1876.] 



on hoard the ' Valorous' in August 1875. 



235 



lentia (Plate 4. Xo. XYII.) on the basis of the serial soundings taken of£ the 

 coast of Ireland in the first cruise of the ' Porcupine ' in 1867, a sounding 

 (Xo. 22) in 1263 fathoms, Lat. 56° 8' X., Long. 13° 34' W., being taken 

 as the principal guide. This being almost on the same parallel with the 

 last serial sounding of the ' Valorous ' (the difference of latitude being 

 only half a degree), and the seasonal difference being rather in favour of 

 the ' Valorous ' temperatures, it is extremely striking to find in this 

 Section the most remarkable contrast yet brought out between the ther- 

 mal condition of the eastern and the western sides of the Xorth Atlantic : 

 for the descent of all the isotherms as they pass from west to east, 

 which has been already pointed out in the ' A^alorous ' portion of the 

 section, continues at an even more rapid rate ; so that the isotherm of 

 40°, which lay at Station XYI. at 380 fathoms, lies at 900 fathoms at 

 Station XYII., 15 degrees to the west ; whilst the isotherm of 45°, which 

 at the first of these stations lay at 80 fathoms from the surface, lay in 

 the second at 640 fathoms. This difference in the thickness of the whole 

 stratiun above the isotherm of 40° is much more remarkable than the 

 difference of surface-temperature, the increase of which between the first 

 and the second station was only from 55° to 59°-6. 



It is clear, therefore, that the heating power of the warm flow which 

 comes up from the S.V^. towards the western shores of the British Isles, 

 and which proceeds onwards to the X.E., so as to ameliorate the climate 

 of the Orkneys and Shetland Islands, but still more markedly to affect 

 that of the coast of Xorway (as has been shown by Prof. Mohn), depends 

 upon its great deiJth. Any such superheated film as the Grulf- stream has 

 been found to be ^\-hen last recognizable as a current (as was long siuce 

 urged by Mr. Fiudlay, and has since been confirmed by Capt. Chimmo's 

 observations) must lose its excess of warmth long before it reaches our 

 shores . Hence, as I have urged on a former occasion (Proc. Eoy. Soc. vol. xx. 

 pp.' 621, 637. §§ 137, 163), the prolonged heatiag power of the X^.E. flow 

 depends much more upon the thickness of its moderately warm stratum 

 than upon its bringing with it a high surface-temperature. A layer of 

 50 fathoms at 60°, flowing X.E. over a bed of ocean-water at 40°, and 

 exposed above to an atmosphere of 40°, would be cooled down to that 

 standard in two or three v>'eeks. But a layer of 900 fathoms thickness, 

 ranging from 40° to 55°, would retain an excess of temperature far 

 longer. 



The advocates of the doctrine that the vis a tergo is the Gidf -stream, 

 which cannot be traced as a current by any distinctive feature further to 

 the X.E. than the parallel of 40° and the meridian of 30°, have to show in 

 what Avay it can raise the temperature of so thick a stratum of ocean- 

 water as we have seen to be affected in the Y*^estern portion of the Xorth 

 Atlantic by a M'arm flow of some kind. Whether, as Prof. Wyville 

 Thomson maintains, the approximation of its boimdaries between the 

 British Islands on one side and Labrador and Greenland on the other 



