Vol. 60.] CAERNARVON EARTHQUAKE OF 1903. 235 



East in County Waterford, Ravensdale in County Louth, Kendal, and 

 Didsbury (near Manchester). The distances of these places from 

 the outermost isoseismal are, respectively, 22, 8, 25, and 13 miles. 

 If we regard the boundary of the disturbed area as passing through 

 Kendal and as concentric with the isoseismal, the disturbed area 

 would include about 40,000 square miles. The observations at the 

 four places mentioned were, however, made in upstair rooms, and, 

 with one exception, by invalids in bed. It seems desirable, therefore, 

 to regard the disturbed area as bounded by the outermost isoseismal, 

 and as containing 25,000 square miles. 



Nature of the Shock. 



In its general features, the nature of the shock was practically 

 uniform throughout the disturbed area ; and the following account 

 from Meyllteyrn (near Xevin) may be regarded as typical for a very 

 large portion of the area. The shock began with a series- of tremors, 

 lasting 4 or 5 seconds, which merged gradually into a single series 

 of principal vibrations of about 3 or 4 seconds' duration, these in 

 turn being succeeded by a brief series of tremors, lasting only 1 or 

 2 seconds. The movement was thus continuous, increased gradually 

 in intensity, and then rather more rapidly died away. At a few 

 places not far from the central area, two maxima of intensity in 

 the principal vibrations were detected by careful observers ; and 

 their evidence, as will be seen, is confirmed by the seismographic 

 record at Birmingham. At a great distance, at Liverpool and 

 Southport and in some parts of Ireland, for instance, the vibrations 

 between these maxima were imperceptible, and the shock seemed 

 to consist of two detached parts. The period of the vibrations also 

 increased with the distance, so that, in Lancashire, Ireland, and 

 elsewhere, the motion was a gentle swaying several times to and 

 fro. The average of 88 estimates of the duration of the shock is 

 6| seconds. 



Seismographic Records. 



The Caernarvon earthquake was recorded by a Milne seismograph 

 at Bidston, near Birkenhead, and by an Omori horizontal pendulum 

 at Birmingham. 



Bidston is 60 miles from the centre in the direction E. 24° N. 

 Mr. W. E. Plummer, the director of the observatory, kindly informs 

 me that the first movements took place at 10 h 5 ,u 5* a.m. The 

 separate oscillations of the pendulum are not shown on the diagram, 

 but there seem, he says, to have been two distinct impulses, the 

 second taking place at 10 h 7 m 30 s . The amplitude of the dis- 

 turbance was even less than in the case of the Derbv earthquake of 

 March 24th, 1903. 



Birmingham lies 111 miles E. 20 J S. from the centre. The record, 

 which is enlarged 9*75 times in fig. 1 (p. 236), gives the component 

 of the motion in the east-and-west direction ; and, as the movement 



u2 



