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.