THE GULF STREAM. 33 



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

 tendency of polar waters toward the tropics and of tropical waters 

 toward the poles. Captain Wilkes, of the United States Explor- 

 ing Expedition, crossed one of these hyperborean under-currents 

 two hundred miles in breadth at the equator. 



24. Assuming tha maximum A^elocity of the Gulf Stream at five 

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

 fore (§ 10), 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 

 between the volume of Gulf water that crosses this sectional line 

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

 ature of the latitude, is fifteen millions of pounds. If these esti- 

 mated dimensions (assumed merely for the purposes of illustration) 

 be within limits, then the force per second operating here to pro- 

 pel the waters of the Gulf tow^ard the pole is the equilibrating ten- 

 dency due to fifteen millions of pounds of water in the latitude of 

 Bemini. 



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 great velocity 

 as that of the Gulf; for, assuming its estimated discharge to be 

 correct, the proposition is almost susceptible of mathematical dem- 

 onstration, that to overcome the resistance opposed in consequence 

 of its velocity would require a force at least sufficient to drive, 

 at the rate of three miles the hour, ninety thousand millions of 

 tons up an inclined plane having an ascent of three inches to the 

 mile.* Yet the very principle from which this agent is derived 

 is admitted to be one of the chief causes of those winds which are 

 said to be the sole cause of this current. 



26. The chemical properties, or, if the expression be admissible, 

 the galvanic properties of the Gulf Stream waters, as they come 

 from their fountains, are different, or, rather, more intense than 

 they are in sea w^ater generally. 



In 1843 the Secretary of the Navy took measures for procur- 

 ing a series of observations and experiments with regard to the 

 corrosive effects of sea water upon the copper sheathing of ships. 

 With patience, care, and labor, these researches were carried on 

 * Supposing there be no resistance from friction. 



c 



