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. [Illustration: FIG. 5.--AN ANCIENT METHOD OF DRAWING WATER.] [Illustration: FIG. 6.--PAPIN'S ENGINE, 1690.] 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.