h 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