138 LIGHT SCIENCE FOR LEISURE HOURS. 



monly supposed that a bed of ordinary sea -water 

 separates the two main currents from each other. 

 Thus the characteristic difference of temperature re- 

 mains unaffected. But in reality we may assume that 

 the cooling effect actually exercised by the arctic cur- 

 rent upon the neighboring sea is altogether dispropor- 

 tionate to the immense amount of heat continually 

 being carried northward by the Gulf Stream. It is 

 astonishing how unreadily two sea-currents exchange 

 their temperatures to use a somewhat inexact mode 

 of expression. The very fact that the littoral current 

 of the United States is so cold a fact thoroughly 

 established shows how little warmth this current has 

 drawn from the neighboring seas. Another fact, men- 

 tioned by Captain Maury, bears in a very interesting 

 manner upon this peculiarity. He says : " If any 

 vessel will take up her position a little to the north- 

 ward of Bermuda, and steering thence for the capes 

 of Virginia, will try the water-thermometer all the 

 way at short intervals, she will find its reading to be 

 now higher, now lower ; and the observer will dis- 

 cover that he has been crossing streak after streak of 

 warm and cool water in regular alternations." Each 

 portion maintains its own temperature even in the 

 case of such warm streaks as these, all belonging to 

 one current. 



Similar considerations dispose of the arguments 

 which have been founded on the temperature of the 



