History/how to of this ancient device: "Air gets very hot when it is compressed under high pressure. A classic example would be the heat that is created when one uses a bicycle pump. But when the air is compressed in a firepiston it is done so quickly and efficiently that it can reach a temperature in excess of 800 degrees Fahrenheit. This is hot enough to ignite the tinder that is placed in the end of the piston which has been hollowed out to accept it."
