544 



Captain Spratt on the Undei 'current 



[June 15, 



the lead at the bottom, as well as before it reached the bottom, so that no 

 incline of the line from a perfectly vertical course of descent could have 

 occurred for several hundred fathoms above the bottom ; all must have 

 been perfectly still water there, for the deviation of the line for a few 

 inches only out of the perpendicular in the lower depths would have 

 prevented the line from coiling itself around the upright lead, and so 

 from this perfect stillness the lead returned to the surface with the 

 excess of 200 fathoms coiled round it*. Now it is perfectly impos- 

 sible that the ship could have been kept stationary over the same spot 

 in the Atlantic, under the most favourable circumstances, even for a 

 few minutes, much less so during the time a sounding-line takes to de- 

 scend in about 2000 fathoms ; for the combined influence of swell and 

 of the smallest superficial current during this time would drift her 

 from it considerably, as must be evident to every one. Hence it will 

 be perceived that, unless perfectly still water existed in the lower depths, 

 no coiling together of the excess of line paid out around the lead could 

 occur; and as it occurred on several occasions, I have been led to in- 

 terpret the fact, as did Capt. Dayman, as being a most interesting and 

 satisfactory test, where it occurred, of the perfect stillness of the ocean 

 deeps there. 



It is shown by the few soundings that have been taken in the Atlantic, 

 that probably a continuous depth of at least 2000 fathoms extends along 

 it between the Arctic and Antarctic circles. 



The consideration of the above points, then, opens up the question of how 

 this low normal temperature of the Atlantic and Pacific deeps is retained 

 in continuity, with a higher terrestrial temperature below, as generally 

 supposed, and a higher atmospheric temperature above — whether it is 

 chiefly, if not entirely, due to the horizontal conduction of this low tempera- 

 ture from the Arctic and Antarctic zones and seas during the long ages 

 the present poles have been the sources of cold, combined also with the 

 great density resulting from this low normal state, and consequent ten- 

 dency of such cold and dense water to remain in the deeps (a view I 

 am more inclined to accept), or whether entirely due to a continuing 

 undercurrent movement between the poles and the equator, as others 

 suppose. 



I only touch upon the question here, and thus merely state, in re- 

 ference to the undercurrent theory, that there seems to be an opposing 

 difficulty in the first thought upon it, — first, because I conceive that the 

 horizontal conduction of extreme cold can evidently occur in a continuing 

 column of equable depth, such as exists in the ocean deeps, and when com- 

 pletely effected remain so, without requiring an appreciable undercurrent 

 to maintain it ; secondly, because the existence of such a current seems to 

 require one of two conditions — either a much less density of the substratum 

 of fluid in continuity before it, so as to cause a horizontal flow, or a pres- 

 * See Capt. Dayman's Report of deep-sea soundings across the Atlantic, pp. 7 & 8. 



