Before describing the instrument which I propose for the recording of strong earth‐motions, I shall, for the benefit of those who have not followed the argument for a short‐period instrument, point out its advantage over the other types. These advantages are stability of the instrumental characteristics, ability to measure large ground‐displacements, and the greater ease with which a complete interpretation of the record may be obtained.The instrumental characteristics of the short‐period instrument are less affected by the acceleration to which they are most certainly subjected during an earthquake. This may readily be seen by substituting the new component of acceleration in the following formula for the period of a horizontal pendulum where L = equivalent length of pendulum, i = the Bine of the angle of inclination of the axis of rotation, T = the period of pendulum, g = the acceleration due to gravity. and a = the component of the Earth's acceleration in a direction perpendicular to the motion of the inertia‐mass and in a plane perpendicular to the axis of rotation.