80 The Hydrography of the Faeroe- Shetland Channel 



waters are : (1) the Norwegian stream (b) is cut off below by the 

 Wvville- Thomson ridge at a depth of about 300 fathoms — north of 

 the ridge its waters are mixed with ice-cold water of slightly lower 

 salinity drawn up by the ' undertow ' to an amount depending partly 

 on the velocity of the stream, and increasing with it (see ' Twelfth 

 Report of the Fishery Board for Scotland,' p. 351) ; (2) the drift 

 current (a) and the European stream are independent of one another, 

 but where the former exists — i.e., as a northward-moving current — the 

 waters of (a) and (b) are likely to be indistinguishable by means of 

 either temperature or salinity observations ; (3) the southward-moving 

 waters (c) and (d) may be independent, and they may or may not be 

 fully mixed before entering the Faeroe- Shetland Channel ; (-4) when the 

 northward-moving currents are strong, they will tend to be surface 

 currents, because of the relatively high temperature of the waters. 

 When they are weak, their waters will be cooled by contact and 

 mixture with the cold underlying waters. The southward-moving 

 waters will tend to be under-currents because of their low temperature, 

 and will only rise to the surface when they are exceptionally strong 

 relatively to the northward-moving currents, or when they contain an 

 unusually large proportion of, on the one hand, warm Norwegian sea- 

 water, or, on the other, fresh Arctic water. 



Taking now the sections in order, the first is that for August, 

 1893, based on the Jackal observations. In the report on these 

 observations I expressed the opinion (p. 352) that the conditions were 

 there ' favourable to an increase of the Atlantic current,' but at the 

 same time it was noted (p. 337) that during the observations the 

 navigating lieutenant of the ship found ' a southerly drift amounting 

 to approximately 10 miles in twenty-four hours.' With the informa- 

 tion available at the time as to the sources from which the waters 

 were derived, it was impossible to identify clearly all the factors 

 involved, or to give a complete explanation of the movements going 

 on. I had to content myself with an attempt to discuss the mechanism 

 of the process of mixture of the northward and southward moving 

 waters, on the assumption that the former (a and b) were one, and the 



