248 WAVES OF THE SEA 



far below this that there is the greatest tidal range 

 (about 40 feet at springs)? It may occur to the 

 reader that a long run is required under bore- 

 forming conditions before the bore becomes appre- 

 ciable ; but both theory and observation show it is 

 not so. If the conditions for forming a bore be 

 fulfilled, the wave attains a considerable size in a 

 very short interval both of time and space. 



An answer to the question, Why does 

 the bore usually originate above and not below 

 Severn Bridge? /.., in the upper, not the lower, 

 half of the steep slope of the estuary is afforded 

 by the following observations made from the com- 

 manding elevation of the Severn Bridge ( 100 feet 

 above low-water mark), on April 27, 1900, two 

 days before the highest of this set of spring tides. 

 At 3.43 p.m. the approach of the flood tide could 

 be discerned by a change of appearance of the 

 water, although there was no actual surface wave. 

 It was 4.5 p.m. before the level of the water was 

 rising at the bridge, so slow is the advance of the 

 first rise. I was stationed near the west end of 

 the bridge, where the principal low -water channel 

 of the ebbing water lies. The flood tide did not 

 force its way up this channel against the strong 

 stream, but was pressed away towards the east, 

 filling a swatchway, or blind channel, which led 



