The history of Sumatra

by William Marsden

SURF.

The surf (a word not to be found, I believe, in our dictionaries) is used in India, and by navigators in general, to express a peculiar swell and breaking of the sea upon the shore; the phenomena of which not having been hitherto much adverted to by writers I shall be the more circumstantial in my description of them.

The surf forms sometimes but a single range along the shore. At other times there is a succession of two, three, four, or more, behind each other, extending perhaps half a mile out to sea. The number of ranges is generally in proportion to the height and violence of the surf.

The surf begins to assume its form at some distance from the place where it breaks, gradually accumulating as it moves forward till it gains a height, in common, of fifteen to twenty feet,* when it overhangs at top and falls like a cascade, nearly perpendicular, involving itself as it descends. The noise made by the fall is prodigious, and during the stillness of the night may be heard many miles up the country.

(*Footnote. It may be presumed that in this estimation of its height I was considerably deceived.)

Though in 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. 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.