1872.] 



'Shearwater* Scientific Researches. 



617 



line of Keys, and 30 or 40 miles to the southward of them. That trie in- 

 ward counter-current, of which this fact affords evidence that may almost 

 be called demonstrative, occupies not only that shallower portion of the 

 channel which follows the coast-line of Florida, but the lower portion of 

 the deepest part of each section, where it underlies the axis of the out- 

 flowing Gulf-stream, will be presently shown to be an equally certain in- 

 ference from the Temperatures met with at different depths in each Section, 

 from the surface downwards (§§ 129-133). 



129. It is on the line of Section taken by the United States Coast Survey 

 in 1866, between the Dry Tortugas and Havana (see Diagrams iv., vi., 

 Plate V.), that our detailed knowledge of the Gulf-stream commences. 

 The breadth of this channel is about 82 miles ; and its maximum depth, 

 which is met with at about 3/ miles from the Cuban shore, is 845 fathoms. 

 On the northern side, the bottom lies in terraces, which are nowhere 

 abrupt ; but on the southern side, about halfway between the deepest part 

 and the Cuban shore, there is a steep ridge rising about 400 fathoms from 

 the sea-bed, so as to come to within about 350 fathoms from the surface. 

 The course of the bathymetrical Isotherms in the different parts of this 

 Section is very remarkable. Instead of lying parallel to the surface, they 

 follow the contour of the bottom ; the colder water being found much 

 nearer to the surface in the shallower than in the deeper portion of the 

 channel. The following is the bathymetrical range of these lines at in- 

 tervals of 5° : — 



Temperature. Range of Depth. 



75° 20 to 130 fathoms. 



70° 60 to 180 „ 



65° 75 to 250 „ 



60° 90 to 320 „ 



55° 150 to 380 „ 



50° 200 to 440 „ 



45° 350 to 500 „ 



40° 480 to 550 „ 



35° 580 to 600 „ 



Thus it appears that the lower half of this channel is occupied by water 

 whose temperature is below 50° ; and that this colder water is thrown up 

 in the shallower parts of it to within 200 fathoms of the surface. Looking 

 to the steady diminution of temperature with depth, until a temperature of 

 35° is reached at about 600 fathoms, it can scarcely, I think, be questioned 

 that the whole of the water below the bathymetrical Isotherm of 50° is an 

 inward prolongation of the Polar Stream ; the o?^flowing Gulf-stream 

 being limited to the stratum of 60° or upwards. This limitation was in- 

 dicated also by the deflection of the sounding-line occasioned by the cur- 

 rent ; which led the U.S. Surveyors to the conclusion that the outwardly 

 moving stratum has not above one third of the actual depth of the channel. 



2 y 2 



