450 SOME RESULTS OF THE INTERNATIONAL FISHERY INVESTIGATIONS. 



varying periodicity of the Atlantic flood and ebb. We have seen that 

 the Gulf Stream circulation is itself periodic in that its intensity is 

 greatest in November and least in March. But that the period of greatest 

 intensity varies slightly from year to year is now tolerably certain, 

 though this problem has by no means received the attention it deserves. 

 So also with the appearance of the Atlantic stream in the ultimate seas 

 of Northern Europe ; on the whole an annual periodicity has been 

 observed. Year after year the Atlantic Hooding occurs at much the 

 same time : the temperature of the water rises, and the salinity 

 increases in such a manner as to eliminate the possibility that these 

 changes are due to local climatic influences, and to render it certain 

 now that they are due to a great oceanic water circulation affecting at 

 nearly the same times areas far apart from each other. But there are 

 perturbations. 



The study of these perturbations belongs to the future, but already 

 there are evidences of regular disturbances in the periodic ebb and flow 

 of the Atlantic current. In thirty-nine years' records of the temperature 

 of the atmosphere in the central part of Sweden, and that of the sea off" 

 the coast of Norway during the cold seasons, a two-yearly period is 

 clearly apparent. Both in the air and in the sea maximum and 

 minimum temperatures occur with great regularity every two years. 

 This is the phenomenon known to meteorologists as that of the "odd 

 and even years." As a rule, the " even " years of the last thirty have 

 had more temperate winters than the " odd " ones. That this observa- 

 tion applies equally to the temperature of the sea indicates that the 

 cause of the biennial periodicity of the air temperature is a hydro- 

 gra})hic one. In addition to this smaller perturbation we can obtain 

 elusive glimpses of other larger disturbances — secular variations due 

 probably to cosmical causes — in the regularity of the yearly flow of the 

 European stream. Biological phenomena afford indications of these 

 larger irregularities. Since the year 859 the a])pearance of winter 

 herrings in the Skagerak has been recorded, and it is observed* that 

 the fishery has returned with intervals of, on the whole, 111 years. 



Both climatic changes and changes in the abundance of the fisheries 

 are thus connected with hydrographic phenomena. One of the most 

 valuable means of research to the meteorologists of the future will be 

 hydrographic investigation, and for the rational study of the fisheries 

 this line of research will prove no less useful. Already it is beyond 

 doubt that hydrographical and biological phenomena are closely related, 

 and the work of the next few years is likely to furnish further instances 

 of this connexion. 



* Pettersson, Eappts. et Procis-verb. , vol. iii., 190fi, pp. 13-19. 



