1876.] 



071 hoard the ' Valorous' in August 1875. 



237 



freedom of its communication with the Antarctic, would have a colder 

 bottom, and that the influence of the Antarctic underflow would probably 

 extend to the north of the Equator. By Sir Wyville Thomson, on the 

 other hand, it was argued from the commencement that the w^hole cooling 

 of the deep stratum of the NoHli Atlantic is due to the Antarctic under- 

 flow ; and this conviction he repeats in his last utterance on the subject, 

 on the ground of the continuity of the isotherms from the South into the 

 North Atlantic*. The question arises, however, w^hy the deep stratum 

 of the North Pacific, which is undoubtedly fed from the Antarctic, should 

 be so decidedly colder, as the 'Challenger' and 'Tuscarora' soundings show 

 it to be, than the deep stratum of the North Atlantic ; and this question 

 appears to me to find an entirely satisfactory answer in the indication fur- 

 nished by the Second Section (Plate 4), that the Arctic Basin is for the most 

 part separated from that of the North Atlantic by an intervening ridge, 

 which (like many similar ridges discovered by the ' Challenger ') allows 

 water of about 36°, but not colder water ^ to pass from the former into the 

 latter. The limited contributions of colder water furnished by Baflln's 

 Bay and the " Lightning Channel " would help to reduce the deep tempe- 

 rature of the North Atlantic generally to the 35°-36° shown in the 

 ' Challenger ' Sections ; but it is only when, on approaching the Equator, 

 a bottom-temperature below this first shows itself, that I can recognize 

 the influence of the Antarctic underflow. 



I forbear, however, to discuss this subject more fully at present, the 

 Admiralty not having yet published the final instalment of the ' Chal- 

 lenger' temperature-sections. And I shall confine myself to an expres- 

 sion of my earnest hope that the ship to be sent next year to communicate 

 with the Arctic Expedition may have, as part of its work, the completion 

 of that which the ' Valorous ' was disabled from performing — namely, 

 the obtaining a continuous temperature-section between Iceland and 

 G-reenland, and another across Davis Strait. 



DESCRIPTION OF THE PLATES. 

 Plate 2. 



Chart showing the track of the ' Valorous ' outward and homeward. The latter is the 

 dotted line. The tints represent different depths. 



Plate 3. 



Soundings I.-IX. and isotherms between Disco and Davis Strait. 

 Plate 4. 



Soundings X.-XVII. and isotherms between Davis Strait and England. 

 * Proceedings of the Eoyal Society, vol. xsiv. p. 632. 



VOL. XXV. 



