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