Currents on the Coast of GUINEA.
The incomparable Sir Isaac Newton solves all the Phænomena of the Tides upon the sole Principle of Gravity. The Earth, Sun, Moon, and all the celestial Bodies, says he, have a Gravitation towards their Centers, in proportion to the quantitys of Matter in each of them. Our Earth being within the Activity of the Sun and Moon’s Attractions; the fluid part of this Globe, the Ocean, feels it, and necessarily swells: and this, by taking off or abating the force of Gravity, wherever the Moon is perpendicular in the Ecliptick; whereby the greater Pressure of Gravity without, carries the Water thitherward, &c. (See the Theory of the Tides, Philos. Transact.) Currents seem to be the same thing (at least on this Coast) and on the same Principle I shall lay down the Facts, and then draw the Conclusions.
The Rivers of Gambia, Sierraleon, Gabon, the Straits and Channels of Benin, and thro’ the whole Coast, the flowings are regular on the Shores, with this difference, that in Rivers or Channels where two Shores contract the Waters into a narrow Compass, the Tides are strong and high, as well as regular; but on the dead Coast, slow and low, not to above two or three foot, increasing as you advance towards the Bay and Channels of Benin; and this is farther evident at Cape Corso, Succonda, Commenda, and other places: for wherever the Land rounds and cheques, the Flowings there will be a Foot or two more than on an evener Coast, tho’ next adjoining.
The Currents here set sometimes two knots an hour along Shore, sometimes with, sometimes against the Wind; tho’ generally to Leeward; sometimes off, sometimes on, ripling like a Tide, at other times a smooth unmoved Surface for days together, and never felt, or imperceptible at 8 or 10 Leagues Offing.
The Currents set in on both Shores, to the Bay of Benin; from the Southward, about and beyond Cape Lopez; and from the Westward along the Papau Coast, that is, to Leeward; for the Winds are as commonly deflected along Shore, as the Currents. This all Ships experience in their Passage to Angola, if they hold the Land on board, or if they endeavour getting Westward on the Papau or Gold Coast.
The reason of this Diversity, I imagine, proceeds from the Formation of the Land, together with the Weather, and the Winds.
The Land being on a strait Line, without Gulphs or Bays, unless that remarkably large one of Benin and Callabar, the Flux of the Sea, when it comes to be bounded by the Shores, have a natural Tendency there, seen in growing stronger as they advance towards it on both sides; because such Gulphs, in a Contraction of the Waters, bear some Resemblance to Channels, which every where in proportion to their Breadth and Depth, and the Sea they stand open with, have more or less Current or Tide along their Shores inward; assisted partly by the Winds, which, as I have observed, are deflected, and tend also on both sides towards the Bay; and partly, by the Weather; clear and hot Sunshines drawing more Vapours from the Seas next Shores in all places, (and especially in Bays with Shoals) to be expended in Exhalations, Mists, Fogs, and Rains; the Rains again may, by being incessant for a Month or six Weeks, and in a Rotation upon different parts of the Coast, contribute to some little diversity of Strength.
Another reason of Currents tending mostly to Leeward here, is the Flood being propagated from a vast Southern Ocean, takes it’s Course along Shore; but the Ebbs revert easily and equally from all parts to the Ocean, and therefore make so little an Alteration of the Stream, as is seldom and scarcely felt at a very little distance from it. The most that we found was open with this Bite of Benin. We left Whydah the latter end of July, where tho’ the Currents in the Road were very strong to Leeward, and the Winds altogether S. W. yet we found we could with ease have weathered any of the Islands; which would have been impossible, had the same Current in the Road extended across the whole Bay; nay, our getting so far to the Southward (i. e. Windward) will be very difficult to account for, unless the Waters received into this Bay by those Currents are allowed to reverberate in the middle Space, tho’ insensibly, towards the main Ocean.
From these light Observations, I think it may be concluded, First, that in all Places, Currents and Tides have a very great affinity. That it is principally the Formation of the Lands drives them into the one or the other; if contracted between two Shores so as to form a Channel, the diurnal Elevation of the Ocean, by the Attraction of the Moon, will make there a Tide, rapid in proportion to it’s Breadth, Depth, and Sea it is open to: And if an open Coast, as Guinea, those Tides become Currents. This agrees with the Voyages I have met to those Parts, and particularly the sixth general one set forth by the India Company of that Channel, made by the Eastern side of the Continent, and the Island of Madagascar; for being too deep and broad for the Direction of a Tide, there are Northern and Southern Currents, as the elevated Sea rowls round the North or South end of the Island: and which is still more agreeable, they are strongest where the Channel is narrowest, and less, and vary on different Points of the Compass, as the Sea spreads more in the Passage cross the Line.
2. That all Currents and Tides are found only on Shores, and indiscernible at 10 Leagues distance from a Coast, or the Mouth of any Channel; are also variable from the same Influence of the Moon, and Change of Weather.