See also pendulums for divination and dowsing. The seconds pendulum was then used as a standard for calibrating pendulums with different periods. In mantel clocks, half-second pendulums, long, or shorter, are used. Other dangers include lava, fireballs, giant sharp pendulums, and various other booby traps. A harmonograph creates its figures using the movements of damped pendulums. Resonances were first characterized in mechanical systems such as pendulums. Huygens was inspired by investigations of pendulums by Galileo Galilei beginning around 1602. Foucault pendulums now swing in museums around the world. These long pendulums required long narrow clock cases. This property, called isochronism, is the reason pendulums are so useful for timekeeping. However, the effects on the pendulums affected by the perturbation are dramatically different. Wiggling one of the upside-down pendulums will have a very different effect from before. There are numerous Foucault pendulums at universities, science museums, and the like throughout the world. Some amusement park rides act as conical pendulums. The complex sine function is also related to the level curves of pendulums.