32 



ANNUAL REPORT OF THE 



clays on which the seismograph stands are not incompatible with 

 its use as a recording station. In only one instance, that of the I 

 earthquake of July 30, 1909, has the agitation of the indicator I 

 needle been so great as to cause it to range the full width of the I 

 chronographic drum (150 mm.)- In the table the times have been I 

 taken usually from one component only, either that which showed I 

 the earliest picking up of the seismic waves or that which gave the I 

 best record, if one of the records was at all undecipherable. The | 

 statistics bearing on distance to the point of origin are calculated I 

 from Laska's rules to which in the later cases Benndorf's correc- i 

 tion for the first rule has been applied. 



Of many unexplained and not wholly understood jars recorded I 

 at the Station no account is taken. Approximate observations con- I 

 cerning certain disturbances lasting from 3.25 min. to 3.75 mins. 

 have shown that a wagon heavily laden with bricks passing down i 

 Oxford St. from which the Station is 250 ft. distant, begins to set I 

 up tremblings and rapid jars at the Station when the wagon is I 

 within 600 ft. of it. None of these minor disturbances have, how- I 

 ever, interfered in any way so far with the recording of earthquake I 

 shocks. 



Plate 3 is reproduced from the significant portion of the record I 

 giving the earthquake of July 30, 1909, whose epicentre was near j 

 Acapulco, Mexico. 



