36 Temperature of the Ocean. 



the number of degrees registered by the thermometer. Thus 

 these lines will be longer or shorter in proportion as the tempera- 

 ture increases or decreases. On joining the upper end of the 

 lines we obtain a curve of a more or less regular shape, gene- 

 rally assuming the form of the letter 3> which is called the 

 temperature curve of the station, and it exactly represents the 

 ^rate of increase or decrease of the temperature of the water in 

 each stratum from the surface to the bottom. 



To exhibit the conditions of temperature ascertained at a 

 number of stations belonofine to a certain section of an oceanic 

 basin, another class of curves may be constructed (Plates 6 to 20). 

 In this case the horizontal base-line forms a scale of miles, or of 

 degrees of latitude or longitude, giving the distance between one 

 station and another. The vertical scale indicates the depth in 

 fathoms, and the curves thus obtained form isotherms exhibiting 

 by their rise and fall the decrease or increase of temperature 

 in the different strata of the oceanic section under consideration. 

 They also show the rate of increase or decrease, spreading out 

 when the temperature decreases more slowly at one station than 

 at another, and closing up when the decrease is more rapid. 

 Instead of a scale of fathoms, the vertical scale may be divided 

 into degrees of temperature, and the curves resulting from this 

 arrangement will form iso-bathymetric lines, exhibiting the 

 variation of temperature for the same depth at different stations. 

 In the plates and tables prepared for this essay, the former 

 arrangement has been given the preference, since, by using a 

 scale ot depths, the diagram represents an actual section 

 of an oceanic basin, although on a scale necessarily much 

 exaggerated. 



Deductions from the Curve. — An examination of the shape 

 of the curve, and of the modifications which it undergoes in the 

 different parts of the ocean, leads to certain conclusions respecting 

 the conditions which determine the distribution of oceanic tem- 



