Which preposition to use with middlemost

of Occurrences 1%

For this island whereon ye stand is no true island, but a great fish stationary a-middlemost of the sea, whereon the sand hath settled and trees have sprung up of old time, so that it is become like unto an island; but when ye lighted fires on it, it felt the heat and moved; and in a moment it will sink with you into the depths of the sea and ye will be drowned.

Which preposition to use with  middlemost