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existence, however, is so problematical that no estimate of the cost of the work is attempted; but it would probably be less than that to be presently proposed.

Passing to the four entrances to the lagoon, it appears that the one between West and Main Islands may, on account of its shallowness, be disregarded, and that the other two minor channels, being only fit for small vessels, carrying, consequently, little or no armoured protection, may, with the additions to be presently submitted, be sufficiently denied by the long range fire of powerful guns. The position which first suggests itself for occupation from which to dispute the main entrance is Middle Island; but a vesssl keeping near in to West Island could pass into the harbour without ever coming within 3,000 yards of the former island.

Two heavy guns mounted in a turret erected on one of the rocky patches near the centre of the four entrances would be capable of commanding them all; but such a construction, besides being very costly, demands more constant and watchful supervision of machinery, &c., than might always be bestowed upon it at an isolated station of this

nature.

Turning to the West Island, it will be seen that it offers a site superior to that first considered; while a work erected thereon might be of a simpler and less costly type than that mentioned in the second suggestion.

No vessel could enter the harbour through the main channel without passing within at least 2,400 yards of guns mounted upon it, while for a minimum of 11⁄2 miles of its course the vessel would be within the 3,000 yards zone of such guns.

Appendix No. 4.

DIEGO GARCIA.

West Island, therefore, appears to be the most eligible site for the requisite defensive works, which should be constructed for four 8-inch breech-loading rifled guns behind iron 4 8-in. B.L.R. guns. shields.

The battery should be designed with its flank and gorge either en glacis, or so thoroughly protected that the severest battering from artillery fire should not expose its casemates to be taken in reverse or flank,

The proposal to erect a work with its gorge turned towards the harbour approaches is somewhat unusual, but so also are the circumstances of the harbour, which, from its great size, would still afford security to vessels taking refuge within it from any hostile fire off the entrance.

If the work were constructed to command the approach as well as the entrance, about twice the number of guns would be necessary, while no corresponding advantage would accrue to the defence.

Works

West Island Battery.

£

60,000 *10,000

Armament

Submarine mines should be laid in the main channel, and also, where requisite, in the Submarine mines- minor channels; and a gun-boat carrying one heavy and two smaller guns should also be provided.

£

Stores

2,970

Boats

4,500

Storehouse

2,500

There does not appear to be a probability of any landing being attempted on the main island in sufficient force to be formidable, as it would be too hazardous an operation to undertake.

Total

9,97

Gun-boat

15,000

The possible debarkation and subsequent operations of one or two boat's crews should be frustrated by the fire of the gun-boat.

A total garrison of 130 men would be required; of whom 40 should be artillery and Garrison- 40 submarine miners, for all of whom barracks would be necessary.

Barracks

£ 13,000

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Works Armament

Submarine mines Gun-boat Barracks

•

May 6, 1881.

May 7, 1881.

Total

RECAPITULATION.

:::::

:

•

•

£

60,000

10,000

9,970

15,000

13,000

•

107,970

(Signed)

J. H. SMITH.

(Signed)

T. L. GALLWEY,

Inspector-General of Fortifications.

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