220 LIGHT SCIENCE FOR LEISURE HOURS. 



Thirdly, its course having carried it into narrow chan- 

 nels, it has required a relatively rapid rate of outflow, 

 insomuch that the surface flow of the current on its 

 outward passage through the Narrows of Bernini, takes 

 place at the rate of from 2J to 4 miles per hour. Its 

 width here is at the surface not more than about 25 

 miles, its maximum depth rather more than a quarter 

 of a mile (about two-fifths of the channel's maximum 

 depth), and its mean rate of flow probably about 50 

 miles per day. 



I shall not follow the Edinburgh Reviewer in con- 

 sidering the details of the progress of the Grulf Stream 

 from the Narrows of Bernini to Cape Hatteras, because, 

 though in themselves of the utmost interest and impor- 

 tance, these details throw no special light on the general 

 subject of oceanic circulation. Suffice it that as far as 

 Hatteras the Grulf Stream remains distinctly recognis- 

 able, and that even off Sandy Hook (New York) its 

 surface temperature is little reduced, and its velocity 

 still amounts to about one mile per hour. Off Nan- 

 brought down by that river, the coarser having been deposited near its 

 (the river's) mouth ; just as the intense blueness of the waters of Lake 

 Geneva depends on its retention of the finest sedimentary particles 

 brought down by the Ehone in the upper part of its course, while that 

 of the waters of the Mediterranean is due to its pervasion by the like 

 particles brought down by the river Rhone and other rivers, which dis- 

 charge themselves into its western basin, and by the Nile into its eastern.' 

 It will be remembered that Prof. Tyndall, by researches carried on 

 during the return of the Urgent from the eclipse expedition of 1870, 

 was enabled to throw considerable light on the cause of the colour and 

 shades of colour in water of greater or less depth. See also Dr. Car- 

 penter's ' Eeport of Researches in the Mediterranean,' in the ' Proceed- 

 ings of the Royal Society,' vol. xix. p. 200. 



