?4 iiEPORT — 1902. 



from the above four stations, which are respectively situated on alluvium, 

 chalk, new red sandstone, and a Palseozoic felstone, with the exception of 

 the month of July, when the instrument at Bidston was not working, 

 relate to the remaining eleven months of the year 1901. 



Earthquake Frequency. 



At Kew 73 records were obtained, 63 of which were noted at other stations. 



AtShide 107 „ „ -„ 90 



At Bidston 133 „ „ „ 94 „ „ 



At Edinburgh 94 „ „ „ 85 



Earthquake Duration. — During the period under consideration fourteen 

 large earthquakes were recorded at each of the four stations, and were 

 also recorded at many other stations throughout the world. The total 

 number of hours and minutes during which the instruments at the four 

 British stations were caused to move by these disturbances were as 

 follows :— Kew, 27 h. 40 m. ; Shide, 31 h. .^6 m. ; Bidston, 30 h. 25 m. ; 

 Edinburgh, 31 h. 59 m. 



Amjjhtudes. — For the above fourteen earthquakes the sum of the 

 amplitudes in millimetres recorded at the four stations were as follows : — 

 Kew, 42-8 mm. ; Shide, > 58-4 mm. ; Bidston, 56"7 mm. ; Edinburgh, 

 40'7 mm. 



The inferences to be drawn from the above three analyses are by no 

 means clear. 



The Frequency Table apparently shows that at Shide and Bidston 

 more earthquakes can be recorded than at Edinburgh and very many 

 more than can be recorded at Kew. Reference to the registers of these 

 four stations shows that the omissions in the Edinburgh and Kew lists 

 relate to earthquakes which were comparatively feeble. 



Not only are the records at Kew few in numbei-, but the duration of 

 a given set of earthquakes as recorded at that station is shorter than the 

 duration of the same set of earthquakes as recorded at other stations. 



A much more marked difference between these four sets of records is 

 to be seen in the Table of Amplitudes, which it must be noted have been 

 entered as horizontal displacements. From these records the inference is 

 that the extent of movement at Edinburgh and Kew is much less than 

 it is at Shide and Bidston. Inasmuch as the foundations at the first two 

 mentioned stations are respectively harder and very much softer than 

 the foundations at the two second stations, it seems improbable that the 

 differences in amplitude here recorded are to be altogether attributable 

 to the geological character of the materials on which these four stations 

 are situated. 



A more likely cause resulting in these apparent differences in ampli- 

 tude, and we may add also the differences in durations of movement and 

 number of records, is to be found in differences in the sensibilities of the 

 instruments at the four stations. 



If as a measure of the sensibility of an instrument we take the angle 

 through which the bedplate of the same has to be tilted to produce a 

 deflection of one millimetre of the outer end of the boom which it carries, 

 then the sensibilities of the instruments at the four stations under con- 

 sideration have been as follows : — 



At Kew the sensibility has varied from 0"-7 to 0"-8, average 0"-75. ■ 



At Edinburgh „ ,, been 0"-71. I 



AtShide „ „ „ 0"-47. 1 



At Bidston „ „ „ 0"'4 up to June 30 and subsequently 0"'3. ^ 



