Such a sight is furnished to the Parrott rifles, and is desirable for all guns.
304. In case the ordinary sights should be lost or rendered useless, tangent firing may be resorted to against ships, by pointing with the wooden dispart-sight at such part of the ship as the Tables indicate for the distance, and according to the class of gun in use at the time.
A Table of this kind is appended, which has been calculated for the 8-inch and some of the heavier of the 32-pounder guns when loaded with single shot and distant-firing charges.
The different classes of sailing ships-of-war, whether of the same or of different nations, are not of the same length, nor are their masts of the same height from the deck, or from the water. They, however, correspond so nearly, for the same class of ships of the same nation, that calculations made from the angles subtended by the average height of their masts, will generally give their distance with sufficient accuracy for general firing.
Tables are inserted at the end of the book, in which the distances corresponding to different angles made by the masts of English and French ships-of-war are shown—from which the intermediate distances due to other angles may be estimated, and the sights regulated accordingly, if circumstances should render it desirable. Also an abridged Table, in which the height of our own mast is used as the base.
305. Officers of divisions and Captains of guns should be occasionally practised in measuring the distances of objects by the eye, at times when opportunities offer of verifying the accuracy of their estimate by comparing it with the distance obtained by the foregoing methods, or any other which will afford the best means of comparison.
306. Within point-blank range, if the hull of an enemy's vessel is obscured by smoke or darkness, the aim may be directed by the flashes of his guns.
307. Most naval guns are now fitted with elevating screws, passing through a hole in the cascabel of the Dahlgren system, and for those of the old system attached to the carriage: but the ordinary beds and quoins are also still in use; they are arranged to allow the extreme elevation and depression of the guns which the ports will admit with safety. When the inner or thick end of the quoin is fair with the end of the bed in place, the gun is level in the carriage; or horizontal, when the ship is upright. The degrees of elevation above this level, which may be given to the gun by drawing out the quoin when laid on its base, are marked on the side or edge, and those of depression on the flat part of the quoin, so that when the quoin is turned on its side for depressing, the marks may be seen. The level mark on the quoin is to correspond with the end of the bed. When the quoin is entirely removed, and the breech of the gun rests on the bed, the gun has its greatest safe elevation; and when the quoin is pushed home on its side, the gun has the greatest safe depression that the port will admit.
Care must be taken that the stop on the quoin is always properly lodged, to prevent the quoin from flying out or changing its position, and that the bed is secured to the bed-bolt.
Porter's bed and quoin has been adopted for all carriages requiring quoins. This quoin, being graduated to whole degrees, requires a small additional quoin for slight differences of elevation in smooth water.