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.