. This rise and fall is similar to the vibrations
of a pendulum and subject to like laws. When a wave is at its height it
descends by the force of gravity, and the momentum acquired in descending
impels the neighbouring particles, which in their turn rise and impel
others, and thus form a succession of waves. This is the case in the open
sea; but when the swell approaches the shore and the depth of water is
not in proportion to the size of the swell the subsiding wave, instead of
pressing on a body of water, which might rise in equal quantity, presses
on the ground, whose reaction causes it to rush on in that manner which
we call a surf. Some think that the peculiar form of it may be plainly
accounted for from the shallowness and shelving of the beach. When a
swell draws near to such a beach the lower parts of the water, meeting
first with obstruction from the bottom, stand still, whilst the higher
parts respectively move onward, by which a rolling and involved motion is
produced that is augmented by the return of the preceding swell. I object
that this solution is founded on the supposition of an actual progressive
motion of the body of water in forming a surf; and, that certainly not
being the fact, it seems deficient. The only real progression of the
water is occasioned by the perpendicular fall, after the breaking of the
surf, when from its weight it foams on to a greater or less distance in
proportion to the height from which it fell and the slope of the shore.

That the surfs are not, like common waves, the immediate effect of the
wind, is evident from this, that the highest and most violent often
happen when there is the least wind and vice versa. And sometimes the
surfs will continue with an equal degree of violence during a variety of
weather. On the west coast of Sumatra the highest are experienced during
the south-east monsoon, which is never attended with such gales of wind
as the north-west. The motion of the surf is not observed to follow the
course of the wind, but often the contrary; and when it blows hard from
the land the spray of the sea may be seen to fly in a direction opposite
to the body of it, though the wind has been for many hours in the same
point.

Are the surfs the effect of gales of wind at sea, which do not happen to
extend to the shore but cause a violent agitation throughout a
considerable tract of the waters, which motion, communicating with less
distant parts, and meeting at length wit