Vol. 60.] DERBY EARTHQUAKES OF 1003. 225 



observers heard the earthquake-sound ; in the surrounding zone (that 

 between the isoseisnials 7 and 6) the percentage of audibility was 

 89 ; in the next (bounded by the isoseismals 6 and 5) 80 ; while, 

 between the isoseismal 5 and the boundary of the sound-area, it 

 fell to 65. In other words, within a radius of about 40 miles from 

 the epicentre, nine out of every ten persons heard the sound ; but, 

 outside a surrounding zone 10 miles in width, the sound became 

 inaudible to all but the most acute observers. 



Xature of the Sound. 



The sound was generally a heavy rumble, deeper than an}' 

 thunder, a quick succession of reports, though sometimes appa- 

 rently continuous. The low grating character of the sound is 

 illustrated in many descriptions, such as its comparison with a 

 number of steam-rollers passing over a very uneven road, a very 

 large barrel rolling over cobble-stones, a peal of thunder in a hilly 

 country, a great fall of rock in underground workings, a confusion 

 of knockings or the trampling of many feet ; the rapid rush of the 

 sound is shown by frequent reference to runaway traction-engines, 

 a number of big vans galloping up a road, or the moving of heavy 

 furniture in a great hurry ; the approach to continuity by compa- 

 risons with a steam threshing-machine at a distance, or the rush of 

 a strong wind. 



The total number of descriptions in the whole sound-area amounts 

 to 745. In 53 per cent, of these, the sound is compared to passing 

 traction-engines, etc., in 21 per cent, to thunder, in 5 to wind, in 8 

 to the tipping of a load of stones, in 4 to the fall of a heavy body, in 

 7 to explosions, and in 3 per cent, to miscellaneous sounds. 



In any one place, many different types of comparison are employed, 

 certain vibrations of the series being audible to some persons and 

 not to others. Thus, at Derby, 61 per cent, of the observers com- 

 pared the sound to passing traction-engines, etc., 11 per cent, to 

 thunder, 6 to wind, 11 to loads of stone falling, 5 to the fall of a 

 heavy body, 3 to explosions, and 2 per cent, to miscellaneous 

 sounds. These proportions also vary in different parts of the sound- 

 area, though (except as regards distance) the law of variation 

 cannot be determined with certainty. The percentage of comparisons 

 to passing traction-engines, etc., is 46 within the isoseismal 7, 53 

 between the isoseismals 7 and 6, 56 between the isoseismals 6 and 

 5, and 59 between the isoseismal 5 and the boundary of the sound- 

 area ; for thunder, the corresponding percentages are 32, 21, 16, 

 and 14. Thus, with increasing distance from the origin, the sound 

 tends to become smoother and more monotonous, owing to the gradual 

 extinction of the limiting sound-vibrations, and especially those of 

 longest period. 



Relation of the Sound to the Double Series 



of Vibrations. 



In many of the detailed accounts, reference is made to two 

 Q. J. G. S. No. 23b. * 



