At the beginning of 1917 four shore hydrophone stations were in use.  During the year eight additional stations were completed and several more were nearing completion.  The first step necessary was a considerable increase in the instructional facilities for training listeners both for the increased number of shore stations and for the large number of vessels that were fitted for hydrophone work during the year.

The greater part of this training took place at the establishment at Hawkcraig, near Rosyth, at which Captain Ryan, R.N., carried out so much exceedingly valuable work during the war.  I am not able to give exact figures of the number of officers and men who were instructed in hydrophone work either at Hawkcraig or at other stations by instructors sent from Hawkcraig, but the total was certainly upwards of 1,000 officers and 2,000 men.  In addition to this extensive instructional work the development of the whole system of detecting the presence of submarines by sound is very largely due to the work originally carried out at Hawkcraig by Captain Ryan.

The first hydrophone station which was established in the spring of 1915 was from Oxcars Lighthouse in the Firth of Forth; it was later in the year transferred to Inchcolm.  Experimental work under Captain Ryan continued at Hawkcraig during 1915, and in 1916 a section of the Board of Invention and Research went to Hawkcraig to work in conjunction with him.  This station produced the Mark II directional hydrophone of which large numbers were ordered in 1917 for use in patrol craft.  It was a great improvement on any hydrophone instrument previously in use.  Hawkcraig also produced the directional plates fitted to our submarines, as well as many other inventions used in detecting the presence of submarines.

In addition to the work at Hawkcraig an experimental station under the Board of Invention and Research was established near Harwich in January, 1917.  The Mark I directional hydrophone was designed at this establishment in 1917, and other exceedingly valuable work was carried out there connected with the detection of submarines.

At Malta an experimental station, with a hydrophone training school, was started in the autumn of 1917, and good work was done both there and at a hydrophone station established to the southward of Otranto at about the same time, as well as at a hydrophone training school started at Gallipoli at the end of the year.

“OTTERS” AND PARAVANES

The “Otter” system of defence of merchant ships against mines was devised by Lieutenant Dennis Burney, D.S.O., R.N. (a son of Admiral Sir Cecil Burney), and was on similar lines to his valuable invention for the protection of warships.  The latter system had been introduced into the Grand Fleet in 1916, although for a long period considerable opposition existed against its general adoption, partly on account