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