GEOPHYSICAL METHODS OF INVESTIGATING 239 



for obtaining greater magnification for smaller earth 

 movements. It comprises a highly sensitive instrument 



Explosion of 1 Lb. Gelignite, Distance 1200 Yards 



Time Vio+h. Second 



F i I . I '. I . \' ^^^ \, 



Explosion of 11/2 Lbs. Gelignite. Distance 1200 Ycirds Time Vioth. Second 



Fig. 159. — Seismographic records. 



for measuring vertical vibrations (the vibrometer) and, 

 a camera for recording them. The vibrometer, shown in 



Fig. 160. — ^^Seismograph. 



Fig. 160 with the outer cover removed, consists of a heavy 

 mass H fixed on a short lever carried on flexible steel 

 hinges at /. This weight is balanced by springs Oi and O2. 



