Horizontal Motion.— A simple pendulum consisting of a massive bob, weighing several pounds in some cases, suspended by a steel wire from a massive support has been extensively employed especially in Italy and Spain as the °steady for a seismograph. The motions of the ground with respect to such a mass must be magnified from 10 to 100 or more times if we are to produce satisfactory records of small distant earthquakes. The means of doing this will be shown later.
A simple pendulum of this character fails to provide a reliable steady mass for two or three reasons. If the length is short, 10 or 20 feet, the mass very quickly responds to the earthquake movements and presently the pendulum is itself swinging in a more or less violent manner. A long pendulum (several hundred feet) cannot be supported with suffi cient rigidity. The top of a lofty tower, even during the absence of an earthquake, wanders all about a fixed point within it, due to the influence of temperature, wind, etc., on the tower, and the effects of moisture, temperature, rainfall, etc., upon its foundation and the adjacent earth. During an earthquake the motions of the top of such a tower cannot be the same as those of the ground and are doubtless much greater, thereby seriously in fluencing the steady mass, notwithstanding the great length of its support.
The Horizontal Pendulum.— The type of pendulum employed first by Ewing for the measurement of earthquake motion in the hori zontal plane is commonly called the horizontal or conical pendulum. A modern design of this form of seismograph is shown in Figs. 1 and 2. The steady mass C (about 40 pounds) is sup ported upon the massive column A by a hori zontal strut and the diagonal wires W. W. so that it is exceedingly free to swing about a pivot point at D and another at B. The two points D and B are not quite in a vertical line, hence the steady mass is in slightly stable equilibrium and if disturbed oscillates like a very long pendulum. As ordinarily adjusted at the Weather Bureau the pendulum illustrated, makes a complete oscillation in about 35 sec onds. This is equivalent to a simple pendulum about 4,000 feet long. The record is made upon the clock-driven by means of the lever L, carried in the heavy yoke piece G. For this purpose the surface of the drum is closely covered with a piece of smooth white paper which is uniformly coated with soot from a wide-flame, smoky, kerosene lamp. The drum makes one revolution an hour, and each minute of time is electrically marked on the record sheet by the time-ticker magnet M. The
distant end of the drum-shaft is cut with a coarse thread which causes the drum to move endwise about 3-16 of an inch with each revo lution, thus separating the lines traced by the stylus.
The manner in which the registration of the earth motion is effected is more clearly seen in The Milne seismograph used in the English work records photographically without the use of mirrors. A small steady mass is suspended on a horizontal strut only an inch or two long, but the strut is prolonged beyond by a very light boom to a length of nearly three feet or more. This long boom at the outer end carries a light aluminum vane pierced with a narrow radial slit. The photographic paper, in the shape of a long ribbon, is placed in a box just under the vane. The box has a slit in its top at right angles to that in the vane. Light from a lamp is thrown upon the aluminum vane and passes into the box at the point where the two slits intersect, thus making the photographic record.
Magneto It has been found that the delicately suspended magnetic needles em ployed for the purpose of recording continu ously the small diurnal and other changes going on in the earth's magnetism are nearly always disturbed by even very small earthquakes. These are not seismographs at all, in the ordi nary meaning of the word, and the records give very little information except the time at which Fig. 2. The lever L is pivoted very freely in the stirrup F. The short end of the lever is forked and engages the slender steel staff f, which is pivoted in the most delicate manner in the frame F, rising from the steady mass C. Observing now that the steady mass is free to remain at rest with respect to the lateral mo tions of its pier and the yoke piece G, it is plain that the stylus at the long end of the lever will trace a magnified record of all the lateral dis placements of the pivot d, because the point of the fork which engages the steady mass at f must remain at rest with the mass. The amount of magnification depends upon the ratio of the lever arms. In instruments of the kind illus trated a 10- or 15-fold magnification is usual. Owing to the fractional resistance offered by the :Riot and at the pivot points greater mag nifications require proportionately greater mass in the pendulum, if the same period is retained.