In special religlous events, visitors may have the chance to see the unique ceremony of the botafumeiro, a spectacular incensory made of silver-plated brass, weighing eighty kilos, whose original purpose was to perfume the church.
It hangs on a rope from the centre of the transept, and it is nudged from vertical by being pushed. As it swings like a pendulum, eight men (called tiraboleiros) let out rope at the apex of the swing and pull on it at the lowest point. This amplifies the incensory’s oscillation swinging it to 21 metres up in the top of the vault, in a 65-metre arc along the transept from the Azabachería to the Praterias doorways. It passes along at floor level at a speed of 68 km/h, leaving behind it a fine trall of smoke and a fragrance of incense.
The magazine Scientific American published a study on “The Physics of the Botafumeiro”. In it, its author, Juan Ramón Sanmartín Losada, stated that the method of handling this huge incensory couid have given rise to the first experiments in “determinist chaos”, a law of physics similar to that which applies for a swing or a pendulum. In the case of the botafumeiro, however, this law is even more obvious thanks to the accelerating action of the tiraboleiros, since they do not push, but pump. The system for pumping the incensory was discovered about 400 years before Galileo and Hyghens analyzed the pendulum, although it was not applied before the 13th century, when a multiplying mechanism was introduced, based on coaxial drums of different diameters, without which the maximum radial movement of the botafumeiro wouid be one and a half metres, equal to the length of the rope pulled by the tiraboleiros, and the maximum allowed in a comfortable pumping action.