356 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



to reach stations which are between Y,000 miles and 10,000 miles of the 

 epicenter, leaving the so-called blind zone as shown in Figure 2. 



In what state is the iron of the core — solid, liquid, or gas? The 

 question is still an open one. The fact that the S wave will not be 

 transmitted through a liquid or a gas suggests that we apply that 

 criterion. This is not as easy as might be supposed. Some seismol- 

 ogists believe the S wave has been identified after transmission 

 through the core. Most are agreed, however, that it has not been 

 positively identified, and some are frankly of the opinion that the 

 central core is liquid or gas. It is supposed to be dense, under high 

 pressure, of course, but not an elastic solid. The velocity of the 

 P wave is high for the layer just outside the central core — about 



FiGUUE 2. — Cross section through the earth, showing the path of propagation of 

 characteristic longitudinal waves. (Adapted from Gutenberg's contribution to 

 Borntracger's llandbuch der Geophysili. See footnote 13) 



13 km (8 miles) per second. Within the core it drops suddenly 

 to 8.5 km (5.3 miles) per second. Does the density suddenly increase 

 as you pass into the core, or does the elasticity become markedly 

 less? Alwa3's we are left with questions, unanswered as yet but not 

 unanswerable. Of such is the kingdom of research. Without them 

 we should develop an orthodoxy of science which would be fatal. 

 The surface of discontinuity at the central core is, as has been 

 noted, well defined. It is indicated by the " blind zone " and also 

 by the sudden drop in velocity, which, in turn, indicates either a 

 sudden increase in density or a rapid falling off in elasticity or a 

 combination of these. The fact that there is a sudden drop in 

 velocity is deduced mathematically from the observational data of 

 earthquake records — the so-called time-distance or travel-time curves. 

 A paper by Knott ^® presents the method by which this is done. A 



'* Knott, C. G., The propagation of earthquake waves through the earth and connected 

 problems, Proc. Roy. Soc, Edinburgh, vol. 39, pt. 2, No. 14, pp. 157-208, 1918-19, 



