Watt spent much time and money in making experiments, but nothing he tried succeeded. "Nature has a weak side," he was fond of saying, "if we can only find it out." So he went on day after day, following now this and now that false hope.

"One Sunday afternoon early in 1765," writes Watt, "I had gone to take a walk in the Green of Glasgow. I was thinking upon the engine and about how to save the heat in the cylinder, when the idea came into my mind that steam was an elastic body and would run into a vacuum. If connection was made between the cylinder and a tank from which the air had been pumped, the steam would pass into the empty tank and might there be condensed without cooling the cylinder. I then saw that I must get rid of the condensed steam and of the water used in condensing it. It occurred to me this could be done by using pumps."

With a separate condenser in mind, to get rid of the steam after it had done its work, without cooling the cylinder, other important improvements were thought of. In Newcomen's engine, the upper end of the cylinder was open to let the air act upon the piston. Watt now planned to put an air-tight cover over the end of the cylinder, with a hole for the piston rod to slide through, and to let steam in above the piston to act upon it, instead of the air. This change made Newcomen's atmospheric engine into a steam engine. In Newcomen's engine the power was the pressure of the atmosphere upon the piston, and this power acted in one direction only. In Watt's engine steam was the power, and the piston was shoved both up and down by it; hence Watt's engine was called a double-acting engine.

"All these improvements," says Watt, "followed in quick succession, so that in the course of one or two days the invention was . . . complete in my mind."

The next step was to make a model, to put the invention into working form. Making the drawings was easy, but to carry them out was hard. A lack of good workmen was the chief difficulty. There were no skilled mechanics in those days, nor self-acting, tool-making machines; everything had to be made by hand. Blacksmiths and tinners were the only men that could be hired, and they were bungling workers even at their own trades. After eight months of racking labor, the model was ready to start. It worked, but despite all Watt's care, it "sniffed at many joints." The condenser did not work well; the cylinder leaked, and the piston was far from being steam tight. To add to Watt's troubles, his "old White Iron man," a tinner and his best workman, died. The cross-beam broke. Nevertheless, Watt saw enough to know that he was on the right track.

Watt's great need was money, for it was necessary to build a trial engine to show the value of steam power. He finally, in 1767, secured a partner who promised, for a two-thirds share in the invention, to pay a debt of five thousand dollars owed by Watt, and to bear the expense of further experiments. The partnership was formed, and Watt turned to the plans for the trial engine.

As the trial engine neared completion, Watt's "anxiety for his approaching doom kept him sleepless at night, for his fears were even greater than his hopes." Alas! the trial engine did not work well. The new condenser acted badly. The cylinder was almost useless. The piston, despite all that could be done, leaked quantities of steam. The whole machine was a "clumsy job." From the way it wheezed, and snorted, and puffed fire and smoke, the engine was named Beelzebub. Months were spent in Wales, overhauling him, but he behaved only slightly better on second trial. Beelzebub was far from being a practical engine, and he was left for the time to rest and rust.

There is little wonder that Watt was downhearted and wrote to his friends: "Of all things in life, there is nothing more foolish than inventing." "I am resolved . . . if I can resist it, to invent no more." "To-day I enter the thirty-fifth year of my life, and I think I have hardly yet done thirty-four pence worth of good in the world."

Watt had by this time spent ten years and several thousand dollars upon his invention, but it was still only a dream. Brighter days were, however, at hand. Matthew Boulton, owner of the largest hardware factory in the world, at Soho near Birmingham, and who had working for him the best mechanics in Europe, became interested in the fire engine. In 1774, he became Watt's partner.

Meanwhile, old Beelzebub was shipped to Birmingham. The best mechanics of Soho set to work upon him. One by one the separate parts were repaired and improved. In a few months, he was ready for trial. Beelzebub puffed as much smoke and fire as ever, but with all his bluster and noise,—thanks to good workmanship, he went surprisingly well. Everyone who saw Beelzebub run felt sure that the invention would prove a success. Even modest Watt wrote to his father: "The fire engine I have invented is now going, and answers much better than any other that has yet been made, and I expect that the invention will be very beneficial to me."

Though success was promised, much remained to be done to make the engine practical. It was found, for example, that if the load Beelzebub was pulling, for some cause became lighter, he would run too fast; if the load suddenly became heavier, he would run too slow.

Some way had to be found to make him run faster when there was need of more power, and to run more slowly when less power was needed. Two heavy balls were fixed to swing around an upright rod. When the engine ran fast, the upright rod turned fast, and the balls swung out and so acted as to admit less steam. When the engine ran slowly, the rod turned slowly, and the balls swung down and let in more steam. By the use of this contrivance, or the governor, Beelzebub was made to run at about the same speed, and when started and set to work, became his own engineer.

