1872.] 



f Shearwater 9 Scientific Researches. 



015 



exit, during the months of November, December, and January, is about 

 76°, there is a loss of heat of about four degrees in the passage of the 

 water that feeds it from the north coast of South America to the south 

 coast of North America. 



12/. Thermal condition of the Gulf-stream, and its influence on that 

 of the North Atlantic. — In considering this part of the subject, we have 

 first to inquire whether the Thermal condition of the Caribbean Sea and of 

 the Gulf of Mexico, which last constitutes the head-water of the Gulf- 

 stream, differs in any essential condition from that of the outside Atlantic : 

 and on this point we derive important information from a series of Ther- 

 mometric observations recently taken in the New York School-ship 

 * Mercury,' which show that a very close correspondence in surface-tem- 

 perature may veil a considerable difference between the temperatures of 

 the underlying stratum. — The 'Mercury' crossed the Atlantic in the 

 month of March from Sierra Leone to Barbadoes, running before a nearly 

 steady E.N.E. Trade-wind, with an Atmospheric temperature ranging 

 from 77° to 83°, the surface-temperature of the Sea ranging from 75° to 

 80°. At a depth of 100 fathoms, the temperature was 62°; while at 

 200 fathoms it was found to be everywhere (save in comparatively shallow 

 water, § 9) between 5 1° and 53°, the mean of eight observations being exactly 

 52°. Having proceeded from Barbadoes to St. Thomas, the ' Mercury ' 

 sailed, during the first fortnight of April, along the southern side of Porto 

 Rico, St. Domingo, and Cuba, finding deep water everywhere ; and then, 

 rounding the western extremity of Cuba, she made her way northwards 

 through the Florida Channel. Now in this part of the course, the tem- 

 perature of the Air ranged from 80° to 84°, and the surface-temperature 

 of the Sea from 83° to 86°, showing a considerable elevation, which is 

 probably attributable to the influence of the Equatorial current, this being 

 at its hottest in March and April. But a still more marked excess shows 

 itself in the temperature of the stratum beneath; for at 100 fathoms the 

 temperature was found to be 72°, and at 200 fathoms 62° (in both cases 

 ten degrees above the temperatures at corresponding depths in the outside 

 Atlantic), at 300 fathoms 54°, at 400 fathoms 50°, and at 500 fathoms 48°. 

 — This, as it seems to me, is an indication that the partial separation of the 

 Caribbean Sea from the Basin of the Atlantic produces somewhat the same 

 effect upon the temperature of its deeper water, that is shown in a much 

 more remarkable degree in the Mediterranean (§ 10) ; for if the bottom- 

 inflow of Polar water be impeded (though not altogether prevented), the 

 temperature of the whole mass of the water that is not directly subjected 

 to solar influence will of course be proportionally higher *. 



* It is remarkable, however, that on the Mosquito Coast, — where a very high sur- 

 face-temperature usually prevails, — the sub-surface temperature seems to correspond 

 more closely to that of the outside Atlantic ; the following observations being given by 

 Capt, Maury from the journals of Mr. Dunsterville : — at 240 fathoms, 48°; at 38(5 

 fathoms, 43° ; at 450 fathoms, 42°. This difference is perhaps attributable to the much 



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