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We report measurements of centripetal accelerations of maser spectralcomponents of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the secondpaper of a series, in which the goal is determination of a new geometric maserdistance to NGC 4258 accurate to possibly ~3%. We measure accelerations using aformal analysis method that involves simultaneous decomposition of maserspectra for all epochs into multiple, Gaussian components. Components arecoupled between epochs by linear drifts (accelerations) from their centroidvelocities at a reference epoch. For high-velocity emission, accelerations liein the range -0.7 to +0.7 km/s/yr indicating an origin within 13 degrees of thedisk midline (the perpendicular to the line-of-sight to the black hole).Comparison of high-velocity emission projected positions in VLBI images, withthose derived from acceleration data, provides evidence that masers trace realgas dynamics. High-velocity emission accelerations do not support a model oftrailing shocks associated with spiral arms in the disk. However, we findstrengthened evidence for spatial periodicity in high-velocity emission, ofwavelength 0.75 mas. This supports suggestions of spiral structure due todensity waves in the nuclear accretion disk of an active galaxy. Accelerationsof low-velocity (systemic) emission lie in the range 7.7 to 8.9 km/s/yr,consistent with emission originating from a concavity where the thin, warpeddisk is tangent to the line-of-sight. A trend in accelerations of low-velocityemission as a function of Doppler velocity may be associated with disk geometryand orientation, or with the presence of spiral structure.

Toward a New Distance to the Active Galaxy NGC 4258: II. Centripetal Accelerations and Investigation of Spiral Structure