Other inventions were made, and the separate condenser, piston, and cylinder were improved. Thus, after years of thought and labor, the steam engine at length stood full grown and ready for all kinds of work.

To make an invention is one thing. To get people to use it and so make it profitable is another. It is difficult to say which is the harder. In any case, Watt's troubles were not over.

All the time that Watt was working on his invention, mines were being flooded with water and had to be given up. Among the first orders for engines was one for a mine in Cornwall. Watt made the drawing with care, and the workmen did their best, for much depended on the first engine.

The engine was ready by the middle of 1777, and Watt went to set it up. The people were eager to get a look at the monster. Mine owners came from far and near to see it work. Many were doubtful, and some even wished that the engine might fail. But to the surprise of all it succeeded. It pumped water as they had never seen water pumped before. The size, the speed, and "the horrible noise of the engine," wrote Watt, "give satisfaction . . . and the noise seems to give great ideas of its powers." In a few days the mine was dry. It was the deepest mine in the district, and orders for engines began to come in. They came so fast that in the course of the next four or five years almost all the mines in England and Scotland were supplied.

Boulton, Watt's partner, felt from the first that the greatest field for the steam engine was in mills and factories. When orders for pumping engines fell off, Watt went to work on a factory engine. The first factory engine was built in 1782, and was for a corn mill.

The use of the steam engine in mills was opposed by the millers. They saw that to put steam engines to grinding corn and wheat would do away in many places with windmills and water mills. The working people also were stirred up. They were led to believe that if the steam engine was put in mills, it would take work away from them.

"It seems," wrote Watt, "the meddlers are determined to be masters of us. To put a stop to fire-engine mills, because they come in competition with water mills, would be as absurd as to put a stop to canals, because they with wagoners. . . . The argument that men are deprived of a livelihood would put a stop to the use of all machines whereby labor is saved. Carry out this argument, and we must do away with water mills themselves, and go back again to grinding corn by hand labor."

So strong was the opposition that Watt and Boulton decided to build a flour mill, to show what could be done. They built one at a cost of sixty thousand dollars, and put into it their newest and best engine. The mill attracted much attention. But it was not allowed to run long. So bitter was the feeling against the steam engine that the mill was set on fire and burned to the ground.

Though the mill was a total loss, it served its purpose. Orders for factory engines came in apace,—orders from France, from Italy, and from America. The advantages of steam power were now apparent. Water mills were stopped in the summer by the lack of water, and in the winter by frost, while steam mills worked on, by day and by night, in all kinds of weather, and in all seasons.

To bring the world to appreciate the value of the steam engine was thus a hard struggle. Down to the year 1785 every penny made from the sale of engines, amounting to more than two hundred thousand dollars, was put back into the business. Besides, large sums were borrowed. So great was the need for money that even the patents were mortgaged. Time and again it seemed as if all would be lost. More than once Watt and Boulton felt that this would be a blessing. The mine owners, for instance, refused to pay for the engines which had saved them thousands of dollars. Dishonest persons stole and used their patents. They were continually annoyed by rumors that a better engine was on the point of being completed. Efforts were even made to get Parliament to take away their patents.

"We are in the state of the old Roman," Watt wrote, "who was found guilty of raising better crops than his neighbors, and was ordered to bring before the assembly of the people his instruments of husbandry, and tell them of his arts. He complied, and when he had done, said, 'These, O Romans, are the instruments of our art, but I cannot bring into the forum the labors, the sweats, the watchings, the anxieties, the cares which produce the crops.' So everyone sees the reward which we may yet probably receive from our labors; but few consider the price we have paid for that reward, which is by no means certain."

Difficulty after difficulty was, however, battled down. Parliament refused to take away the patents. Persons who used them without right were punished. The mine owners were forced to pay what they owed. The business, after long waiting and untold distress, began to pay.

The partnership between Watt and Boulton came to an end in 1800. Watt was now well-to-do. Relieved of business cares and worry his health improved. He built a beautiful country home at Hearthfield. From there he made trips to different parts of Scotland, Wales, and England. To Hearthfield came old friends and the greatest men of England to visit him. Inventing continued to give him the greatest pleasure. A room was fitted up in the attic of the house, and there he would work for days at a time. This room remains just as it was in 1819.

On a monument erected to Watt's memory in Westminster Abbey are these inspiring words: