470 



Messrs. Carpenter, Jeffreys, and Thomson [Nov. 18, 



the same scale as the two preceding figures of the same kind (pp. 457, 458) 

 enables the relation between Depth and Temperature in the Atlantic 

 Basin to be compared with the like relation in the Warm and Cold Areas 

 respectively. For the sake of convenience, the Surface-temperature is here 

 taken at 54°, this (as shown in Table III.) having been its average at those 

 Stations in which the "superheating" did not conspicuously manifest itself. 



116. When the rates of decrease of Temperature in successive strata 

 of this deep Atlantic Basin are compared with those which have been shown 

 to exist in the thinner strata of our comparatively shallow Cold area, a very 

 remarkable relation presents itself, the Thermometric changes requiring in 

 the former case a much greater Bathymetric descent than in the latter, but 

 corresponding very closely with them when this allowance is made. This 

 relation may be presented to the mind by ideally extending Diagram II. in 

 a vertical direction, so that its horizontal lines should be separated by four 

 times their interval. It has been shown (§ 98) that in the latter the 

 stratum of about 100 fathoms which lies below the superficial 50 shows 

 but a very slight decrease of temperature, presenting almost exactly the 

 same rate of descent as the stratum between similar depths in the 

 neighbouring Warm area. Now with this 100 fathoms' stratum, a 

 stratum of about 500 fathoms beneath the superficial 100 in the deep 

 Atlantic very closely corresponds, the reduction down to 500 fathoms 

 being at an extremely slow rate. Between 150 and 300 fathoms in the 

 Cold area, however, the rate of reduction becomes very much greater ; 

 and this is just what presents itself in the Atlantic Basin between 500 and 

 1 000 fathoms ; so that as in the Cold area we come down at very little 

 below 300 fathoms upon a stratum of ice-cold water, so in the Atlantic 

 basin we come down at 1000 fathoms upon a stratum averaging 38°*6. 

 And further, as there is below this a slow progressive diminution of about 

 2° as we descend through the lower 300 fathoms of the Cold area, so a 

 like progressive diminution is shown as we descend through the lower 1000 

 fathoms of the deep Atlantic Basin. 



117. The significance of these facts becomes yet more apparent, when the 

 varying rates of diminution of temperature in successive strata of the deep 

 Atlantic Basin are reduced to a curve (Diagram VI.), in the same manner as 

 the corresponding rates in successive strata of the Cold area ; but with a re- 

 duction in the scale of depths in the former case, so as to make 500 fathoms 

 in the deep basin correspond with 150 in the comparatively shallow channel. 

 It is true that there is by no means the same absolute reduction in the 

 one case as in the other ; but this difference is just what would be antici- 

 pated on the hypothesis we have been advocating. For if it be supposed 

 that the body of ice-cold water brought down from the Arctic basin by the 

 various Polar currents is discharged into the wide and deep Atlantic Basin, 

 it will tend to diffuse itself over its bottom, partly displacing and partly 

 mingling with the water which previously occupied it, so as to form a 

 stratum of considerable thickness, which, while much colder than the 



