1872.] 



' Shearwater 3 Scientific Researches. 



619 



more than 40 miles, the Polar stream occupies 1 5 miles of this hreadth on the 

 Florida side; and while its greatest depth scarcely exceeds 300 fathoms, even 

 this excess only extends over a breadth of about ten miles, forming a valley 

 on either side of a submarine ridge (Diagram ix.). The Florida slope 

 shows a well-marked succession of terraces, whilst the Bernini slope is very 

 steep. It is most singular to see how the bathymetrical Isotherms here 

 follow the undulating contour of the bottom, instead of lying parallel to 

 the surface ; plainly indicating that the colder and heavier water has a 

 motion of its own, by which it is carried up the slopes of the hills, instead 

 of finding its level in the valleys. The 75° line, which lies at the surface 

 on the Florida side, sinks to nearly 100 fathoms in the deepest part of the 

 channel ; and at ten miles from Cape Florida, where the whole depth but 

 little exceeds 160 fathoms, the water has a temperature of 70° even at 

 75 fathoms, clearly showing the extension of the warm stratum to that 

 side of the channel. But below 75 fathoms, on this terrace, the tempera- 

 ture falls so rapidly, that 45° is reached at 140 fathoms; though in the 

 deeper channel the water at that depth has a temperature of 65°, sinking 

 to 45° at 250 fathoms. No temperature below 45° is recorded as having 

 been observed in this section ; but if the rate of bathymetrical reduction 

 between 200 and 250 fathoms be continued downwards to a bottom below 

 300, the temperature of the lowest stratum will be found as low as 35°. 



133. A\ though the temperature of 35° was not actually observed either 

 in the Carysfort or in the Bernini sections, the fact that it showed itself in 

 the deepest water of both the Sombrero and the Tortugas sections, and in 

 the latter at a depth of less than 400 fathoms, — taken in connexion /with 

 the difficulty, if not the impossibility, of otherwise accounting for the pre- 

 sence of water of 35° within the Narrows, — seems fully to justify the assump- 

 tion that it has flowed over what may be called 'the submarine watershed 

 between Cape Florida and the Bernini Isles ; and it is clear that in so 

 doing it must have flowed up a very considerable ascent, which it could 

 only do in virtue of a constantly acting propulsive force. This is strikingly 

 shown in Diagram in., Plate IV., which represents the longitudinal course 

 of the bathymetrical Isotherms of 50°, 45°, 40°, along the axis of the 

 Gulf-stream in its passage through the Florida Channel. — The soundings 

 taken in the subsequent course of the Gulf-stream do not anywhere give 

 the depth at which temperatures below 45° exist beneath it ; but as that 

 temperature is reached in the Polar current at 200 fathoms, while the 

 thermometer sinks at 300 fathoms to 40°, and at 400 fathoms to 38°, it 

 seems clear that the water occupying the lower stratum of the Florida 

 Channel is derived from the same source. 



134. That the Polar current which passes into the Gulf of Mexico beneath 

 the outflowing Gulf-stream is due to the excess of density in the outside 

 column, depending on the depression in the temperature of its lower stratum, 

 was maintained in 1807 by Mr. Mitchell*, who had been engaged in the 



* Silliman's American Journal, vol. sliii. (18G7) p. 74. 



