The presumed-unvarying flow of time is measurable differentially by observing a constant-rate process such as the movement of the sun's shadow on an equatorial sundial, or integratively by counting a series of evenly repeated events such as meridian transits of the sun (solar time) or of a star (sidereal time). For multiples of whole days, time has always been measured integratively, and the necessary counting of days and their multiples of weeks, months and years has been assisted by the use of tallies, diaries, calendars and historical records. Precision in differential time measurement of fractions of days was dependent first upon the development of accurate means of measuring the constant-rate apparent angular displacement of the sun and stars. The availability of alternative differential methods followed the discovery of such other constant-rate processes as flows of liquids under constant pressure. Precision in the use of these methods depended in the first instance on accurate measurements of length. The first use of an integrative method for measuring periods of time less than a whole day was probably the mental counting, or tallying, of the constant periods required for the repeated slow filling or emptying of a vessel full of water, or later sand. More general use of integrative methods was, however, dependent upon the eventual discovery of unvarying repetitive processes such as the vibrations of a foliot, spring-controlled balance, pendulum, tuning fork, quartz crystal, or atomic particle, and the development of mechanically or electrically operated apparatus for maintaining such processes.