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 with resistance from the shore, occasions the sea to swell and break in the manner described? To this I object that there seems no regular correspondence between their magnitude and the apparent agitation of the water without them: that gales of wind, except at particular periods, are very unfrequent in the Indian seas, where the navigation is well known to be remarkably safe, whilst the surfs are almost continual; and that gales are not found to produce this effect in other extensive oceans. The west coast of Ireland borders a sea nearly as extensive and much more wild than the coast of Sumatra, and yet there, though when it blows hard the swell on the shore is high and dangerous, is there nothing that resembles the surfs of India.
PROBABLE CAUSE OF THE SURF.
These, so general in the tropical latitudes, are, upon the most probable hypothesis I have been able to form, after long observation and much thought and inquiry, the consequence of the trade or perpetual winds which prevail at a distance from shore between the parallels of thirty degrees north and south, whose uniform and invariable action causes a long and constant swell, that exists even in the calmest weather, about the line, towards which its direction tends from either side. This swell or libration of the sea is so prodigiously long, and the sensible effect of its height, of course, so much diminished, that it is not often attended to; the gradual slope engrossing almost the whole horizon when the eye is not very much elevated above its surface: but persons who have sailed in those parts may recollect that, even when the sea is apparently the most still and level, a boat or other object at a distance from the ship will be hidden from the sight of one looking towards it from the lower deck for the space of minutes together. This swell, when a squall happens or the wind freshens up, will for a time have other subsidiary waves on the extent of its surface, breaking often in a direction contrary to it, and which will again subside as a calm returns without having produced on it any perceptible effect. Sumatra, though not continually exposed to the south-east trade-wind, is not so distant but that its influence may be presumed to extend to it, and accordingly at Pulo Pisang, near the southern extremity of the island, a constant southerly sea is observed even after a hard north-west wind. This incessant and powerful swell rolling in from an ocean, open even to the pole, seems an agent adequate to the prodigious effects produced on the coast; whilst its very size contributes to its being overlooked. It reconciles almost all the difficulties which the phenomena seem to present, and in particular it accounts for the decrease of the surf during the north-west monsoon, the local wind then counteracting the operation of the general one; and it is corroborated by an observation I have made that the surfs on the Sumatran coast ever begin to break at their southern extreme, the motion of the swell not being perpendicular to the direction of the shore. This manner of explaining their origin seems to carry much reason with it; but there occurs to me one objection which I cannot get over, and which a regard to truth obliges me to state. The trade-winds are remarkably steady and uniform, and the swell generated by them is the same. The surfs are much the reverse, seldom persevering for two days in the same degree of violence; often mountains high in the morning and nearly subsided by night. How comes a uniform cause to produce effects so unsteady, unless by the intervention of secondary causes, whose nature and operation we are unacquainted with?
It is clear to me that the surfs as above described are peculiar to those climates which lie within the remoter limits of the trade-winds, though in higher latitudes large swells and irregular breakings of the sea are to be met with after boisterous weather. Possibly the following causes may be judged to conspire, with that I have already specified, towards occasioning this distinction. The former region being exposed to the immediate influence of the two great luminaries, the water, from their direct impulse, is liable to more violent agitation than nearer the poles where their power is felt only by indirect communication. The equatorial parts of the earth performing their diurnal revolution with greater velocity than the rest, a larger circle being described in the same time, the waters thereabout, from the stronger centrifugal force, may be supposed to feel less restraint from the sluggish principle of matter; to have less gravity; and therefore to be more obedient to external impulses of every kind, whether from the winds or any other cause.
TIDES.
The spring-tides on the west coast of Sumatra are estimated to rise in general no more than four feet, owing to its open, unconfined situation, which prevents any accumulation of the tide, as is the case in narrow seas. It is always high-water there when the moon is in the horizon, and consequently at six o'clock nearly, on the days of conjunction and opposition throughout the year, in parts not far remote from the equator.* This, according to Newton's theory, is about three hours later than the uninterrupted course of nature, owing to the obvious impediment the waters meet with in revolving from the eastward.
(*Footnote. Owing to this uniformity it becomes an easy matter for the natives to ascertain the height of the tide at any hour that the moon is visible. Whilst she appears to ascend the water falls and vice versa; the lowest of the ebb happening when she is in her meridian. The vulgar rule for calculating the tides is rendered also to Europeans more simple and practical from the same cause. There only needs to add together the epact, number of the month, and day of the month; the sum of which, if under thirty, gives the moon's age--the excess, if over. Allow forty-eight minutes for each day or, which is the same, take four-fifths of the age, and it will give you the number of hours after six o'clock at which high-water happens. A readiness at this calculation is particularly useful in a country where the sea-beach is the general road for travelling.)