A mean value calculus is presented. It is an extension of the Duration
Calculus.  The calculus reasons about mean values of durations of
Boolean functions instead of integrals. With mean values, we can
define integrals and specify point values of functions, and therefore
instant actions, including state transitions and events. A state
transition of a system determines the system states in a neighbourhood
of the transition, and is called a state germ using terminology from
topology. State transitions of an automaton are caused by events. Thus
the behaviour of an automaton can be formulated by predicates of
events and state germs. As an application of the calculus, state germs
and their inference rules are introduced and are used to specify and
reason about combinational circuits and automata.

{\bf Keywords:}Duration Calculus, Mean Value Calculus, State Germs,Combinational Circuit, Automata,Real-Time System.