618 



Dr. W. B. Carpenter on the 



[Jane 13, 



In the northern half of this section, above the terraces south of the Florida 

 Reefs, the water lies almost motionless ; and the eastward flow of the Gulf- 

 stream is limited to the deeper southern half of the channel. 



130. Proceeding about 120 miles to the eastward, we find that while 

 the channel (between the Sombrero Lighthouse and the Salt Key Bank) 

 is narrowed to 45 miles (see Diagram iv.), and its maximum depth is re- 

 duced to 600 fathoms, the evidence of a division between two strata is still 

 more distinctly marked by the course of the bathymetrical Isotherms (Dia- 

 gram vii.). The northern slope, as in the previous instance, is the more 

 gradual, the depth being only about 120 fathoms at a distance of 13 miles 

 from the American shore ; and here we find that while the temperature at 50 

 fathoms is 75°, so rapid a reduction shows itself with increase of depth, that 

 the bottom at 1 20 fathoms is below 50°. In mid-channel, on the other hand, 

 where the depth is the greatest, the line of 50° sinks to 175 fathoms, while 

 towards the Salt Key Bank it is found at 200 fathoms ; and the whole 

 mass of water that occupies the deeper portion of the channel has a tem- 

 perature below 50°, the thermometer showing 45° at 250 fathoms, 40° at 

 350, and 35° at less than 400. — Thus it seems clear that the colder water 

 which underlies the warm Gulf-stream surges up on the shallower bottom 

 of the northern side of it, so as to rise to within 120 fathoms of the sur- 

 face, though overlain by a thin stratum of water having nearly the tempe- 

 rature of the Gulf- stream proper. 



131. After passing Sombrero Lighthouse, the channel of the Gulf-stream 

 begins to turn northwards (Diagram iv.) : and the next line of section, 

 taken between Carysfort Lighthouse and the Great Bahama Bank (Dia- 

 gram viii.), shows it to have somewhat widened and at the same time 

 become shallower. The total breadth of the channel is here about 60 

 miles; but of this, a band of about 15 miles broad on the Florida side 

 appears to be occupied by the Polar Stream. The maximum depth in mid- 

 channel is about 500 fathoms ; and the portion of it that exceeds 400 

 fathoms in depth is about 35 miles broad. The general character of this 

 section very closely approximates to that of the Havana section, except 

 that the slope of its banks is more equal. It is only on the American 

 side, however, that the Isotherms down to 50° run upwards, showing the 

 approach of the colder water to the surface. Thus the temperature of 55° 

 is there found at 125 fathoms, whilst on the other side it is not reached at 

 250. At 350 fathoms in mid-channel, the temperature is below 50°, and 

 in 60 fathoms more it is reduced to 40°. Although the temperature 

 does not seem to have been observed at depths much greater than 400 

 fathoms, it can scarcely be doubted that it would be found as low as 35° 

 in the deepest part of this channel, as it is at a less depth in the Sombrero 

 section. 



132. The channel again contracts, and at the same time diminishes in 

 depth, as far as Cape Florida (Diagram iv.), between which and the Bernini 

 Isles its sectional area is smallest : for, while its whole breadth is not much 



