Time and its measurement has always preoccupied man. First he utilised natural time keepers: the rotation of the earth round the sun, giving the day and the year. But to divide the days into minutes and hours an accurate device was required. Galileo realised that he had one available in his own body-his heartbeat; and by counting his pulses while watching the incense-burning pendulum swinging in Padua Cathedral he made a fundamental discovery.