at the end of the book). They were changed
occasionally when suspicion was aroused that their limits were known to
the enemy, or as submarine attack in an area became intense.

[Transcriber's note: Chart A is a navigational map of the waters
southwest of England, with approach routes marked.]

The approach areas were patrolled at the time, so far as numbers
admitted, by patrol craft (trawlers, torpedo-boat destroyers, and
sloops), and ships with specially valuable cargoes were given directions
to proceed to a certain rendezvous on the outskirts of the area, there
to be met by a destroyer or sloop, if one was available for the purpose.
The areas were necessarily of considerable length, by reason of the
distance from the coast at which submarines operated, and of
considerable width, owing to the necessity for a fairly wide dispersion
of traffic throughout the area. Consequently, with the comparatively
small number of patrol craft available, the protection afforded was but
slight, and losses were correspondingly heavy. In the early spring of
1917, Captain H.W. Grant, of the Operations Division at the Admiralty,
whose work in the Division was of great value, proposed a change in
method by which the traffic should be brought along certain definite
"lines" in each approach area. Typical lines are shown in Chart B.

[Transcriber's note: Chart B is a navigational map of the waters
southwest of Ireland, with approach routes marked.]

The idea was that the traffic in, say, Approach Route B, should,
commencing on a certain date, be ordered by the Routeing Officer to pass
along the line Alpha. Traffic would continue along the line for a
certain period, which was fixed at five days, when it would be
automatically diverted to another line, say Gamma, but the traffic along
Gamma would not commence until a period of 24 hours had elapsed since
discontinuance of the use of the line Alpha. This was necessary in order
to give time for the patrol craft to change from one line to the other.
During this period of 24 hours the arrangement for routeing at the ports
of departure ensured that no traffic would reach the outer end of any of
the approach lines, and consequently that traffic would cease on line
Alpha 24 hours before it commenced on line Gamma. After a further period
of five days the line would again change automatically.

It was necessary that Shipping Intelligence Officers should have in
their possession the orders for directing