depend upon the requirements of each individual stroke, and the work it has to do, and afford to the piston a greater or less push, as the emergencies of that particular instant may require. This arrangement would be one of regularity of movement and of economy in the use of steam. That which is needed is supplied, and no more. This is the principle and the object of the Corliss cut-off, and of all others similar to it in purpose. Their principle is that only the escape is automatically controlled by the movements of the piston-rod, occurring always at the same time with reference to the stroke, while the supply is under control of the movement of the governor, and regulated according to the emergencies of the movement. The governor, in any of its forms, as ordinarily applied, performs only half of this function. It regulates the general supply of steam to the cylinder, but the supply-valve continues to be opened, always to full width, and always at the same moment with reference to the stroke. With the two separate sets of automatic machinery required by engines of the Corliss type, the piston does not always receive its steam at the beginning of the stroke, and the supply may be cut off partially or entirely at any point in its passage along the cylinder, as the work to be done requires. The economic value of such an arrangement is manifest. No attempt is made here to explain by means of elaborate diagrams. It is believed that if the reason of things, and the principle of action, is clear, the particulars may be easily studied by any reader who is disposed to master mechanical details.