Sentence:  The bureau's researches in the war involving electrical principles covered many subjects—the location of enemy batteries by means of sound-recording mechanisms, the electrical control of airplane guns to fire between the propeller blades, time studies of gunfiring, variation of gun pressures, ejection-velocity studies by recoil meters, the development of <WSD>kinemometers</WSD> to measure the jumps and whip of a gun on firing, the measurement of projectile velocities inside the gun, and scientific studies of the gun blast. 
Meaning of the word marked with <WSD> tags:  A highly sensitive type of tachometer.