thimble

To relieve the stern in a measure from the great weight of the [diving-]bell, a short, strong mast was stepped in the usual place, and steeving forward; from its head to the head of the davit was a span, which set up with two thimbles [footnote: A sort of iron ring.] and a lanyard, and the whole was supported by a strong stay from the mast-head to the stern of the boat, and two shrouds on a side, leading forward.