222 DR. CHARLES DAY1S0N ON THE L^ av I 9°45 



paper travelling at the rate of 10*8 millimetres per minute. Owing 

 to the short period of the vibrations at Birmingham, the heavy bob of 

 the pendulum acted almost as a steady point, the slight swinging of 

 the pendulum being evident in the large curve on which the seismic 

 waves are superposed. The movements of the ground in such a case 

 are magnified 13*7 times by the pendulum ; and, as the original 

 record is also magnified 28*1 times by the enlargement in tig. 1, it 

 follows that the latter represents the actual movements multiplied 

 by 385. 



An examination of the record under the microscope shows not the 

 slightest trace of movement before the first abrupt disturbance to 

 the east, which took place at l h 30'" 19 s p.m., G.M.T. The diagram 

 is chiefly remarkable for the two prominent displacements to the 

 west, which occurred at l h 30 m 23 s and l h 30 m 28 s , and which no 

 doubt correspond to the two parts of the shock so widely observed. 

 It is difficult to determine accurately the periods of these two large 

 waves, owing to the width of the trace made by the recording pointer, 

 but in each case it seems to have been about OS sec. At l h 30 ra 31% 

 another oscillation of some importance took place, followed by a 

 series of 13 ripples with an average period of 0*84 sec. These are 

 all that are shown in fig. 1, but the original record continues with 

 a series of 79 still smaller ripples, with a slightly longer average 

 period of 1*03 sees., the last visible under the microscope occurring 

 at l h 32 m 3 s . The total duration of the disturbance as registered 

 in Birmingham was l m 44 s . 



Making allowance for the width of the trace and the swinging of 

 the pendulum, the range of motion of the ground from east to west 

 was '078 millimetre during the first prominent displacement, and 

 •075 mm. during the second. Birmingham, however, lies S. 11° W. 

 from the epicentre, and therefore, if we ma)* assume that the 

 resultant movement was directed from that point, the total displace- 

 ments registered by the pendulum must have been -41 and *39 

 millimetre respectively. These, with periods of "8 sec, would corre- 

 spond to maximum accelerations of 12-6 and 12*0 millimetres per 

 sec. per sec, showing how nearly equal in strength were the two 

 principal parts of the shock. The values given seem to be too small 

 to produce a shock sensible in the centre of a busy city ; and it is 

 therefore probable that the recorded range of motion is less than the 

 actual movement of the ground, owing to the unavoidable friction 

 between the pointer and the smoked paper and between the different 

 parts of the apparatus. 



Bidston lies 65 miles west-north-west of the epicentre, and 8| miles 

 south-west of the rectilinear band. The method of registration in 

 the Milne seismograph being photographic, the paper is made to 

 travel much more slowly than in the Omori pendulum, and con- 

 sequently the diagrams are less detailed and the times of different 

 epochs are ascertainable with less accuracy. I am informed by 

 Mr. W. E. Plummer that 



' The record of the Derby earthquake is small, both in amplitude and duration. 



