For the purpose of comparing the different bearings of the guns, two points K, K, Figs. I and 2, Plate CCCCXCV1I. were assumed in the longitudinal axis XV of the vessels, at the distance of 17 feet from the efterpart of the counter of the square stern, and also from the afterpart of the lower stool of the curvilineal stern. From these points as centres, and with radii of 27 feet, two arcs or circles a b eeb a,abeffeba were described, the former, as the figure indicates, surrounding- the square stern, and the latter that of the curvilineal form. To these circumferences, the various arcs or ranges swept over by the guns, in their translation from one bearing to another, were in all cases referred.
The first experiments were performed on board the square stern vessel. An eighteen pounder was placed at the aft..•r broadside port, and trained to its greatest possible angle Wore the beam, as denoted by the lines A a, Fig. 1, or Fig. 3, Plate CCCCXCV11.; forming, in the first mentioned figure, an angle a AV of 61'. with the principal longitudinal axis XV of the vessel; the outer extremity of the muzzle of the gun being at the same time within the external edge of the port four inches. • This bearing- being determined, the gun was text brought into the position denoted by the line B b, Fig. I, being the greatest possible angle at which it could be trained abaft the beam; the line of fire forming with the principal axis of the vessel the angle b BX of 351°. The arc al), intercepted between the two bearings, amounted to 46'; and it hence followed. that an object placed in any part of it could be hit be a shot from the after broadside port, confined of course to the limits prescribed by the ordinary charge of powder. The circular dots introduced in the arc a h, as also in all the arcs which may be hereafter al luded to, are designed by Mr. Harvey to indicate, that every part of the space contained between the extreme bearings, can be perfectly defended.
The exact position of the point b having been de termined by the last experiment, the gun was next re moved to the adjacent port or the stern, and traMed to its greatest possible, angle, as denoted by the line C c, Fig. I, or Fig. 4, and forming with the axis XV the angle c CX of 2 . To obtain this bearing. the muzzle was brought 4 feet within the fore part of the rail, creating thereby great danger from fire. It will
also be observed, by referring to the former figure, that the truck of the gun was brought into immediate contact with the rudder head, so that the utmost bearing was determined.
This being the greatest bearing that could be ob tained with a stern gun directed towards the adjacent quarter or the ship, necessarily left the arc b c, Fig. 1, amounting to entirely undefended; and it was also remarked, that the bearings B b and C c were not in directions parallel to each other, but in a state of divergency, amounting to three degrees; and that therefore the extreme lines of fire proceeding from the after broadside port. and the adjacent port in the stern, could not, under the present circumstances, be made to "cross," and consequently, that a "point of impunity" existed.
Desirous, however, of discovering if it would be possible, under any circumstances. consistently with the preservation or the frame of the ship, to make the lines of fire issuing from the last-mentioned ports intersect each other, an estimate was made by Mr. Harvey and the naval officers present, to determine what alteration would be produced in the bearing of the gun at the stern port, by supposing the rudder head removed. The utmost effect, however, that could • be produced by this arrangement in the bearing of the gun, amounted only to a diminution of a degree and a half of the divergence before determined, the new line of fire being the direction E e, and which, therefore, still kept the bearings of the two guns from a state or parallelism, and consequently preserved a "point of impunity" between them.
The undefended arc b c was of course diminished by the same quantity as the divergence of the stern gun was altered, the arc b e in this new condition amounting to thirty-one degrees. And hence it ap pears that the lines of fire proceeding from the after broadside port, and the adjacent port in the stern, cannot be made to cross even when the rudder-head is removed, unless by destroying one of the sides of the former port, or a port of the stern frame; and that a point of 'impunity therefore exists on the quarter of a square stern vessel. which it is impossible altogether to remove, unless by injuring very materially the strength of the ship.