THE GULB^ STREAM. 377 



different Streams, the under part having been doubled one way, and the 

 upper part the contrary." 



It is, therefore, somewhat surprising that a claim should have been 

 made for Dr. Franklin as the real discoverer of its nature and its warmth 

 in 1770. The tale is this : — Being in London in that year, he was con- 

 sulted by the Treasury as to why the Falmouth packets were generally a 

 fortnight longer to New York than common traders were from London to 

 Providence, Rhode Island ? He therefore consulted a Nantucket whaler, 

 who laid its course on a chart, and explained that the Rhode Island cap- 

 tains, being acquainted with the Gulf Stream, avoided it, while the Fal- 

 mouth commanders, being ignorant of it, were set back 60 or 70 (! !) miles 

 a day by it. Without stopping to refute this altogether, which may be 

 done by Dampier and his predecessors, it will be evident that something 

 else than the Gulf Stream must have retarded them 60 or 70 miles a day 

 in that latitude, if they were thus delayed. 



(362.) Causes. — We have already given some general remarks (248, 249) 

 on the origin and causes of surface currents. There have been very many 

 speculations as to the cause of this great river in the Ocean, but they were 

 promulgated before there was any increase of knowledge upon which to 

 found arguments, and, as said before, recent investigations have overturned 

 most of those which have been advanced. It has been supposed that it 

 runs out of the Gulf of Mexico from the superior level of that sea, and 

 facts are not wanting for such an assumption. Captain Manderson, R.N., 

 gave an opinion in his "Examination as to the true course of the Florida 

 Stream," that it was owing to the Mississippi and the floods from the 

 other rivers falling into the Gulf of Mexico. It was still further argued 

 that the velocity of the Gulf Stream might be determined by the floods 

 from thest. rivers. But Captain Andrew Livingston, in our earlier editions, 

 overturnefi this hypothesis by showing that what is poured into the sea by 

 the River Mississippi is not a three-thousandth part of the volume of the 

 Gulf Stream. He thought that it might be accounted for by the motion of 

 the sun in the ecliptic, and its influence on the Atlantic waters. 



The effect of Temperature is also advanced as the prime mover ; in- 

 creasing its heat, the water expands, and thus becomes higher than the 

 cooler waters beyond it, and as the Gulf of Mexico has the highest tem- 

 perature, here is the head water of the Gulf Stream on that account. Sir 

 John Herschel says on this point : — " Let us see what this declivity, formed 

 by unequal temperature, would amount to. The Equatorial surface-water 

 has a temperature of 84° F. ; at 7,200 feet the temperature is 39°, the level 

 of which temperature rises to the surface in lat. 56 ^ Taking the dilata- 

 bility of sea-water to be the same as fresh, a uniformly progressive increase 

 of temperature from 39° to 84° would dilate a column of 7,200 feet by 10 

 feet (or 9-971 feet more exactly), to which height, therefore, above the 

 spheroid of equilibrium (or above the sea level in 56°), the Equatorial 

 surface is actually raised by this dilatation. An arc of 56° on the earth's 

 surface measures 3,360 geographical miles, so that (were the water to run 

 direct North) we have a slope of l-28th part of an inch per mile for the 

 water so raised to run down. As the accelerating force corresponding to 



N. A.O. 49 



