138 OUR HERITAGE THE SEA 



of the ocean is susceptible not only of surface suasion, 

 such as is exerted by a long-continued gale, diverted 

 or retarding or accelerating an immense and well- 

 known current, but it may be, and often is, affected by 

 submarine disturbances, such as earthquakes or vol- 

 canoes, of whose occurrence the mariner can have no 

 knowledge. 



It has already been pointed out that warm water 

 is lighter than cold, and that consequently a sudden 

 change of temperature in the bed of the ocean must 

 cause a corresponding displacement of a large body 

 of water, in other words, set up a new and temporary 

 current. This being borne in mind, it will be readily 

 understood how, if by some great opening in the sea- 

 bed admitting the cold water into an incandescent 

 abyss and thereby suddenly raising the temperature 

 of the lower stratum of water many degrees, the whole 

 body of water for hundreds of miles around may be 

 driven or drawn in a totally different direction from 

 the normal. These immense changes are continually 

 taking place in some part of the watery world, but, in 

 the nature of things, they must be unknown to the 

 sailor, and, let him be as careful as he will, they will 

 occasionally land him in some disaster, for which he 

 will be held to blame, but which he was powerless to 

 foresee and consequently to prevent. 



Fortunately for the world, such changes, immense 

 as they are, have but a comparatively trifling effect 

 upon the great regular currents of the globe. The 

 ocean areas are so enormous that, although such a 

 catastrophe as I have mentioned may temporarily 

 affect an area of many thousand square miles, that 

 effect will still be but local and last but a short time. 



