Engine, Diesel, 240 Field-guns, 81 Fire broom, 105 liquid, 103 Forts, machine-gun, 58 Fuse, grenade, 28 Gas, 85 American, 102 Gas attack, boomerang, 92 first, 89 Gas, chlorine, 87 diphosgene, 96 exterminating rats, 94 grenades, 26 helium, 164 hydrogen, 150 lock, 97 masks, 99 mustard, 98 phosgene, 93 pouring like water, 86 shell, 95 sneezing, 98 tear, 95 vomiting, 98 Gasolene-tank, airplane, 130 Gate, barbed wire, trench, 9 Gates through barbed wire, 15 Gatling gun, 43 Geologists, Messines Ridge, 19 Glass, non-shattering, 100 Grapnel shell, 16 Graveyard, submarine, 314 Grenade, disk-shaped, 33 fuse, 28 gas, 26 hair brush, 34 history of, 23 Mills, 29 parachute, 31 range of, 25 rifle, 28 throwing implement, 27 Grenade, wind-vane safety device, 32 Gun, aircraft, 131 American, 50-mile, 63 big, hiding, 226 caliber, 68 disappearing, 77 double-end, 145 18-inch, monitors, 306 elastic, 73 field, 81 42-centimeter, 79 how made, 76 120-mile, 70 long range, German, 62 non-recoil, 145 on submarine, 249 16-inch, coast defense, 78 Skoda, 81 spotting by sound, 181 three-second life, 73 12-inch, submarine, 251 ways of increasing range, 67 wire-wound, 76 Hand-grenade, see Grenade Helium, 164 Hospital, airplane, 147 Horizon, seeing beyond, 219 Howitzer, 79 Hush ships, 304 Hydroaëroplanes, see Seaplanes Hydrogen, weight of, 150 Hydrophone, 270 Illusions, optical, 215 Kilometer, length in miles, 6 Kite balloons, 174 Kite, water, 283 Liberty motor, 142 Liquid-fire, 103 Locomotives, gasolene, 10 _Lusitania_, 316 Machine-gun, 112 airplane, 127 Benèt-Mercié, 52 Browning, 53 Colt, 44 forts, 58 Gatling, 43 history, 41 Hotchkiss, 49 Lewis, 50 Maxim, 42 water-jacket, 47 worth in rifles, 58 Machine-rifle, 55 Magnets, lifting, salvage, 334 Maps, making with camera, 175 Marne, first battle of, 4 Messines Ridge, mine, 19 Metal-cutting under water, 329 Microphone detectors, mines, 18 Mine-field, North Sea, 290 Mine laying, North Sea, 292 Mine-laying submarine, 287 Mine railroad, 294 Mine-sweeping, 281 Mines, 276 anchored, 278 and counter-mines, 17 automatic sounding, 278 drift of, 285 electric, 277 floating, 284 Messines Ridge, 19 Mines, paravanes, 288 Monitors, 306 Mortars, 79 depth bomb, 266 flying, 23 Mortars, See also Trench mortars Mother-ships for airplanes, 305 Motor-boat, electric, 308 sea Tank, 299 Motor torpedo-boats, 298 Mystery ships, 220 Net, North Sea, 290 Ocean currents, 285 Optical illusions, 215 Oxy-hydrogen torch, submarine, 329 Paint in war, 209 Papier mâché heads, 13