Plate II. fig. 20. is a perspective view of the compass, when in order for observation; the point of view being the center of the card, and the distance of the eye two feet. A B. is the wooden box in which it is usually contained. K. is a cat-gut line drawn from the inside of the box for determining the degree upon the brass edge. L, M, N, O. is the index bar with its two stiles, and cat-gut threads, which being taken off from the top of the box, is placed in two pieces P Q, notched properly to receive it. The other parts of the figure, with their references, are explained in the article COMPASS. B. BACK _of the post_. See the article STERN-POST. To BACK _an anchor_, _empeneller_, to carry out a small anchor, as the stream or kedge, ahead of the large one, by which the ship usually rides, in order to support it, and prevent it from loosening, or _coming home_, in bad ground. In this situation, the latter is confined by the former, in the same manner that the ship is restrained by the latter. _To BACK astern_, in rowing, _scier à culer_, is to manage the oars in a direction contrary to the usual method, so as that the boat or vessel impressed by their force, shall retreat, or move with her stern foremost, instead of advancing. _To BACK the sails_, is to arrange them in a situation that will occasion the ship to retreat or move astern. This operation is particularly necessary in narrow channels, when a ship is carried along sideways by the strength of the tide or current, and it becomes requisite to avoid any object that may intercept her course, as shoals, or vessels under sail, or at anchor: it is also necessary in a naval engagement, to bring a ship back, so as to lie opposite to her adversary, when she is too far advanced in the line. See ABACK. BACK-BOARD, a piece of board of a semicircular figure, placed transversely in the after-part of a boat, like the back of a chair, and serving the passengers to recline against whilst sitting in the stern-sheets. See BOAT. BACK-STAYS, _cale haubans_, (from _back_ and _stay_) long ropes reaching from the topmast-heads to the starboard and larboard sides of the ship, where they are extended to the channels: they are used to support the top-masts, and second the efforts of the shrouds, when the mast is strained by a weight of sail in a fresh wind. They are usually distinguished into breast-back-stays and after-back-stays; the intent of the former being to sustain the top-mast when the force of the wind acts upon the ship sideways, or, according to the sea-phrase, when the ship sails upon a wind; and the purpose of the latter is to enable it to carry sail when the wind is further aft. There are also back-stays for the top-gallant-masts, in large ships, which are fixed in the same manner with those of the top-masts.