We have now traced the steps by which man mastered the art of kindling a fire quickly and easily and have followed the progress that has been made in the most common uses of fire. But the story of a most important use of fire remains to be told, the story of its use in doing man's work. How important this use is, how much of the world's work is done through the agency of fire, a little reflection will make plain. Fire makes steam and what does steam do? Its services are so many you could hardly name all of them. The great and many services of steam are made possible by the fire-engine, or steam-engine, and the story of this wonderful invention will now be told.

That steam has the power to move things must have been learned almost as soon as fire was used to boil water. Heat water until it boils and the steam that is formed is bound to move something unless it is allowed to escape freely. It will burst the vessel if an outlet is not provided. That is why a spout has been placed on the tea-kettle. Where there is cooking, steam is abundant and the first experiments in steam were doubtless made in the kitchen (Fig. 1). It has been said that the idea of the steam-engine first occurred to Adam as he watched his wife's kettle boil.

Whatever may have happened in ancient kitchens, we are certain that there were no steam-engines until many centuries after Adam. The beginnings of this invention are not shrouded in so much mystery as are those of the match and the lamp and the forge. In giving an account of the steam-engine we can mention names and give dates from the very beginning of the story. We know what the first steam-engine was like and we know who made it and when and where it was made. It was made 120 B.C. by Hero, a philosopher of Alexandria in Egypt. It was like the one shown in Figure 2. The boy applies the fire to the steam-tight vessel p and when steam is formed it passes up through the tube o and enters the globe which turns easily on the pivots. The steam, when it has filled the globe, rushes out of the short tubes w and z projecting from opposite sides of the globe and bent at the end in opposite directions. As it rushes out of the tubes the steam strikes against the air and the reaction causes the globe to revolve, just as in yards we sometimes see jets of water causing bent tubes to revolve. This was Hero's engine, the first steam-engine ever made.

Hero's engine was used only as a toy and it seems to represent all the ancients knew about the power of steam and all they did with it. It is not strange that they did not know more for there is no general rule by which discoveries are made. Sometimes even enlightened peoples have for centuries remained blind to the simplest principles of nature. The Greeks and Romans with all their culture and wisdom were ignorant of some of the plainest facts of science. It is a little strange, however, that after Hero's discovery was made known, men did not profit by it. It would seem that eager and persistent attempts would have been made at once to have steam do useful work, as well as furnish amusement. But such was not the case. Hero's countrymen paid but little attention to his invention and the steam-engine passed almost completely out of men's minds and did not again attract attention for nearly seventeen hundred years.

About the end of the fifteenth century Europe began to awaken from a long slumber and by the end of the sixteenth century its eyes were wide open. Everywhere men were now trying to learn all they could. The study of steam was taken up in earnest about the middle of the sixteenth century and by the middle of the next century quite a little had been learned of its nature and power. In 1629 an Italian, Branca by name, described in a book a steam-engine which would furnish power for pounding drugs in a mortar. There was no more need for such a machine then than there is now and of course the inventor aroused no interest in his engine. You can easily understand how Branca's engine (Fig. 3) works. The steam causes the wheels and the cylinder to revolve. As the cylinder revolves, a cleat on it catches a cleat on the pestle and lifts the pestle a short distance and then lets it fall. Here the pestle instead of being raised by a human hand is raised by the force of steam. This engine would be more interesting if an engine had actually been made, but there is no reason to believe that Branca ever made the engine he described. We owe much to him, nevertheless, for suggesting how steam might be put to doing useful work.

It was not very long before an Englishman put into practice what the Italian had only suggested. Edward Somerset, the Second Marquis of Worcester, in 1663 built a steam-engine that raised to the height of forty feet four large buckets of water in four minutes of time. This was the first useful work ever done by steam. Figure 4 shows the construction of Worcester's engine.

In this engine there was one improvement over former engines which was of the greatest importance: there was one vessel in which the steam was generated and another in which the steam did its work. The steam-engine now consisted of two great divisions, the boiler and the engine proper.

