364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



Honzontal component instrument. — In most of these the principle 

 of the horizontal pendulum is used. For stability combined with 

 high sensitivity the line joining the upper and lower points of sup- 

 port must be slightly out of the vertical. Adjustments are provided 

 to make this possible. 



Damping. — In order to prevent the seismograph from oscillating 

 in its own natural period and producing a very complicated record, 

 the pendulum is damped, which means that a suitable vane attached 

 to it moves in an oil w^ell, an air pocket, or in a magnetic field, any 

 of which will serve as a brake. If the period of the earthquake and 

 that of an undamped pendulum are equal or nearly equal, resonance 

 is set up and the pendulum movement as recorded has no relation to 

 the earth movement. Damping can not be perfectlj^ effective, but 

 the aim of the seismologist is to make the damping arrangement as 

 effective as possible. 



Magnification. — In the case of distant earthquakes the actual 

 ground movement, if it were directly recorded, would be invisible 

 for all but the very strongest earthquakes. For near earthquakes of 

 considerable strength, the magnification must be small. There is 

 in practice a wide range of requirements. Various instruments use 

 magnifications of approximately 1,200, 700, 150, and 5 or 2. For 

 special purposes magnification of 1,000,000 has been used. By optical 

 or mechanical means the actual movement of the mass or, rather, of 

 the supporting frame with respect to the mass, can be multiplied to 

 a considerable degree, and this is known as static magnification. For 

 a record the magnification to be used differs from this and the so- 

 called harmonic magnification varies with periods of earth wave and 

 instrument and with the damping, and therefore for the same instru- 

 ment magnification will vary with the earth period. 



Period. — The period is the interval of time between two successive 

 passages of a pendulum through the position of rest in the same 

 direction. For recording near-by earthquakes, short-period instru- 

 ments are used, both on account of the short periods to be recorded 

 and because of the necessary optical and mechanical requirements to 

 obtain the desired magnification. In the case of distant earthquakes 

 there is a wide range of periods and several might be chosen for 

 the instrument. Periods of 12 to 15 seconds are in quite general use. 

 Vertical cormponcnt instruments. — In these a mass is supported by 

 a spring, and the difficulty is to secure stability along with great 

 sensitivity, large magnification, long period, and freedom from 

 changes due to temperature. 



Time. — The accuracy of the time is of great importance. Uniform 

 speed of rotation is one element and absolute time is another, and 

 both must be satisfactory. Even with the best of apparatus small 



