certain large sails extended across the topmasts, by the topsail yard above, and by the yard attached to the lower mast beneath; being fastened to the former by robands, and to the latter by means of two great blocks fixed on its extremities, through which the topsail-sheets are inserted, passing from thence to other two blocks fixed on the inner part of the yard close by the mast; and from these latter the sheets lead downwards to the deck, where they may be slackened or extended at pleasure.