Among the earliest inventions were the abacus, the public toilet, and the "shadow clock".[29] Joseph Needham noted the "Four Great Inventions" of China as among some of the most important technological advances; these were the compass, gunpowder, papermaking, and printing, which were later known in Europe by the end of the Middle Ages. The Tang dynasty (AD 618 - 906) in particular was a time of great innovation.[29] A good deal of exchange occurred between Western and Chinese discoveries up to the Qing dynasty.