February 24, 19 10] 



Na ture 



THE HYDROGRAPHY OF THE NORTH SEA 



AND ADJACENT WATERS. 

 "T^IIE fourth report of the North Sea Fisheries Investiga- 

 -'• tion Committee contains a number of papers on hydro- 

 graphical researches in the northern part of the North Sea 

 and the Fseroe-Shetland channel which are of special 

 interest, inasmuch as they provide a resume of the work 

 done since the committee began its labours, and a statement 

 of certain results and conclusions which may now be 

 accepted as definitely established and used as standards for 

 comparison with future observations. These papers are : — 

 (i) and (6) on hydrographical investigations in the North 

 Sea and Fieroe-Shetland channel during the years 1906-7-8, 

 by Dr. A. J. Robertson ; (2) on the temperature of the 

 surface waters of the North Sea during the years 1906 

 and 1907, by Mr. Frank G. Young ; Co) on the salinity of 

 the North Sea, and (4) on surface-temperature observations 

 between Hull and Hamburg during the years 1877-83, by 

 Prof. D'Arcy Thompson ; and (5) on the deep currents of 

 the North Sea as ascertained by experiments with drift 

 bottles, by Captain C. H. Brown. Dr. Robertson dis- 

 cusses the observations made during the periodic cruises 

 executed by the SS. Goldseeher on lines laid down by 

 the International Council. Mr. Young subjects to 

 harmonic analysis temperature observations made i)y 

 captains of passenger steamers and officers in charge of 

 certain lightships and lighthouses. Prof. D'.^rcy Thompson 

 reviews in his first paper a long record of surface-tempera- 

 ture observations made between Hull and Hamburg by 

 Captain W. Barron!* and examines the relation of the sea 

 temperature in the southern part of the North Sea to the 

 air temperature of the adjacent coasts, and in his second 

 paper gives an account of the mean values of salinity in 

 the waters of the North Sea — the general distribution of 

 salinity, its mean periodic variation, and the epochs of 

 .naximal and minimal salinity. Captain Brown reports 

 upon experiments with the drift bottle devised by Mr. 

 G. P. Bidder, which is so constructed as to float a few 

 inches above the sea bottom, being carried along by the 

 bottom current, and in the course of time scooped up by 

 a trawl-net or found stranded on a beach. 



In the summary which concludes his report on the 

 observations of 1906, Dr. Robertson makes use of the 

 results obtained by the other investigators, and lays dovifn 

 certain general rules. Tidal action is sufficiently active in 

 the southern part of the North Sea to effect a thorough 

 mixing of w.aters from surface to bottom ; hence over this 

 area, the northern boundary of which, by the way, seems 

 somewhat uncertain in position, surface observations alone 

 will henceforth be deemed sufficient. In the northern 

 section the conditions are altogether different, and no 

 uniformltv exists in the surface to bottom distribution. 

 Over the North Sea area the temperature decreases from 

 the shore to the open sea in summer and increases in 

 winter. In summer the warmest water (15° to 18° C.) 

 occurs along the Belgian and Dutch coasts, and the coldest 

 in the deep channel off Norway, while in winter the coldest 

 water is, as a rule, along the Danish coast (2° to 3° C), 

 and the warmest between Scotland and Shetland (7° C). 

 The greatest annual variation at the surface occurs 

 along the Belgian, Dutch, and German coasts, where it 

 amounts to 13°, while between Scotland and Shetland it 

 is some 9° less. In the deeper layers over the northern 

 area of the North Sea the value is only 1°, while the 

 smallest variation of all takes place in the deepest parts 

 of the Skagerak, where it amounts to only 02°. 



Mixing bv tidal currents is so strong that water of less 

 salinity than 33 per mille is rarely found more than a 

 few miles from shore ; over the North Sea area the varia- 

 tions in salinity are greatest at the surface, and the greatest 

 mean deviation from the average occurs where salinity is 

 lowest. In the northern area the variation seldom exceeds 

 0-2 per mille. The changes in salinity are thus too small 

 to have anv direct effect upon the occurrence or wander- 

 ings of food-fishes; they are mainly of interest as a guide 

 in studying the movements of the waters. 



