Long-term trends and changes in the hydrography of the Faroe-Shetland Channel region 485 



metal tripod illustrated in Fig. 2, deposited within one metre of the sea bottom, 

 independently of the ship from which it was lowered, at no fewer than three well- 

 spaced positions along the summit of the Ridge. The meter was activated and re- 

 locked at precise instants by the operation of a moveable foot-plate, P, which, through 

 its connecting rod, R, and an attached cord, released and withdrew the operative 

 messenger M immediately on contact with, and on withdrawal from, the sea-bed. 

 In all three instances zero current was registered during an interval of thirty minutes. 



In view of the uncertainties of the normal dynamic computational method, however, 

 the need was recognised from the outset of some form of corroboration of results 

 obtained by its means. This was sought in similar dynamic computations of the 

 volume transport of oceanic water through an approximately parallel and, within 

 a matter of not more than two weeks, contemporary, hydrographic section across 

 the debouchment of the Faroe-Shetland Channel proper into the Norwegian Sea, 

 that is, between the Faroe and the Shetland Islands. As illustrated in Fig. 1, this 

 section to begin with was situated in a more southerly position than that in which, 

 from 1934 onwards, it became established. 



For computation of the absolute oceanic volume transport through this more 

 northerly section, in the absence of such a convenient feature as a submarine ridge 

 as in the case of the southern section, the initial difficulty of determining the level of 

 zero horizontal current was overcome in the Hght of the following reasoning. 



Throughout the length of the Faroe-Shetland Channel to the Wyville Thomson 

 Ridge, the situation has long been known, of an upper relatively warm and salt 

 water-mass, the oceanic water-mass, over-riding a substantially colder and fresher 

 water-mass which emanates from the deep waters of the Norwegian Sea ; and also 

 the fact that these two different water-masses move in contrary directions along the 

 Channel. The more so, therefore, as cross-sectional representations of temperature 

 and sahnity distributions almost invariably reveal a well-marked boundary between 

 these two water-masses, it is legitimate and reasonable to assume the existence of an 

 interface of zero or neghgible horizontal motion between them. The same diagrams 

 in considerable number, by the obvious and increasing concentration of both iso- 

 therms and isohalines towards the isohahne of 35-00 °/oo, which of course was zero. 

 Pettersson's definition of the lower limit of demarcation of oceanic water, indicate 

 that this isohaline probably represents in cross-section also the said interface in the 

 Faroe-Shetland Channel of neghgible or zero horizontal current. In point of fact, 

 after trials with the isohalines of 35-05 °/oo and 34-95 °/oo as zero reference bases, and 

 subject to one other consideration which is apphcable to both sections but not 

 altogether relevant to the present issue, but which will be mentioned in another 

 connection below, the closest agreements between the resultant oceanic volume trans- 

 ports through the two sections above cited when traversed within short intervals of 

 each other, were obtained on the basis of zero horizontal potential along the 35-00 "/oo 

 isohaline on the northern section. 



The fact of these agreements, most of which are very close as will be seen from 

 Table I, in no fewer than twenty-one instances between the years 1927 and 1952, 

 that is, in all cases save one in which during this period the two sections were accom- 

 pUshed more or less together, reflects favourably on the accuracy of the above assump- 

 tion. Its experimental verification by current meter with the means then at disposal- 

 it may the more readily be accomplished now since the ingenious inventions of 



