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Sect. VIL] 



EARTHQUAI^E PKENOMENA. 



203 



Observations of eacli of these classes of waves Viiicli we 

 have thus brieflv described may be made either dirently 





hj the aid of instruments, specially provided or ex- 

 temporaneously formed, or indirectly by proper notice of 



which they produce, on objects upon the 



certam 



earth^s surface. 



Direct observations l^y complete self-registering Seis- 

 mometers do not come within our present scope. We, 

 therefore, proceed to direct observations with extempora- 

 neous instruments on the earth-wave or shock. The ele- 

 ments necessary to be recorded aro such as will enable us 

 to calculate — 1. The direction in azimuth of the v/ave's 

 motion ; and (if it have an upward motion) also its direc- 

 tion of emergence at the points of observation. 2. Its 

 velocity of transit. 3, Its form — i. e. its amplitude and 



altitude. 



If a common barometer be moved a few inches up and 



down by the hand, the column of mercury v/ill be found to 

 oscillate up and dow^n in the tube with the motions of the 

 instrument and in opposite directions, the range of the 

 mercury depending upon the velocity and range of mo- 

 tion of the whole instrument. A barometer 6xed to the 

 earth, therefore, if we could unceasingly w^atch it. would 

 give the means of measuring the vertical element of the 

 shock-wave ; and if w^e could lay it down horizontally, it 

 would do the same by the amplitude or horizontal ele- 

 ment. Tliis we cannot do ; but the same principle may 

 be put into use by having a few pounds of mercury, and 



some 



las: 



R 



L 



one eud, and open at the other ; the bore behig about 



three-eighths of an inch in diameter, and each limb about 



■^Z^i^^- 



