MR. T. HEELIS ON OCEAN SWELL. 277 



servation. In such a case there are, in fact, two swells — 

 one arising firom the wind, and one from the current. 



The remaining causes of swell are : — 



The undulations thrown off from a cyclone in its pro- 

 gress. These occasion a very confused sea in the area oc- 

 cupied by the cyclone itself, and a confused swell at a short 

 distance beyond its limits ; but this description of swell has 

 the peculiarity of becoming more regular as its distance 

 from the cyclone which causes it is increased. The reason 

 of this will be obvious from a consideration of the dia- 

 grams given in Lieut.-Colonel Reid's account of the Pro- 

 gress of the Development of the Law of Storms and of 

 the Variable Winds (Ed. 1849), PP- 3^^ 37- 



The swell thrown off by cyclones often travels to great 

 distances. Several instances of this are given in Reid^s 

 work, and can be supplied by the experience of every 

 seaman who has had any experience of tropical navigation. 



The undulations impressed upon the waters of the ocean 

 by an earthquake form the last class of swells whose 

 causes are known to us. One of the most remarkable 

 instances of undulations arising from this cause is afforded 

 by the swell caused by the earthquake in Japan, which 

 destroyed the Russian frigate ' Diana.^ These undulations 

 travelled across the whole breadth of the Pacific Ocean, 

 and were detected by the tide-gauges of the United States 

 Coast Survey at San Francisco. 



All well-kept log-books of ships record the occurrence 

 of swell, noting also its direction. In my own log I have 

 generally added the observed height of it when considerable. 



A heavy swell in open water is much more impres- 

 sive, and gives to the thoughtful observer a much greater 

 idea of power than even the waves in a heavy gale of 

 wind. Instead of the ridges being formed by detached 

 summits with intervening depressions, as in the case of a 

 wave-ridge, they extend in one unbroken wall of water as 

 far as the eye can reach. 



