is possible for submarines to carry on wireless conversations under water when at a distance of several hundred feet apart, and to pick up the characteristic sounds of different types of surface ships at considerable distances. Sound detectors are of greater benefit to submarines lying in wait for their enemies than they are to surface vessels, as they enable the submarine to lie at rest, submerged and invisible, herself giving no betraying sound, while no surface ship can come within the zone of her receiving apparatus without betraying its presence. =Submerged Sound Detectors.=--It has been stated that sound detectors connected to shore stations have been able to detect submarines when passing in their vicinity, and, by the triangulation method as applied to the intensity of sounds, observers have been able to tell approximately the location of the U-boat from the sound of the U-boat's machinery. The obvious thing for submarine designers and commanders to do to offset this danger to the submarine is to use noiseless machinery in the U-boats, or to send other U-boats with a wire-cutting grapnel to cut the shore connections of the sound transmitter. It is apparent that this method of attack is applicable only to points close to shore or in places like the English Channel. [Illustration: A GROUP OF GERMAN U-BOATS Note their broad decks, due to buoyant superstructure.] [Illustration: RUSSIAN-LAKE TYPE CRUISING SUBMARINE "KAIMAN" MAKING A SURFACE RUN IN ROUGH WEATHER IN THE GULF OF FINLAND] =Deck Guns.=--The mounting of deck guns on merchantmen for defence against the submarine has proved of slight value. When it was first proposed to mount guns on American merchant ships I wrote the Navy Department on March 11, 1917, in part as follows: "I have tried, in the interest of this country, to impress this fact upon the people (that the submarine, because it is invisible, is invincible), but I find in talking with many intelligent people, that they do not and cannot comprehend the possibilities of the submarine when it is taken seriously and the effort is made to get all there is out of it, without reference to political, financial, or prejudiced interests. The _destructiveness_ of the submarine is growing; devices which were effective in detecting and trapping submarines early in the war are now becoming useless. The theory that putting a gun on a merchant ship is going to protect that ship, her crew and passengers, will, I fear, be equal to the signing of the death-warrant of all that are on that ship if we are at war, as the slogan in to-day's headlines (as per copy clipping enclosed)--'Sink any ship you see'--will be met, I fear, by a German slogan of 'Sink every ship you meet, but don't let them _see you_ do it.'"