234 DEEP BOREHOLE SURVEYS AND PROBLEMS 



earth as seismic waves. In air and water only longitudinal 

 condensation and rarefaction waves are formed, and in 

 these every particle oscillates to and fro about its position 

 of rest in a direction parallel to the direction of propagation. 

 In air the velocity of propagation v, under a pressure p, 

 density p, and specific heat ratio x at constant tempera- 

 ture and constant pressure, is v equals Vxp/p, while in 

 liquids it is v equals Vk/p, where k is the compressibility. 

 But in solids the relations are very complicated, and, 

 indeed, not yet fully comprehended but are determined, 

 as said, with the aid of Poisson's constant o-, which is the 

 ratio of the extension of a pulled bar to its accompanying 

 decrease in cross-section being between 0.2 and 0.5 for the 

 different solid substances; and also with Lame's coefficients 

 X and jjL, particularly the latter, which indicates the stiffness 

 or rigidity modulus and is therefore of great practical 

 significance. These must be known from laboratory tests, 

 because the speed of the waves varies so greatly with differ- 

 ent media; for instance, soft friable rocks, like sand, propa- 

 gate earthquake shock waves at about 400 m. per second, 

 while hard primitive rock shows a velocity of about 

 4,000 m. per second; in general, from 1 to 4 km. per second, 

 and these figures, of course, vary with the differing densities 

 and elasticities of the different media traversed. Doctor 

 Mintrop^ has undertaken observations collecting and 

 developing usable methods of investigation through the firm 

 Seismos, Ltd., in Hanover. 



All workers in this field are indebted to the pioneering 

 work of Wiechert^ and his able pupil, Gutenberg,- the result 

 of whose labors, combined with the very extensive experi- 

 mental material of many earthquake observatories, have 

 brought about practical conclusions upon which modern 



^ MiNTROP, L., "tjber kiinstliche Erdbeben," Intrn. Kong. Diisseldorf, 

 1910. Abtlg. IV, Vortrag No. 14, Seismos-Gesellschaft. Mttlgn. d. I. "Erfor- 

 schung von Gebirgsschichtcn und nutzbaren Lagerstatten nach dem seis- 

 michen Verfahren," Hanover, 1922 (the Seismos Company's own 

 publication). 



2 Repts., Imp. Soc. Sci., p. 195, Gottingen, Berlin, 1899; also Sieberg, 

 A., "Applied Earthquakes, Geological-Physical," p. 283, Jena. 



