The Mark III optical interferometer has been in routine operation on Mt. Wilson, near Los Angeles CA, since 1988. Because it employs active fringe-tracking, seeing measurements are a natural byproduct of any astronomical observation. An automated seeing analysis program has allowed us to extract measurements of the coherence time, t(omicron ), and the high-frequency spectral index of the power spectrum of fringe motion for all clear nights in the observing seasons 1989-1991. This provides a unique database of the seeing; very few other measurements of the temporal properties of astronomical seeing exist, and none with such a representative sampling of nights. We examine various statistical properties of the seeing and infer quantities of interest to the design of future interferometers.