The words 'crusade' and 'crusader', which I have used here for clarity, are relatively modern terms, derived from the twelfth-century term crucesignati (signed by the cross).

The historical 'turning point' at Poitiers, AD 732, was regarded by Edward Gibbon as 'an encounter which would change the history of the world'. In his Decline and Fall of the Roman Empire (1776-1788), Gibbon opined that following an Arab victory, 'Perhaps the interpretation of the Koran would now be taught in the schools of Oxford, and her pulpits might demonstrate to a circumcised people the sanctity and truth of the revelation of Mahomet.' A more recent essay covering Poitiers is by Barry S. Strauss in What If? (Putnam's, 1999); in the same volume Cecelia Holland speculates on the turning-back of the Mongols in 1242.

An account of the English opposition after the Norman Conquest is Peter Rex's The English Resistance (Tempus, 2004).

The eleventh-century 'flying monk' Aethelmaer of Malmesbury (or Eilmer, Elmer or Oliver) was an historical character, mentioned in William of Malmesbury's twelfth-century history Gesta Regum Anglorum ('The History of the English Kings'). I have based some of Aethelmaer's (fictitious) engine designs on the sketches of Leonardo da Vinci (1452- 1519), including the gunpowder engine; see for example Leonardo da Vinci's Machines by Marco Cianchi (Becocci Editore, 1984). The career of the ninth-century Moorish aviator Ibn Firnas is described in, for example, the Dictionary of Scientific Biography (Scribner). An airport to the north of Baghdad is named after him, as is a crater on the Moon.

Roger Bacon, born c. 1220, was another historical character, a philosopher, educational reformer and an early proponent of experimental science. Bacon was indeed the first European to describe the properties of gunpowder, and he speculated on its use in weaponry. This was largely based on a study of Chinese firecrackers brought to him from the Mongol court by a missionary called William Ruysbroek, a few years after the date of Bacon's fictitious encounter with the Engines of God shown here (Part II Chapter XXII). However (in our timeline) the manufacture of gunpowder remained unknown in the west for decades after Bacon, and the flintlock musket was not developed until c. 1550. See Brian Clegg's The First Scientist: A Life of Roger Bacon (Constable, 2003), Gunpowder by Clive Ponting (Chatto amp; Windus, 2005), and Joseph Needham's Science and Civilisation in China (Cambridge University Press, 1962).

A recent reference on the Black Death is The Great Mortality by John Kelly (Fourth Estate, 2005). The notion that the Chinese in their great age of exploration in the fifteenth century might have reached Australia and perhaps gone much further is set out in Gavin Menzies's 1421: The Year China Discovered America (Transworld, 2002).

A solid reference on the life of Columbus, based on the primary sources, is The Worlds of Christopher Columbus by William D. Phillips and Carla Rahn Phillips (Cambridge, 1992). A recent study of the events of the years leading up to 1492 is James Reston's Dogs of God: Columbus, the Inquisition and the Defeat of the Moors (Doubleday, 2005).

My research for this book took me to Ronda, Granada, Cordoba, Seville and elsewhere in Andalucia, as well as locations closer to home such as Harbottle in Northumberland. As I noted in the first book of this series, there is no substitute for visiting such wonderful places.

Any errors or inaccuracies are my sole responsibility.

Stephen Baxter

Northumberland

February 2007