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obtained for a turret battery on that spot, and which might possibly supersede the necessity of erecting the south-east battery in the somewhat secluded position near Half Die.
21. Referring to this plan of distributing the three new batteries, it seems clear that any vessel passing up mid-channel would not only be engaged much sooner from both sides, but would also be exposed to the simultaneous fire of all three, irrespective of that from the present works of defence, while a passage nearer to Fort Bullen (to escape the artillery fire from the Bathurst side) could be made very hazardous, if not leading to certain destruction, by means of submarine torpedoes.
22. Without losing sight of the desirability of furnishing whatever information I may be able to give in reference to the cost of erecting here the works previously proposed, I beg to conclude the subject of turret batteries by giving a general description of the plan on which I would propose them to be carried out, if thought desirable. In the first place, I would, for the sake of economy and simplicity in erection, propose that all three be made alike, and each adapted to carry five 10-inch rifled muzzle-loaders, but the principal characteristics of the proposed plan would lie in utilizing the bearing capacity of water, in place of the great power-absorbing and expensive mechanism which would otherwise be necessary to produce a revolving motion were the battery to rest on rollers, so as to bring the guns successively into the line of direction. As shown in the drawing representing a vertical section of the battery,* the stationary central pivot which passes up through the well forms a simple means of causing the battery to revolve as soon as sufficient force is applied to the horizontal lever attached to it. The fact that the resistance to be overcome in causing a heavy circular body floating in water to revolve round its vertical axis is very trifling as compared to the force required to effect the same motion in a similar but non-floating body moving on rollers, leads me to give the preference to a rotary floating battery, and especially so where, as in this case, the low level of the ground, the vicinity of the river, and a moderate rise of tide, greatly favour the application of that system.
23. I propose also to make use of the necessarily very strong armour-plating covering the turret battery for the support of the guns, to save thereby, firstly, the expense of carriages; secondly, the space they would occupy in loading and firing positions (which saving of space also allows a con- siderable reduction in the size of the battery); and thirdly, their weight. The kind of hang-gear I propose will be best explained by the drawing appended to this Report.*
24. The battery would not be subject to wave motions, as it would float in a kind of reservoir surrounded by massive masonry, which, being flush with the ground and protected by an iron ring curb, may safely be considered to withstand heavy artillery fire without serious damage, and the size and weight of the battery itself would completely counteract any tendency to produce oscillations, while the central pivot would prevent any part of the battery touching the surrounding masonry.
25. Having given a general description of the herein-proposed turret batteries, and referring to the appended drawings* for dimensions, I merely beg now to add that I believe their cost, inclusive of armament, would not exceed 50,0007. each, which is about equal to the estimated cost of the batteries and armaments named in the printed Synopsis received.
26. Reverting to the previously-proposed construction of the north-east, south-east, and Fort Bullen batteries, I beg to refer to paragraph 18 of this Report, in which I pointed out the expediency of making such new works as effective and as safe as possible, and taking into consideration that very probably only a small number of efficient artillerymen may actually become available to be distributed among the various works of defence within the district, I think that they ought to be securely sheltered to enable them to do their work uninterrupted by casualties which may deprive us of the use of the guns just when it is most needed; and owing to the exposed position of the proposed new works of defence, it may be advisable to construct gun casements with iron-shielded embrasures in place of open batteries, which form no protection from vertical fire, and only partly from the effects of exploding shells.
27. The maximum range of direction of fire which can be allowed for in an embrasure being 60°, it necessarily follows that, to obtain the whole amount of sweep which the intended position of those batteries will allow, the shape of each battery or gun casemate will have to be semi-circular, or nearly so, and, judging from the diagram shown in red lines on the Chart appended to the printed Synopsis, I believe such shape was originally intended.
28. In the appended design for a gun casemate,* I have arranged the total range of direction of fire to correspond with the amount of sweep shown in red lines on the original Chart, and have also, for the sake of economy, limited the size of the gun casemate to the smallest diameter in which five 10-inch rifled muzzle-loaders can be worked. I propose some brickwork in rear of the semi-circular battery, in order to support and strengthen the bomb-proof covering, and also to provide some compartments which may be used as shell-rooms and for defensive purposes. Besides the racers, steel pivots are intended to be fixed below the iron embrasures, to give additional means of guiding the trucks on which the sloping platforms rest. Small shell recesses are provided within the masonry on each side of the guns, for the convenience of loading; and, to obviate the necessity of using very long girders for the roof, it is proposed to build an iron-cased central pillar, to support the inner ends of radial wrought- iron girders, which are to have arches built between to carry the covering materials.
29. In concluding the above general description of the gun casemate, I beg to give an abridged estimate of the cost of erection. The whole of the masonry, inclusive of concrete, &c., I estimate at 6,4007.; 125 tons of iron-work in shields and armour-plating, at 507, per ton, 6,2501.; 25 tons of iron- work in rolled girders, at 101. per ton, 2501.; and 5 tons in the racers, at 201, per ton, 100%.; making a total of 13,000l., exclusive of armament; but, as stated in the foregoing paragraph of this Report, the dimensions of this gun casemate are limited to the smallest size at which five 10-inch rifled muzzle- loaders, running on racers, can be worked. To increase the size so far as to allow a clear space between the gun platforms in any position would bring the external diameter of the casemate from 80 feet to 120 feet, and would consequently almost double the cost. * Not printed.
Appendix No. 4.
GAMBIA.
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