4S4 



John B. Tait 



(1905), Hessflberg and Sverdrup (1915), Sverdrup (1933), Jakelln (1936), Jacob- 

 sen (1943), and others, from the Bjerknes' Circulation Theorem, of volume transport 

 computation through two roughly parallel hydrographic sections laid approximately 

 at right angles to the course of the Atlantic Current through the Faroe-Shetland 

 Channel, scene of the pioneering oceanographical investigations of C. Wyville 

 Thomson in 1868 and 1869, which led up to the classic Challenger Expedition. 



There are special physical features of the Faroe-Shetland Channel region, as 

 illustrated in Fig. 1, and of its waters, which make this a peculiarly appropriate site 

 for an investigation of the kind indicated. As Mohn, the Norwegian meteorologist, 

 first pointed out in 1887, by far the greater bulk, if not almost the entire mass of 

 the oceanic water which pervades north-west European seas, flows within relatively 

 restricted lateral limits through this Channel. It possesses, moreover, in the Wyville 

 Thomson Ridge, a well-defined threshold from the North-Eastern Atlantic Ocean, 

 thus defining, at its mean summit depth of about 550 metres beneath the sea surface, 

 the thickness of the oceanic water-mass which, so far as is known, continuously 

 passes in an east to north-easterly direction over it. Initially, then, as a first approxi- 

 mation, the level of zero horizontal current — the basis of conversion of relative into 



Fig. 2 



absolute computational values— was assumed to lie at this depth of 550 metres over 

 the major part of a hydrographic section between the Butt of Lewis, (the northmost 

 point of the Outer Hebrides off" the west Scottish coast), and Faroe Bank, the section 

 thus passing obliquely across the Wyville Thomson Ridge and, as nearly as may be 

 judged, at right angles to the general course of the Atlantic Current over it. Between 

 those stations of the section which lie over the continental shelf and slope, zero 

 horizontal potential was assumed at the mean bottom depth between successive 

 pairs of stations, and the modification of the computational method introduced by 

 Helland-Hansen (1934) to suit such circumstances, applied. 



As regards the assumption of zero horizontal current along the summit of the Wy- 

 ville Thomson Ridge, this was experimentally confirmed in May 1953. In remarkably 

 calm conditions, and after repeated testing in each instance at higher levels where 

 considerable current of an order to be expected was in fact registered, an old-style 

 but technically efficient Ekman current meter was, by means of the large, tubular. 



