On the general Causes of the Ocean-Currents. 5 



the difference of pressure only extends to a certain depth, than when it 

 extends to the bottom: and in general a greater time is required, when 

 the difference of pressure extends to a less depth, than when to a greater. 



Frtim these theorems, the truth of which I need not stop to de- 

 monstrate, I shall now endeavour to draw some general conclusions, re- 

 specting the effect, which differences of specific gravity between the 

 waters in different parts of the ocean can have on the origin of oceanic 

 streams. 



If we define the normal sea-level as that, which would result from 

 the force of gravity and the earths rotation with the mean barometric 

 pressure, certain deviations from that level must take place in conse- 

 quence of different specific gravity in different parts of the ocean, and 

 these deviations ought to approximate to, but not fully reach those, 

 which would be determined by the inverse ratio of the sp. gravities of 

 the water-strata '). 



Let us first suppose the case of the water in a certain part of the 

 ocean being lighter than the rest dowii to a distance, n, below the sur- 

 face, but that the surface itself is everywhere normal. Two different 

 kinds of motion would immediately be produced. The lighter water 

 would be lifted by the surrounding heavier, till the difference of pres- 

 sure at the new level of its lower side, n', vanished, but, as its pressure 

 at all levels above 71' have now become greater than that of the heavier 

 water, it would instantly begin to displace and overflow the latter. This 

 movement of the lighter water would however require considerably more 

 time than the movement of the heavier water, whereby the lighter was 

 lifted and continues to be lifted, as its level sinks by lateral diffusion. 

 For the sum of the differences of pressure, which caused the elevation 

 of the lighter water, were in the first place greater, than the sum of the 

 difterences, that caused its diffusion towards the sides. Secondly the 

 differences of pressure, which caused the first motion, must extend all 

 the way to the bottom, whereas those, which cause the latter, extend no 

 deeper than the lighter stratum itself, and even within the extent of 

 that have their chief effect confined to the superficial strata. With re- 

 gard to the velocity of these motions, that of the heavier water will be 

 relatively little sensible, because it takes place in a section of larger 

 area, but the motion of the lighter water will be seen to form a more 

 rapid and more clearly defined stream. 



') Some impoitant exceptions of this last statement will be hereafter noticed. 



