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 Pl. 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 
