Few people know that Papin was the real inventor of the steam-engine. But everybody acknowledges his invention of a kettle for cooking food under high pressure. He called it a “digester.” When water is boiling it cannot be made any hotter by putting greater heat under it. Greater heat simply makes it boil away more quickly. But it can be made hotter by preventing the steam from escaping. When the lid is stuck on tight, and the spout is stopped up, the steam cannot escape until it reaches a high pressure, and this means a higher temperature at which the water boils. Papin, therefore, constructed a kettle which was closed at the top by means of a heavy weight. He did not close up the kettle with a screw, since that would have led to an explosion. He only made sure that the steam could not escape until it was hot enough and strong enough to lift the heavy weight. People thought at that time that if they could cook things at a very high temperature, they could get nourishment not only out of meat, but out of the bones as well. They thought they could make the bones eatable. Papin thought so too, and he cooked some bones in his new kettle. The hardest bones of beef and mutton were made as soft as cheese, and the extract from them made thick jelly, though the bones themselves were not good to eat. The digester is used to obtain gelatine from bones in this way at the present time. [Illustration: Fig. 44.--Papin’s Cylinder, in which the Piston was raised by boiling water under it.] When Papin seriously set about inventing the steam-engine, he was guided by what he remembered about a machine invented by Huygens. In the year 1674 he had himself constructed this machine. It was intended to raise water by an explosion of powder. The water was intended to be used for the great water-works in the palace of the King of France. A long cylinder containing a heavy piston was fixed in an upright position. In the bottom of the cylinder was a plug which could be screwed in. Before it was screwed in, some gunpowder and a slow match were placed on the plug. Shortly after the plug was screwed in, the match lighted the powder, the powder exploded, and drove the piston up to the top of the cylinder. When the cylinder cooled down, the pressure of the air drove down the piston. The piston was made to pull a chain going over a pulley to which buckets were attached.