The force of the surf is extremely great.  I have known it to overset a country vessel in such a manner that the top of the mast has stuck in the sand, and the lower end made its appearance through her bottom.  Pieces of cloth have been taken up from a wreck, twisted and rent by its involved motion.  In some places the surfs are usually greater at high, and in others at low, water; but I believe they are uniformly more violent during the spring-tides.

CONSIDERATIONS RESPECTING THE CAUSE OF THE SURF.

I shall proceed to inquire into the efficient cause of the surfs.  The winds have doubtless a strong relation to them.  If the air was in all places of equal density, and not liable to any motion, I suppose the water would also remain perfectly at rest and its surface even; abstracting from the general course of the tides and the partial irregularities occasioned by the influx of rivers.  The current of the air impels the water and causes a swell, which is the regular rising and subsiding of the waves.  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.