Worcester spent a large part of his fortune in trying to improve the steam-engine, yet he received neither profit nor honor as a reward. He died poor and his name was soon forgotten. His service to the world was nevertheless very great. In his time the mines of England had been sunk very deep into the earth; and the deeper they were sunk the greater was the difficulty of lifting the water out of them and keeping them dry. The water was lifted up from the mines by means of buckets drawn by horses or oxen (Fig. 5). Sometimes it took several hundred horses to keep the water out of a single mine. It was Worcester's object to construct an engine that would do the work of the horses. The engine he built could not do this, yet it furnished the idea—and the idea is often the most important thing. It was not long before engines built upon Worcester's plan were doing useful work at the mines. At the opening of the eighteenth century the steam-engine had been put to work and was serving man in England and throughout the continent of Europe.

The first engines were not safe. Often the steam pressed too heavily upon the sides of the vessel in which it was compressed and there were explosions. About 1680 Denis Papin, a Frenchman, invented the safety valve, that is a valve that opens of its own accord and lets out steam when there is more in the vessel than ought to be there. About ten years later Papin gave the world another most valuable idea. In Worcester's engine the steam in the steam chest pressed directly on the water that was to be forced up. Papin showed a better way. He invented the engine shown in Figure 6. In this engine a small quantity of water was placed in the bottom of the cylinder A. Fitting closely in the cylinder was a piston B such as Papin had seen used in ordinary pumps. We will suppose that the piston is near the bottom of the cylinder and that a fire is built underneath. The bottom being made of very thin metal the water is rapidly converted into steam and thus drives the piston up to the top as shown in the figure. Here a latch E catches the piston-rod H and holds the piston up until it is time for it to descend. Now the fire is removed and the steam, becoming cold, is condensed and a vacuum is formed below the piston. The latch E now releases the rod H and the piston is driven down by the air above it, pulling with it the rope L which passes over the pulleys TT. As the rope descends it lifts a weight W or does other useful work. As the inventor of the piston Papin ranks among the greatest of those whose names are connected with the development of the steam-engine.

Our story has now brought us to the early part of the eighteenth century. Everywhere men were now trying to make the most of the ideas of Worcester and Papin. The mines were growing very deep. As the water in them was getting beyond control something extraordinary had to be done. Now it seems that whenever the world is in need of an extraordinary service someone is found to render that service. The man who built the engine that was needed was a humble blacksmith of Dartmouth, England, Thomas Newcomen. This master mechanic in 1705 constructed the best steam-engine the world had yet seen. We must study Newcomen's engine (Fig. 7) very carefully. The large beam ii moved freely up and down on the pivot v. One end of the beam was connected with the heavy pump-rod k by means of a rope or chain working in a groove and the other end was connected with the rod r in the same way. When steam from the boiler b passed through the valve d into the cylinder (steam-chest) a it raised the piston s and with it the piston-rod r thus slackening the rope and allowing the opposite end of the beam to be pulled down by the weight of the pump-rod k. As soon as the piston s reached the top of the cylinder the steam was shut off by means of the valve d and the valve f was turned and a jet of cold water from the tank g was injected into the cylinder a with the steam. The jet of cold water condensed the steam rapidly—steam is always condensed rapidly when anything cold comes in contact with it—and the water formed by the condensation escaped through the pipe p into the tank o. As soon as the steam in a is condensed, a vacuum was formed in the cylinder and the atmosphere above forced the piston down and at the same time pulled the pump-rod k up and lifted water from the well or mine. When the piston reached the bottom of the cylinder the valve d was opened and the piston again ascended. Thus the beam is made to go up and down and the pumping goes on. Notice that steam pushes the piston one way and the atmosphere pushes it back.

In Newcomen's engine the valves (f and d) at first were opened and shut (at each stroke of the piston) by an attendant, usually a boy. In 1713 a boy named Humphrey Potter, in order to get some time for play, by means of strings and latches, caused the beam in its motion to open and shut the valves without human aid. We must not despise Humphrey because his purpose was to gain time for play. The purpose of almost all inventions is to save human labor so that men may have more time for amusement and rest. Humphrey Potter ought to be remembered not as a lazy boy but as a great inventor. His strings and latches improved the engine wonderfully (Fig. 8). Before his invention the piston made only six or eight strokes a minute; after the valves were made to open and shut by the motion of the beam, it made fifteen or sixteen strokes a minute and the engine did more than twice as much work.

