The work of all these men provided the world with a solid scientific foundation upon which it was possible to build even the most complicated of engines, and a number of practical men made good use of it. The Middle-Ages had used wood for the few bits of necessary machinery. But wood wore out easily. Iron was a much better material, but iron was scarce except in England. In England therefore most of the smelting was done. To smelt iron, huge fires were needed. In the beginning, these fires had been made of wood, but gradually the forests had been used up. Then “stone coal” (the petrified trees of prehistoric times) was used. But coal as you know has to be dug out of the ground and it has to be transported to the smelting ovens and the mines have to be kept dry from the ever invading waters. These were two problems which had to be solved at once. For the time being, horses could still be used to haul the coal-wagons, but the pumping question demanded the application of special machinery. Several inventors were busy trying to solve the difficulty. They all knew that steam would have to be used in their new engine. The idea of the steam engine was very old. Hero of Alexandria, who lived in the first century before Christ, has described to us several bits of machinery which were driven by steam. The people of the Renaissance had played with the notion of steam-driven war chariots. The Marquis of Worcester, a contemporary of Newton, in his book of inventions, tells of a steam engine. A little later, in the year 1698, Thomas Savery of London applied for a patent for a pumping engine. At the same time, a Hollander, Christian Huygens, was trying to perfect an engine in which gun-powder was used to cause regular explosions in much the same way as we use gasoline in our motors. [Illustration: THE MODERN CITY] All over Europe, people were busy with the idea. Denis Papin, a Frenchman, friend and assistant of Huygens, was making experiments with steam engines in several countries. He invented a little wagon that was driven by steam, and a paddle-wheel boat. But when he tried to take a trip in his vessel, it was confiscated by the authorities on a complaint of the boatmen’s union, who feared that such a craft would deprive them of their livelihood. Papin finally died in London in great poverty, having wasted all his money on his inventions. But at the time of his death, another mechanical enthusiast, Thomas Newcomen, was working on the problem of a new steam-pump. Fifty years later his engine was improved upon by James Watt, a Glasgow instrument maker. In the year 1777, he gave the world the first steam engine that proved of real practical value.