might be considered serious were it not for the fact that the whole plan of the barrage is based upon the assumption that an effective mine field can be laid in 1,000 feet of water. If this assumption be true, then whether a portion of the mine field be in 40 or in 60 fathoms of water is not material, except as the change of plan introduces delay. If the assumption be not true, then the barrage is doomed to partial failure anyway.

It will be noted that the original line extended from mainland to mainland, while the new line extends from island to island and has in it passages completely navigable to submarines. This condition is, in our opinion, undesirable. We believe it wrong to accept a plan that provides in advance a way by which the plan may be defeated. This point will be discussed more fully when the character of the barrage is considered.

CHARACTER OF THE BARRAGE.

The proposed character of the barrage does not provide for the full accomplishment of the mission. The proposed barrage will not close the northern exit from the North Sea, because

(a) The barrage is not complete in a vertical plane in Areas B and C.

(b) The barrage is not deep enough.

(c) The Pentland Firth is open.

(d) The waters east of the Orkney Islands, for a distance of miles, are open.

(e) Patrol vessels on the surface are not sufficiently effective in barring passage to submarines, as witness the Straits of Dover.

The barrage is to be a great effort. It is our opinion that nothing short of a sound design will justify the effort.

The requirements of a sound design are the extension of the barrage complete in the vertical plane from coast to coast. If it be impracticable to carry the barrage up to the Orkneys, and to close the Pentland Firth, then the western end of the barrage should turn south to the Aberdeen Promontory.

The necessity for an opening in the surface barrage is recognized, but it is held that this opening should be in the surface barrage only, and that the deep barrage should be widened so that the difficulties of navigating the opening submerged may be practically prohibitive. Deep mines should cover for a considerable distance all approaches to the barrage opening.

The Norwegian coast presents special difficulties both in mining and patrol, but all of these difficulties will be greatly reduced by carrying the surface barrage up to the 3-mile limit. It will then be practicable to concentrate the strength of the patrol in the very near vicinity of Norwegian territorial waters.

The deep barrage in Norwegian waters should be extended so as to porcupine the coast both north and south of the surface barrage for a considerable distance. The submarine must be taught to fear all Norwegian territorial waters.

The above points concerning the character of the barrage are points to which we attach great importance.

LENGTH OF ANTENNE.

British experiments indicate that a length of antennæ greater than 70 feet will not assure the destruction or disablement of an enemy submarine. This length requires three lines of mines in the vertical plane. Three lines permit the vertical barrage to be extended vertically to a depth of 235 feet. It is essential that the upper tier of mines have antennæ of such length that vessels traveling on the surface may not escape, otherwise vessels might escape by the simple plan of making the passage on the surface in Area A. The necessity for short antennæ is not so pronounced for the deeper mines, as the probability that submarines may make contact with the upper end of the deep antennæ is much less than it is in the case of the shallow mines. The length of the antennæ is related directly to the vertical width of the barrage, as follows:

(1) Three 70-foot antennæ cover 235 feet.

(2) Three 100-foot antennæ cover 325 feet.

(3) One 70-foot and two 100-foot antennæ cover 295 feet. Add 25 feet to each of the above depths and get the prohibited vertical zone for submarines.

We are of the opinion that the third combination should be used, as this combination provides for destruction on the surface and for reasonable certainty of destruction up to 300 feet submergence.

SEQUENCE IN LAYING.

While the sequence of laying the mines is an operating matter, it seems desirable that the situation on the Norwegian coast be cleared up by laying the fields there as early as possible.

In Area A it may be desirable to lay the southern system first and to lay all deep mines before any shallow mines are laid.

TENTATIVE DECISIONS.

1. To accept the new position of the barrage as outlined by the British Admiralty.

2. To urge that the barrage be complete in the vertical plane from coast to coast, except an opening in the surface barrage at the western end and in Norwegian territorial waters.

3. To carry the barrage to a depth of 295 feet.

4. To have surface mines fitted with 70-foot antennæ and other mines with 100-foot antennæ.

5. To urge that deep mine fields be laid at numerous points on the Norwegian coast.

6. To urge that all approaches to barrage openings be mined with deep mine fields for a considerable distance, so as to make the navigation of these openings by submerged vessels as hazardous as possible.

(See British comment in Memorandum No. 3.)

A. Concur.

COMMENT OF BRITISH ADMIRALTY.

B. It is considered this assumption is true as far as can be judged with the knowledge in our possession.

The question as to the greatest depth to which the enemy submarines may be expected to dive was discussed with our submarine experts when the depth of the barrage was decided on.

The matter has been again discussed with them since the receipt of your memorandum and they confirm their former opinion that submarines will not of their own free will dive to a depth exceeding 200 feet.

