There he could reach the foot of the shrouds of the foremast, which happened to be immediately above the port.

He swung himself up, and, placing his hands on the edge of the bulwark, cautiously looked over.
At present all was quiet there; the signal from below had not been given, and the troops on deck--for, owing to the numbers on board, one fourth were always on deck in fine weather--were standing about or sitting in groups.

Keeping his feet on the ledge which ran round level with the deck, and his fingers on the top of the bulwark, Jack managed to edge his way aft until he reached the line of the quarterdeck.

Here the line of the bulwark ceased, the cabins of the officers rising, as was usual in those days, in a double tier high about the waist.
The nearest porthole, which was open, was but three feet long, and Jack, reaching forward, put one hand in it and continued his way.

The porthole was but just large enough for him to squeeze through.