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 t