To dive under the barrage the submarines would have to dive to 240 feet in the American mine field and to 215 feet in the British mine field, measuring to the bottom of the boat, which is the German practice.

It is the considered opinion of the submarine experts that it is of more importance to effectively mine from the surface to 200 feet rather than to mine deeper with a loss of efficiency down to 200 feet.

A question which must be taken into account is whether the explosion of a charge at a depth of 200 feet has a greater radius of destruction than a similar charge at a depth of 70 feet.

Opinions differ much on this point and without direct proof, which is difficult to obtain, it is considered the effect must be assumed to be equal.

A point which requires careful consideration is whether the American mine as now being constructed will withstand the external pressure to which it would be subjected if laid at 300 feet.

The possibility of having to lay mines at 300 feet has been taken into consideration in future orders of British mines.

Taking the above points into consideration, it is requested that you will put forward any proposals you may wish to make regarding the length of the antennæ.

C. The stopping power of a mine barrage such as we propose to lay should not be overrated.

It is considered that if we relied on any mine barrage across such a great width to entirely stop submarines passing out of the North Sea our hopes would be foredoomed to failure, at any rate until the barrage had become very thick.

It is the patrol craft, armed with various antisubmarine devices, on which we must rely to actually kill the submarines.

Now, the efficiency of the patrol depends on its intensity, and it is on the mine field that we rely to give us this intensity.

The introduction of the "Acoustic mine" may, and we hope will, give us an instrument which will enable us to absolutely deny large areas to submarines unless they accept the probability of almost certain destruction.

The acoustic mine is not yet a fait accompli and therefore we can not base our plans on it.

Assuming that we are correct in considering the mine field only as an accessory to the patrol, we must arrange the mine fields to that end.

When looking at the plan of the Northern Barrage it immediately occurs to one, Why not extend the surface mine field right up to the Norwegian coast!

Until we have proved the efficiency of the American mine field we must look upon it as a bluff.

It is not suggested that the American mines will not be efficient, but only whether any system of existing mines will deny an area 150 miles in width to submarines.

We notify an area 150 miles in width as dangerous and hope that the enemy submarines will be diverted into the areas on each side where our patrol craft can deal with them.

If we attempt to put the bluff too high, which it is considered would be the case if we mined the eastern area up to the surface, there would be a chance of forcing the submarines to pass through the mine field, which they might find they could do without prohibitive loss. We should then be faced with the problem of patrolling the whole area between Orkneys and Norway-a task beyond

our resources.

By the end of the summer the mine fields in the notified area will, it is hoped, be so dense as to make the danger of passing through them prohibitive, in which case we could then mine the eastern area up to the surface.

It would be desirable to do this, if possible, before next winter, as our patrol craft will find it next to impossible to efficiently patrol the eastern area during the stormy winter days with long nights.

D. (a) The reason for not making it complete in Areas B and C has been explained under C.

(b) Already discussed.

(c) The navigation of the Pentland Firth by submarines when diving is not considered to be a practicable proposition. Patrol craft should prevent submarines passing through it on the surface. Also, as already explained, the patrol areas thoroughly cover the approaches to the Firth, and as it is on the patrol craft we rely to destroy the submarine the fact that it is not covered by the mine field is not considered to be of vital importance. It is clearly recognized, however, that once the barrage has one or more systems completed right across, our subsequent mine laying must be adjusted to meet any new tactics on the part of the enemy. It may for instance be necessary to continue the deep mine field down to the coast of Scotland or to mine an area to cover the western end of the Pentland Firth.

(d) The patrol craft in the Straits of Dover are not at present fitted with up-to-date hydrophone gear, nor are strong tidal waters, such as the Straits of Dover, suitable for hunting with the fish hydrophone.

The efficiency of the patrols on the Northern Barrage should not, for the above reasons, be based on results obtained at Dover up to the present time.

Submitted: Question whether the barrage should be completed on the surface up to the Norwegian coast.

The American idea of having a surface mine barrage from the Orkneys to Shetlands is presumably based on the assumption that a mine field 220 miles in length can be made so effective that it will stop submarines passing through it.

The experience of the war, it is claimed, does not bear out this assumption.

Neither do we yet know whether the American mine is efficient.

When the design of the barrage was originally considered, it was estimated that three lines of mines at each depth would be required to make it efficient.

Now, three lines of mines at each depth will not be in place until well on in the summer, even if there are no more delays than we know of at present.

Hence the mine barrage can not be considered really effective until later on in the summer, and therefore we should not attach too much importance to it.

Now, if the above assumption is correct, we should almost certainly create a situation we could not deal with if we followed the American suggestion to mine up to the surface right across, for the following reasons:

(a) The submarines would break through the mine field without prohibitive loss.