  A simply structured distributed observer is described for estimating the
state of a continuous-time, jointly observable, input-free, linear system whose
sensed outputs are distributed across a time-varying network. It is explained
how to design a gain $g$ in the observer so that their state estimation errors
all converge exponentially fast to zero at a fixed, but arbitrarily chosen rate
provided the network's graph is strongly connected for all time. A linear
inequality for $g$ is provided when the network's graph is switching according
to a switching signal with a dwell time or an average dwell time, respectively.
It has also been shown the existence of $g$ when the stochastic matrix of the
network's graph is chosen to be doubly stochastic under arbitrarily switching
signals. This is accomplished by exploiting several well-known properties of
invariant subspaces and properties of perturbed systems.
