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Arecibo observations of the conal triple pulsar B1918+19 at 0.327- and1.4-GHz are used to analyse its subpulse behaviour in detail. We confirm thepresence of three distinct drift modes (A,B,C) plus a disordered mode (N) andshow that they follow one another in specific cycles. Interpreting the pulsar'sprofile as resulting from a sightline traverse which cuts across an outer coneand tangentially grazes an inner cone, we demonstrate that the phase modulationof the inner cone is locked to the amplitude modulation of the outer cone inall the drift modes. The 9% nulls are found to be largely confined to thedominant B and N modes, and, in the N mode, create alternating bunches of nullsand emission in a quasi-periodic manner with an averaged fluctuation rate ofabout 12 rotation periods ($P_1$). We explore the assumption that the apparentdrift is the first alias of a faster drift of subbeams equally spaced aroundthe cones. This is shown to imply that all modes A, B and C have a commoncirculation time of 12 $P_1$ and differ only in the number of subbeams. Thistimescale is on the same order as predicted by the classic {\bf E}$\times${\bfB} drift and also coincides with the N-mode modulation. We therefore arrive ata picture where the circulation speed remains roughly invariant while thesubbeams progressively diminish in number from modes A to B to C, and are thenre-established during the N mode. We suggest that aliasing combined withsubbeam loss may be responsible for apparently dramatic changes in drift ratesin other pulsars.

Drifting, moding, and nulling: another look at pulsar B1918+19