286 THE DEPTHS OF THE SEA. [chap. vii. 



perature in the North Atlantic we are indebted for 

 the singular mildness of our winter climate. The 

 chart PI. VII., the general result reduced from many- 

 hundreds of thousands of individual observations, 

 gives the distribution of the lines of equal mean 

 temperature for the surface of the North Atlantic 

 for the month of July ; and it will be seen that 

 the isotherms, instead of passing directly across the 

 ocean, form a series of loops widening and flatten- 

 ing northwards, all participating in certain secondary 

 deflections which give them a scalloped appearance, 

 but all of them primarily referred to some common 

 cause of the distribution of heat, having its origin 

 somewhere in the region of the Straits of Florida. 



These peculiarities in the distribution of tempera- 

 ture on the surface of the sea may usually be very 

 immediately traced to the movement of bodies of 

 water to and from regions where the water is exposed 

 to different climatal conditions ; — to warm or cold 

 ocean currents, which make themselves manifest like- 

 wise by their transporting power, their effect in 

 speeding or retarding vessels, or diverting them from 

 their courses, frequently, however, the current, 

 although possibly involving the movement of a vast 

 mass of water, and exerting a powerful influence 

 upon climate, is so slow as to be imperceptible ; its 

 steady onward progress being continually masked 

 by local or variable currents, or by the drift of the 

 prevailing winds. 



The Gulf-stream, the vast 'warm river' of the 

 North Atlantic, which produces the most remark- 

 able and valuable deviations of the isothermal lines 

 which we meet with in any part of the world, is in 



