708 



DARBYSHIRE 



[CHAP. 20 



Bermuda are shown in Fig. 6 for 6-sec waves coming from directions 0°, 30°, 

 60° . . . 330°. There is a very wide divergence of wave energy at the island for 

 the 30°, 210° and 240° directions and a concentration at 180° and 270°. This 

 must affect greatly the amplitude of microseisms received from a storm in a 

 particular direction and may be responsible for some of the microseism barriers 

 which some writers suggest prevent the passage of microseisms. For instance. 

 Carder (1955) quotes two storms of approximately equal intensity, one to NE 

 of Bermuda and the other to NW; the NW storm gave appreciable microseism 



Fig. 6. Refraction diagrams for microseisms approaching Bermuda. (After Darbyshire, 

 1955.) 



activity but the NE one gave very little. This agrees exactly with the deductions 

 from the refraction diagrams. 



Similar diagrams (1957) have been constructed for the British Isles. There is 

 here a marked divergence for microseisms coming from a westerly direction. 

 This is borne out by observations. 



These diagrams have been drawn by assuming an original long straight wave 

 front far out in the ocean and plotting successive wave fronts by using 

 Huyghens's principle. An alternative method is to assume a point source. 



