Sentence:  For that an <WSD>"undercurrent</WSD>" (which ſome have beleived,) in the "ſtraights-mouth", will not ſolve this difficulty, unleſs occaſioned by a vaſt "Gulf" that muſt be placed ſomewhere in the "Atlantic" near the "Mouth" of the "ſtraight", which though overflown and hidden by that mighty ſea, yet may poſſibly abſorb the deeper waters, and ſo cauſe a contrary <WSD>"undercurrent</WSD>". 
Meaning of the word marked with <WSD> tags:  A current of water which flows under the surface, and often in a different direction from surface currents.