Prague overcame the difficulty in 1525 by the invention of the fusee, a kind of conical pulley interposed between the barrel, or circular drum containing the mainspring, and the train of wheels which the spring has to drive. The principle of the "drum and fusee" action will be understood from Fig. 201. The mainspring is a long steel ribbon fixed at one end to an arbor (the watchmaker's name for a spindle or axle), round which it is tightly wound. The arbor and spring are inserted in the barrel. The arbor is prevented from turning by a ratchet, B, and click, and therefore the spring in its effort to uncoil causes the barrel to rotate.
A string of catgut (or a very fine chain) is connected at one end to the circumference of the drum, and wound round it, the other end being fixed to the