We present observations of dispersion measure (DM) variability in the directions of pulsars PSR 1821-24, PSR 1855+09, PSR 1937+21, and PSR 1951+32, and relate these results to previously reported work. Temporal variations of pulsar dispersion measures provide a direct sample of the integrated spectrum of electron density fluctuations in the intervening plasma. The spectrum is sampled over a range of length scales from 10 13 to 10 15 cm owing to the relative peculiar motion of the Sun and the pulsar with respect to their local standards of rest. The amplitude of the variations over a fixed time interval appears to depend on the square root of the distance