34 THE PHYSICAL GEOGRAPHY OF THE SEA. 



scribed would now be interrupted by obstructions and local causes 

 of various kinds, such as unequal depth of water, contour of shore- 

 lines, &c. ; and we should have at certain places currents greater 

 in volume and velocity than at others. But still there would be 

 a system of currents and counter currents to and from either pole 

 and the equator. Now do not the cold waters of the north, and 

 the warm waters of the Gulf, made specifically lighter by tropical 

 heat, and which we see actually preserving such a system of coun- 

 ter currents, hold, at least in some degTce, the relation of the sup- 

 posed water and oil ? 



23. In obedience to the laws here hinted at, there is a constant 

 tendency (Plate IX.) of polar waters toward the tropics and of 

 tropical waters toward the poles. Captain Wilkes, of the United 

 States Exploring Expedition, crossed one of these hyperborean 

 under-currents two hundred miles in breadth at the equator. 



24. Assuming the maximum velocity of the Gulf Stream at five 

 knots, and its depth and breadth in the Narrows of Bemini as be- 

 fore (§ 9), the vertical section across would present an area of two 

 hundred millions of square feet moving at the rate of seven feet 

 three inches per second. The difference of specific gravity be- 

 tween the volume of Gulf water that crosses this sectional line in 

 one second, and an equal volume of water at the ocean tempera- 

 ture of the latitude, supposing the two volumes to be equally salt, 

 is fifteen millions of pounds. If these estimated dimensions (as- 

 sumed merely for the purposes of illustration) be within limits, 

 then the force per second operating here to propel the waters of 

 the Gulf toward the pole is the equilibrating tendency due to fif- 

 teen millions of pounds of water in the latitude of Bemini. This 

 is in one scale of the balance. In the other, the polar scale, there 

 is the difierence of specific gravity due an equal volume of water 

 in the polar basin, on account of its degree of temperature as well 

 as of saltness. 



25. In investigating the currents of the seas, such agencies 

 should be taken into account. As a cause, I doubt whether this 

 one is sufficient of itself to produce a stream of such velocity and 

 compactness as that of the Gulf; for, assuming its estimated dis- 

 charge to be correct, the proposition is almost suf^ccptible of math- 



