302 PHYSICAL GEOGRAPHY. 



"We do not know the depth to which oceanic currents 

 (warm or cold) extend: the deflection of the South 

 African current by the Lagullas bank, which is from 

 sixty to seventy fathoms deep, would indicate it to be 

 considerable. The presence of sand banks and shoals, not 

 situated in any line of current, may be recognised by the 

 low temperature of the water over them; and the knowledge 

 of this fact may often conduce to the safety of navigation. 

 It was first discovered by the justly celebrated Benjamin 

 Franklin, who may be said to have thereby transformed the 

 thermometer into a sounding line; and its explanation ap- 

 pears to be, that when in the general movement of the 

 water forming the current, the deeper situated and colder 

 particles strike upon a bank, their motion is inclined up- 

 wards, and they mingle with and chill the upper stratum of 

 water. My late illustrious friend, Sir Humphry Davy, 

 attributed the phsenomenon rather to the descent of the 

 surface particles, cooled by nocturnal radiation, and to their 

 being prevented from sinking deeper by the shoal, which 

 thus retained them in closer proximity to the surface. Mists 

 are frequently met with over shoals, from the influence of 

 the cooled water in condensing the vapour in the atmo- 

 sphere. I have seen such mists to the south of Jamaica, 

 aud in the Pacific, which have shewn the outline of the 

 shoals beneath so well defined, as to be distinctly recog- 

 nised from a distance; thus forming to the eye aerial 

 images reflecting the form of the bottom of the ocean. A 

 still more remarkable effect of the cooling influence of shoals 

 is shewn sometimes in the higher regions of the atmosphere. 

 At sea and in very clear weather, clouds are often seen sus- 

 pended above the site of sand banks or shoals, as well as 

 over low coral or sandy islands. Their bearings may be 



