1872.] 



6 Shearwater ' Scientific Researches. 



623 



movement in the sub-surface stratum ; and all our knowledge of double cur- 

 rents would lead to the belief that it gradually diminishes from above down- 

 wards, until the water becomes motionless, or nearly so, as we approach the 

 plane of reversal. Hence, taking the medium between two miles and zero, 

 the average rate of the whole outflowing stream would be no more than 

 one mile per hour. 



141. If we assume the limit of the stratum above 60° as that of the 

 real Gulf-stream current, we shall find its average temperature to be 

 somewhat higher than it has been stated by Mr. Croll, who seems to 

 have taken 65° as the average of the water flowing through the entire 

 channel. The average swr/ace-temperature of the Florida Channel for 

 the whole year is 80° ; and we may fairly set the average of the entire 

 outgoing Stream, down to the plane of 60°, at 70°, instead of 65° as esti- 

 mated by Mr. Croll. 



142. The prevalent doctrine, however, of the persistence of this Tempe- 

 rature, with but very trifling reduction, nearly as far as the Banks of New- 

 foundland, is based on observations made during the Summer ; when the 

 Isotherm of 70° extends north of the parallel of 40°, and the cooling 

 influence of the atmosphere is consequently at its least. In Winter, on 

 the other hand, when the Isotherm of 60° follows nearly the same line, the 

 surface-temperature of the Gulf-stream is reduced almost to that degree 

 before it reaches the Banks ; as is shown in the following Table, derived 

 from the Admiralty Chart : — 





Winter. 



Spring. 



Summer. 



Autumn. 



Mean of 

 Year. 





o 



o 



o 



o 





73 



77 



83 



80 



78% 



Florida Channel 



77 



78 



83 



82 



80 



Off Charleston 



75 



77 



82 



81 



78| 





72 



73 



80 



76 



7H 



S.E. of Nantucket Shoals . . 



67 



68 



80 



72 



71| 





62 



67 



78 



69 



69 



Thus it appears that instead of a loss of only 5° in the northward flow 

 of the Gulf-stream from Lat. 25° to Lat. 35°, the average loss for the 

 whole year is 11°. And the cooling influence of the superincumbent Air 

 on the Gulf- stream, even at its deepest and strongest, is manfested in its 

 loss of 13° of surface-temperature in the Autumn, and of 15° in the Winter, 

 although its passage thus far is accomplished in from forty to fifty days. — 

 During this time, it must be remembered, it continues to lose heat by 

 evaporation as well as by radiation ; the large amount of vapour which is 

 being continually given off being made manifest by its precipitation in the 

 form of fog when the Gulf-stream encounters the Arctic current which 

 meets it before it reaches the Banks of Newfoundland. 



143. It is, again, on the contrast in Temperature between the Gulf- 



