TIDAL WAVES. 



613 



wind has subsided the rolling, or " swell," remains, a long, lazy, undulating 

 motion a rocking to sleep of the billows of the sea. Without a ripple on 

 the surface these huge rollers will glide towards the shore and break upon 

 the shingle with a roaring sound which can be heard for miles, dragging the 

 pebbles after them as they recede with a rattling like bones and marbles. 

 The pebble ridge at Westward Ho ! will illustrate this vividly at times, the 

 sound being heard far inland like continuous thunder, and on a calm night, 

 when there is no wind stirring, the roar of the ground swell is weird and 

 mysterious in the gloom. 



The height of waves is very varied. Observers say that forty-four feet 

 is about the highest-known wave from hollow to crest. Waves of thirty-five 



Fig. 699. The Piroroco on the Amazon. 



feet have been often met with, and off the Irish coast and in the Atlantic 

 sailors tell of waves " as big as houses." But houses differ in size as do 

 waves. 



The rate which waves are estimated to travel varies with the wind- 

 propelling force. The average hurricane wave travels at about forty-five 

 miles an hour. But earthquake waves those set in motion by subaqueous 

 disturbance have been known to travel at the rate of six hundred feet in a 

 second for thousands of miles across the ocean. Such a one occurred after 

 the earthquake which destroyed the town of Arica in August 1868, and the 

 wave crossed the Pacific to Chetham Islands, 5,520 miles, in fifteen hours 

 and twenty minutes. We have many of us seen the 1 great tidal waves, or 

 <; bores," which at certain seasons rush up our rivers the Severn, for instance 

 with great violence, and at times forty feet high. 



