278 MR. T. HEELIS ON OCEAN SWELL. 



There are other swells noticed in different parts of the 

 world, which, so far as our present knowledge extends, 

 cannot be traced to any of the above causes. Such swells 

 occur particularly in the Southern Atlantic, and form the 

 rollers at Ascension, St. Helena, Tristan d^Acunha, and 

 the Island of Ichaboe, on the coast of Africa. They roll 

 heavily into the Bay of Jamestown at St. Helena, which 

 is open to the westward, often doing much damage to the 

 shipping lying there. They are pretty regular in their 

 occurrence, but are heavier on some occasions than others. 

 The anchorage at Ascension, which is subject to them, is 

 also open to the N.W. But little mention is made of 

 them in Horsburgh. I have only heard of one instance in 

 which these rollers have been met with at sea, and I am 

 now unable to refer to it. The ship which encountered 

 them on that occasion had her decks swept. Their cause 

 is unknown; attempts have been made to explain them, 

 by the supposition that they are caused by submarine 

 earthquakes, which are known to be common a little to 

 the south of the equator ; but their occurrence is too regu- 

 lar to admit of such an explanation. 



The altitude of the successive undulations in the case 

 of swell observed in open water is variable; the largest 

 observed usually occur in pairs or triplets — every tenth or 

 twelfth undulation, or thereabouts, being large, while the 

 intermediate ones are smaller, with a tendency in the sixth 

 or seventh to be above the average of the small ones in 

 altitude. Some observations which I have made in light 

 winds, give altitudes for the largest undulations of 14 feet 

 measured from the troughs, with a width of trough of 

 some 600 feet, and a speed of translation in one case of 

 twenty -two miles per hour. 



An exact knowledge of the phenomena presented by 

 swell (of which at present we know little or nothing) is 

 a necessary foundation for any enlarged study of waves. 

 The ocean from the Cape of Good Hope to 37° south, a 



