THE GULF STKEAM. 407 



peiTnanent, as on some sections its absence was in some years conspicuous. 

 Lieutenant Pillsbury says: — "I am convinced that the so-called "Cold 

 Wall " is not the inner edge of the Stream, but is near the dividing line 

 between the Stream proper and the outside Atlantic current, and that the 

 maximum velocity will always be found some miles Northward of it. 

 The current outside the Stream is not comparable with the latter in point 

 of velocity; its speed probably is never much over 1 knot, and usually 

 much less." 



(397.) The earlier surveyors were of opinion that the "Cold Wall" 

 appeared at the line separating the Gulf Stream from the Arctic Current, 

 their observations showing the separation between the two currents to be 

 , so well marked beneath the surface, and to the greatest depths, as to have 

 obtained for it the title of the " Cold Wall," being, in fact, an upright 

 division between them. This peculiarity was found to exist almost along 

 the whole coast of the United States, where the Stream skirts the bank of 

 soundings. Without diagrams the features cannot be made quite intelligible ; 

 but the main fact, so interesting to the physical geographer, is as above 

 stated — that there appears to be a marked non-aflinity between the waters 

 of the two currents. 



It was at first supposed the " Cold Wall " was cut off at Cape Hatteras, 

 but subsequent researches have qualified this notion. The cold water has 

 been traced as far as the Tortugas. Off Sombrero Kay, in some of the 

 earlier researches, the "Cold Wall " was strongly marked at depths vary- 

 ing from 70 to 100 fathoms, while everywhere the warm water overflowed 

 the " Cold Wall," and reached quite to the shore. 



(398.) Subsurface Temperatures. — It was formerly held that the Gulf 

 Stream flowed on in one majestic current of warm water from its surface 

 to its bed. The magnitude of its effects, and the extent of the area attri- 

 buted to it, seemed to leave room for no other conclusion. But the first 

 observation of ice-cold water at a small depth, in its narrowest and strongest 

 part, followed by the discovery of the parallel warm and cold bands, over- 

 turned all these hypotheses, and left philosophers in a most perplexed 

 condition to know where to look lor an explanation.* How this cold 

 water, flowing directly contrary to the course of the upper strata, should 

 preserve its Polar characteristic almost unimpaired to such an enormous 

 distance from its origin, and under such apparently adverse circumstances, 



• In the earlier experiments made by the United States Coast Survey, the tempera- 

 ture was gained at all depths, from the surface down to 600 fathoms. At great depths 

 a peculiar thermometer was used, constructed for the purpose, whose principle of action 

 depended on the differing expansion of two metals. It is a spiral coil, composed of two 

 strips of silver and platinum soldered together, which, from their unequal contraction 

 and expansion by the effects of temperature, act on an index, which registers the ex- 

 treme temperatures. 



But later experiments have shown that many of these results are fallacious, both as 

 respects this differential evil, and especially with the ordinary thermometer with un- 

 protected bulb as formerly used ; and, therefore, all of them will require revision, as the 

 probability, nay certainty, is that the recorded temperatures at great depths have been 

 placed much too high. Of this more hereafter. The more recent investigations have 

 been carried on with the Miller-Casella Thermometer and with Siemens' Electrical 

 Thermometer. 