Much information has been acquired with regard to the 

 general circulation of the waters within the area, and the 

 extent to which this undergoes changes of periodic and 

 irregular kinds. Large volumes of Atlantic water are 

 normally streaming northward as a surface current through 

 NO. 2104, VOL. 82] 



the Fsroe-Shetland channel into the Norwegian Sea, com- 

 paratively little entering the Norwegian Sea between 

 Fajroe and Iceland, where the east Iceland current comes 

 southwards. Under exceptional conditions Polar water 

 extends far enough south to enter the regions of the 

 channel. (Dr. Robertson cites 1908 as one in which this 

 occurrence was very well marked, and it was observed in 

 1902, as appears from Dr. VVolfenden's observations YGeo- 

 graphical Journal, April, 1903] and Dr. Robertson's report 

 to the North Sea Committee, 1902-3 [p. 11]. The con- 

 ditions observed by H.M.S. Jackal in 1893 were also 

 probably somewhat similar.) The deeper layer north of 

 the WyviUe Thomson ridge is normally flooded with cold 

 water of salinity 349 per mille, which is in direct con- 

 nection with the bottom area of the Norwegian Sea, but 

 in the southern parts of the channel at least these bottom 

 layers are occasionally displaced by warmer and Salter 

 water, showing that marked changes may occur even at 

 the greatest depths. C'l'his also appears, from Dr. 

 Wolfenden's observations, in the summer of 1900.) 



Between the Fasroes and Fair Isle the centre of the 

 Atlantic stream is situated between 3° and 5° west longi- 

 tude, where the mean annual temperature is 9-5° C. and 

 salinity 3529 per mille. Within the regions of the channel 

 its direction of flow varies from north-east to east, and 

 the speed of the surface waters apparently averages about 

 fourteen miles in twentv-four hours. Branches are thrown 

 off which enter the No'rth Sea round the north and south 

 of Shetland, and of these the latter is certainly subject 

 to seasonal variation. A scanty winter salt-water distri- 

 bution is normally followed by more vigorous inflo\V during 

 earlv spring, increasing to a maximum in the beginning' 

 of summer, and gradually decreasing again on the approach 

 of the following winter. As exceptional seasons. Dr. 

 Robertson quotes (i) the winter 1905-6, when an unusually 

 powerful Atlantic inflow took place; (2) the summer of 

 1907, when the maximum inflow was unduly delayed ; and 

 {3) the whole of 1908, when the inflow was very scanty. 



The greater proportion of the Atlantic water entering 

 the northern part of the North Sea area bends eastward 

 before reaching the 57th parallel of latitude, and after 

 throwing off a branch which enters the Skagerak as an 

 undercurrent is carried back northwards. This rotational 

 movement, due to the configuration of the bottom, gives 

 rise to a cold, deep-water area, an area with a great 

 temperature phase delay over which the maximum value 

 in the bottom layers is not reached until near the close 

 of the year. (This laver appears in the Jackal observa- 

 tions, 1803.) A fresh-water current continually streams 

 northward along the Norwegian coast, being exclusively 

 confined to the in-shore regions during the winter months, 

 but extending in summer far out to sea as a thin surface 

 layer ; similar movements occur in summer off the Scottish 

 coast. 



From the above summary of Dr. Robertson's conclusions 

 it appears that the normal distribution and circulation of 

 the waters in the North Sea area may now be regarded" 

 as definitely known. The departures from the normal are, 

 as was siipposed, very considerable, but it would seem 

 that the years 1905-6 and 1908 may be taken as repre- 

 senting the nature of the extreme variations which are 

 likelv'to occur, and it is noteworthy that the older observa- 

 tions of H.M.S. Jackal and other vessels indicate conditions 

 which, while showing some abnormal features for the years 

 to which they refer, agree satisfactorily with the more 

 recent and niore adequate work as regards the type of 

 distribution and general movement, and fall within the 

 limits of departure from that type which thev have them- 

 selves recorded. H. N. Dickson. 



THE THOMSEN MEMORIAL LECTURE.' 

 A MONG the Danes whose names are inscribed as men 

 ■^^ of science on the eternal bead-roll of fame, that of 

 Julius Thomsen stands pre-eminent — linked indeed with that 

 of Oersted. It is significant of the position which Thomsen 

 acquired in physical science, and of the respect which th.at 

 position secured for him in the eyes of his countrymen, 

 that his statue should have been erected during his lifetime 



I Delivered before the Chemical Socieey on February 17, by Sir Edward 

 Thorpe, C.B., F.R.S., past-president of the Society. 



