352 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1931 



But one swallow does not make a summer. It was desirable to 

 check the value of the velocity by means of other explosion records, 

 especially as the velocity as determined for waves generated by earth- 

 quakes of presumably shallow focus was found to be 7.1 km (4.4 

 miles) per second. Accordingly, in May, 1924,^^ four explosions, the 

 first two of 10 metric tons each, of melinite, the second two of 5 

 metric tons each, were exploded at La Courtine, in central France. 

 The explosions being predetermined, arrangements were made to 

 have precise timing and fast-speed chronographs, so that the records 

 obtained were spread out sufficiently to be readily legible.^ The 

 tremors were registered at three stations ranging in distance from 

 3y2 to 151/2 miles. The mean value of the determined velocity for 

 the most rapid tremors was found to be 5.5 km (3.4 miles) per sec- 

 ond, confirming substantially the results obtained from the records 

 of the Oppau explosion. 



This raises a further point. We have referred only to the veloci- 

 ties of the first movement as registered on the seismograph. The 

 fact that high-speed chronographs were used at La Courtine to 

 spread out the record implies that there were other onsets of value. 

 There were. 



Waves propagated in an elastic body (that is to say, a substance 

 which has the power of recovering its shape if it is not strained 

 beyond certain limits — and the earth is such a body) are of two 

 kinds. The first is known as a longitudinal or dilatational type and 

 the waves so propagated are called P waves, since they are the pri- 

 mary or first registered. The other type is called transverse or 

 distortional, the waves being termed S waves because they are the 

 secondary registration in point of time. The P wave and the S 

 wave each travel through the earth from the focus to the station 

 by paths which may be for the present described as being somewhat 

 concave upward, or as sagging below the chordal line joining the 

 focus and the station. The velocity of each depends on the elastic 

 properties of the earth, being greater in each case if the elasticity 

 of the material along the path increases, but slowing down for an 

 increase in density. The S wave has the added important charac- 

 teristic that it can not be propagated through a liquid. 



The velocity of the first, fast tremor, found to be 5.4 km (3.35 

 miles) per second in the case of the Oppau explosion, and 5.5 km 

 (3.4 miles) per second, on the average, at La Courtine, was that of 

 the longitudinal vibrations — the so-called P wave. The velocity of 

 the S wave was found to be 3.1 km (1.9 miles) per second at Oppau 

 and 2.8 km (1.7 miles) per second in the mean at La Courtine — 



^'Maurain, Charles, Eble, L., and Labrouste, H., Sur les ondes sismiques des explosions 

 de la Courtine, Journ. Phys. et le Rad., vol. 6, No. 3, pp. 65-78, March, 1925. 



