Vol. 62.] DONCASTER EARTHQUAKE OF 1905. 9 



Seismographic Record. 



The earthquake was registered by- the Omori horizontal pendulum 

 at Birmingham, which city is 75 miles S. 28° W. of the principal 

 epicentre. The record is too minute and ill-defined for reproduction, 

 the range or double amplitude of the largest waves (the second, 

 third, and fourth) being less than y^- - inch, and corresponding to 

 a movement of the ground of about .> * inch. 1 The remaining 

 vibrations are represented by mere notches on the trace, and in 

 parts these cannot be separated. Altogether, there appear to be 

 about thirty vibrations in about 15 seconds. The exact time of the 

 first movement I am unable to determine. 



Sound-Area. 



The boundary of the sound-area is indicated on the map (PI. II) 

 by the dotted line. Except towards the north-east, it overlaps the 

 isoseismal 4 by a few miles, extending 11 miles beyond it towards the 

 south-west, and falling short of it towards the north-east by 3 miles. 

 It is 135 miles in length, 115 miles in width, and about 12,000 square 

 miles in area. 



For a strong earthquake, the sound was heard by an unusually- 

 large proportion of the observers : the percentage of audibility for 

 the whole disturbed area being 93. The same high percentage is 

 also maintained to a considerable distance from the epicentres : 

 being 94 within the isoseismal 7, 94 between the isoseismals 7 

 and 6, 93 between the isoseismals 6 and 5, 86 between the iso- 

 seismals 5 and 4, and 67 between the isoseismal 4 and the boundary 

 of the sound-area. Nor is there any sensible variation with the 

 direction from the epicentres : for, in attempting to draw isacoustic 

 lines by the method employed for the Derby earthquakes of 1903 

 and 1904, the change in audibility from one square to another was 

 found to be small and irregular. 2 There is no trace of any strength- 

 ening of the sound in the directions at right angles to the longer 

 axes of the isoseismals — a result which is in agreement with the 

 absence of a synkinetic band. 



Nature of the Sound. 



The total number of descriptions of the earthquake-sound is 1061. 

 In 43 per cent, of these, the sound is compared to passing traction- 

 engines, motor-cars, etc., in 29 per cent, to thunder, in 12 to wind, 

 in 4 to loads of stones falling, in 4 to the fall of a heavy body, in 6 

 to explosions, and in 2 per cent, to miscellaneous sounds. These 



1 It is probable that the actual displacement of the ground was greater than 

 this, for the deposit of soot on the paper clearly hindered the movement of the 

 pendulum. 



2 With a vei*y large number of observations, it would of course have been 

 possible to draw the isacoustic lines. But when, as in this case, the number of 

 records from places within any square is not great and the number of negative 

 records very small, one more or one less negative record causes a considerable 

 change in the percentage of audibility. The irregular variations in audibility 

 are probably to be explained in this manner. 



