284 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 9 



recognizable offset (or in some cases, a short interruption) occurs on 

 the line. A diagram of one sort of seismograph is shown in figure 1 

 together with a small section of a record showing the timing marks. 

 All we wish to emphasize here is that an instrument has been devel- 

 oped which enables the inertia of a weight to give us a record indi- 

 cating that an impulse has arrived at the recording station and telling 

 us the exact moment of its arrival. 



The earth is an elastic body — almost as elastic as steel. That is 

 to say, if any part of it be deformed slightly by force, that deforma- 

 tion will vanish when the force is removed. It is the elastic properties 



ELEVATION 



'R 



— Jfi 



-R-- 



PLAN 



.-ctl3 



-n — t 



Figure 1. 



-Schematic diagram of a typical horizontal seismograph and a section of a 

 record showing time breaks and an earthquake. 



of steel, for example, which cause a suspended bar of the material to 

 give off a ringing note when struck by a hammer. It is the elastic 

 properties of the earth which cause it to transmit earthquake waves. 

 As early as 1849 it was deduced mathematically that an elastic body 

 should transmit two kinds of waves, and mathematical expressions 

 for the speeds of the two waves were found. These two classes of 

 waves are known as longitudinal and transverse. The first is propa- 

 gated by the back-and-forth motion of the earth particles in the 

 direction of the wave, somewhat after the manner in which a bump 

 from an engine is transmitted down a long line of freight cars. The 



