* {{cite book | last=Dales | first=Richard C. | title=The Scientific Achievement of the Middle Ages | publisher=University of Pennsylvania Press | publication-place=Philadelphia, Pa | date=1973-11 | isbn=978-0-8122-1057-6}}
<ref name="p892">{{cite book | last=Dales | first=Richard C. | title=The Scientific Achievement of the Middle Ages | publisher=University of Pennsylvania Press | publication-place=Philadelphia, Pa | date=1973-11 | isbn=978-0-8122-1057-6 | page=}}</ref>