196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 4 



any undamped oscillatory system has a period of its own irrespective 

 of the period of the vibrations which are forced upon it, and in 

 such a case the motion recorded bears no definite relation to the 

 true earth motion. For magnifying the movement of the pendu- 

 lum mechanical lever arrangements are tending to be displaced by 

 optical or electromagnetic methods of recording which have the 

 advantage of eliminating friction. In some of the older types of 

 seismographs the weight of the main mass of the pendulum often 

 ran into hundredweights or even tons; it is now realized that such 

 heavy systems are not necessary, and the tendency now is to go to 

 the other extreme. The moving mass of the Wood-Anderson seis- 

 mograph, for instance, weighs no more than a few grams, and whereas 

 most other horizontal instruments use gravity as the restoring force 

 this instrument empleys the torsion of a stretched fiber. In vertical 

 seismographs a coiled spring is generally used to control the motion 

 of the pendulum, and the chief difficulty associated with these instru- 

 ments is the effect of temperature on the elastic constants of the 

 spring. This has been overcome to some extent by the use of elinvar 

 alloy, but there is still plenty of room for improvement in the design 

 of vertical seismographs. A point of very great importance in the 

 recording of earthquakes is the accuracy of the timing of the records. 

 There is no difficulty in getting sufficiently accurate time marks from 

 a control clock which can be checked by wireless time signals, but 

 unfortunately some seismograms suffer from the lack of uniformity 

 of rotation of the recording drums. 



EARTHQUAKE WAVES 



The record of a strong earthquake obtained on a modern seis- 

 mograph at fairly large distances from the epicenter ^ usually 

 presents a characteristic appearance. Three distinct portions can 

 be recognized ; the first has a sharp commencement followed by 

 rapid but irregular oscillations. After a certain interval there is 

 a second abrupt pulse and rather larger but still irregular vibrations. 

 In the last phase the maximum movement is reached, and it is 

 characterized by smoother oscillations of longer period and greater 

 amplitudes. The disturbance then gradually dies away after lasting, 

 in many cases, for several hours. Now in the theory of elastic vibra- 

 tions in a homogeneous solid it had long been known that two types 

 of waves could be transmitted through the medium with different ve- 

 locities. One of these is known as the compressional or longitudinal 

 wave, in recognition of the fact that the displacement of the medium 

 occurs in the direction of propagation, as in the case of sound waves. 



» The point within the earth at which the shoclt originates is called the focus ; the 

 epicenter is the point on the surface vertically above the focus. 



