n the rising and formation of the surf the water seems to have a
quick progressive motion towards the land, yet a light body on the
surface is not carried forward, but, on the contrary, if the tide is
ebbing, will recede from the shore; from which it would follow that the
motion is only propagated in the water, like sound in air, and not the
mass of water protruded. A similar species of motion is observed on
shaking at one end a long cord held moderately slack, which is expressed
by the word undulation. I have sometimes remarked however that a body
which sinks deep and takes hold of the water appears to move towards
shore with the course of the surf, as is perceptible in a boat landing
which seems to shoot swiftly forward on the top of the swell; though
probably it is only after having reached the summit, and may owe its
velocity to its own weight in the descent.

Countries where the surfs prevail require boats of a peculiar
construction, and the art of managing them demands the experience of a
man's life. All European boats are more or less unfit, and seldom fail to
occasion the sacrifice of the people on board them, in the imprudent
attempts that are sometimes made to land with them on the open coast. The
natives of Coromandel are remarkably expert in the management of their
craft; but it is to be observed that the intervals between the breaking
of the surfs are usually on that coast much longer than on the coast of
Sumatra.

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