Newcomen's engine as improved by Potter and others grew rapidly into favor. It was used most commonly to pump water out of the mines but it was put to other uses. In and about London it was used to supply water to large houses and in 1752 a flour mill near Bristol was driven by a steam-engine. In Holland Newcomen's engines were used to assist the wind-mills in draining lakes.

For nearly seventy-five years engines were everywhere built after the Newcomen pattern. Improvements in a small way were added now and then but no very important change was made until the latter part of the eighteenth century, when the steam-engine was made by James Watt practically what it is to-day. This great inventor spent years in making improvements upon Newcomen's engine (Fig. 9) and when his labors were finished he had done more for the steam-engine than any man who ever lived. We must try to learn what he did. We can learn what Watt did by studying Figure 10. Here P is a piston working in a cylinder A closed at both ends. By the side of the cylinder is a valve-chest C into which steam passes from the pipe T. Connecting C with the cylinder there are two openings, one at the top of the cylinder and the other at the bottom. The valve-chest is provided with valves which are worked by means of the rod F, which moves up and down with the beam B, thanks to Humphrey Potter for the hint. The valves are so arranged that when steam enters the opening at the top of the cylinder it is shut off from the opening at the bottom, and when it enters the opening at the bottom it is shut off from the opening at the top. When the opening at the bottom is closed the steam will rush in at the upper opening and push the piston downward; when the piston has nearly reached the bottom of the cylinder the upper opening will be closed and steam will rush in at the bottom of the steam chest and push the piston upwards. Here was one of the things done by Watt for the engine: he contrived to make the steam push the piston down as well as up. You have observed that in Newcomen's engine steam was used only to push the piston up, the atmosphere being relied upon to push it down. Thus we may say that Watt's engine was the first real steam-engine, for it was the first that was worked entirely by steam. All engines before it had been worked partly by steam and partly by air.

Watt's greatest improvement upon the steam-engine is yet to be mentioned. In Newcomen's engine when the cold water was injected into the cylinder it cooled the piston and when steam was let into the cylinder again a part of it, striking the cold piston, was condensed before it had time to do any work and the power of this part of the steam was lost. Watt did not allow the piston to get cold, for he did not inject any cold water into the cylinder. In his engine as soon as the steam did its work it was carried off through the pipe M to the vessel N and there condensed by means of a jet of water which was injected into N (called the condenser) by means of a pump E worked by the motion of the beam, thanks again to Humphrey Potter for the idea. This condensation of the steam outside of the cylinder and at a distance from it prevented the piston (and cylinder) from getting cold. In other words, in the Watt engine when steam entered the cylinder it went straight to work pushing the piston. No steam was lost and no power was lost and the cost of running the engine was greatly reduced.

It cannot be said that Watt invented the steam-engine—no one can claim that honor—yet he did so much to make it better that he well deserves the epitaph which is inscribed on his monument in Westminster Abbey. This inscription is as follows:

But the story of the steam-engine does not end with Watt. It will be remembered that in the engines of Nero and of Branca the steam did its work by reaction or by impulse. Now soon after the time of Watt, inventors turned their thoughts to the old engines of Nero and Branca and began to experiment with engines that would do their work by a direct impact of steam. After nearly a century of experimenting and after many failures there was at last developed an engine known as the steam-turbine. In this engine the steam does its work by impinging or pushing directly upon blades (Fig. 11) which are connected with the shaft which is to be turned, and it does this in much the same manner that we saw the steam do its work in Branca's engine. One of the greatest names connected with the steam turbine is that of Charles Algernon Parsons of England. In 1884 this great inventor patented a steam-turbine which proved to be a commercial success and since that date the steam-turbine has been constantly growing in favor. So great has been its success on land and on sea that there are those who believe that the engine invented by Watt will in time be cast aside and that its place will be taken by an engine which is the most ancient as well as the most modern of steam motors.

Within the cylinder are thousands of blades upon which the steam acts directly in the turning of the shaft. In the largest turbines there are as many as 50,000 blades.