mystical energy required to conjure up one small white pigeon, or three normal-sized billiard balls. Several SI-modifiers have been applied to it (e.g. millithaum, kilothaum) in the books. Magic can be measured with a thaumometer, which is described as a black cube with a dial on one side. A standard thaumometer is good for up to a million thaums, beyond that level, Discworld reality starts to break down. Parodying the introduction of the metric system, later Discworld novels refer to the introduction of the newer unit, the Prime, to avoid arguments over the standard sizing of pigeons. It is defined as the magical energy required to move one pound of lead one foot, referencing the Joule. The thaum also appears to be a particle, the Discworld physics equivalent of the atom. "Splitting the thaum" revealed that it was in fact composed of numerous sub-particles, called resons ("thingies") which in turn are created from a combination of up to five "flavours": up, down, sideways, sex appeal, and peppermint (parodying the real-world quarks). In the opening books, the number eight is generally significant and has magical properties on the Disc, (e.g. the number of the colour of Magic, octarine) and should never be spoken by wizards, especially in certain places. Doing so may allow the ancient dungeon dimension creature "Bel-Shamharoth the sender of eight" to break through (this is taken to a somewhat extreme end in one book, as even the narrator takes great pains to avoid saying the word). On the other hand, eight turns up in many places one would expect the number seven in the real world (e.g. the Discworld week contains eight days, not seven). After The Colour of Magic, both the colour and the number eight no longer appear as dangerous. The Disc's magical field is centred on Cori Celesti. Everyday natural forces, such as light and magnetism, are muffled by the power of the Disc's magical field, and rather than a magnetised needle, navigators on the Disc use a compass with a needle of the magical metal octiron, which will always point towards Cori Celesti. Light is so oddly affected by magic that, as it passes into the Disc's atmosphere, it actually slows down from millions to hundreds of miles an hour. One odd effect of this is that the Disc has time zones, however, as a flat world, it should not. Another effect is that,