236 Dr. W. B. Carpenter on Physical Investigations [June 15^ 



can possibly produce this result, is a point on which it is for Hydro- 

 graphers to decide. For myself, I cannot regard it as probable that a 

 spent stream of 50 fathoms thickness can give motion to a vast layer of 

 900 fathoms depth. 



On the other hand, the doctrine I advocate that a thick upper stratum 

 of the Korth Atlantic is slowly moving Polewards, to fill up the void left 

 by the gravitation-underflow of the coldest water towards the equator, and 

 that this stratum will also have an Easterly tendency in virtue of the excess 

 of easterly momentum which it brings with it from a lower latitude, seems 

 adequately to account for the facts now brought to light. The pro- 

 gressive closing in of the boundaries of this poleward upper flow will 

 obviously tend to deepen it, so as to give it a more persistent heating 

 power*. In the South Atlantic and Southern Indian Oceans, on the 

 other hand, the progressive opening-out of the ocean-boundaries, as we 

 pass Southwards from the Equator, will tend in the same measure to 

 reduce the thickness of the Poleward upper flow, thus diminishing the 

 persistence of its heating power. And in this, as it seems to me, we 

 have the true explanation of the marked difference between the chmate 

 of Kerguelen s Land (Lat. 50° S.), for example, or that of Heard Island 

 (Lat. 53° S.), and that of Ireland (lying between the parallels of 51 2° 

 and 54|° N. Lat.), the summer temperature of the former being but 

 little above the w^inter temperature of the latter. 



The 'Challenger' temperature-sections have most conclusively shown 

 that the entire warm upper stratum in the South Atlantic is very much 

 thinner than that of the ISTorth Atlantic ; and while I fully admit that a 

 part of this difference is due to the fact that a far larger portion of the 

 Equatorial current is deflected into the latter than into the former, I 

 cannot see that the Gulf-stream by any means accounts for the descent 

 of the isotherm of 40° in Lat. 56° to a depth of 900 fathoms. 



The ' Valorous ' temperature-soundings seem to me to be of peculiar 

 interest and value, in furnishing a satisfactory explanation of the com- 

 paratively high bottom-temperature of the Korth Atlantic. I have 

 always attributed this to the comparative narrowness of the channels of 

 communication between the Arctic and the Xorth- Atlantic basins, which 

 restrict the flow of the coldest Polar water from the former into the 

 latter ; and long before the ' Challenger ' Expedition sailed, I had ven- 

 tured the prediction that the South Atlantic, on account of the perfect 



^ This position may seem inconsistent with the objection just taken to the doc- 

 trine of Sir Wy villa Thomson. But the inconsistencj- is only apparent, I cannot 

 conceive that after the Florida Current has spread itself out like a fan over the Mid- 

 Atlantic, it can retain enough vis a tergo to give a N.E. movement to a mass of water 

 nearh^ 2000 miles wide and 700 or 800 fathoms deep, the impelling force being pro- 

 gressively weakened by the obstacles to that movement. On the other hand, the force 

 which (on the doctrine of a Thermal circulation) acts as a vis a fronte, grows stronger as 

 the water which it puts in motion approaches the Polar area, and thus is fvilly compe- 

 tent to deepen the poleward stratum iu proportion to the redaction of its breadth. 



