19 



APPENDIX A ; PETTERSSON 



The statements made above disclose the near connection of the disturbance in the 

 annual hydrographie periodicity with important chmatic and biologic occurrences. The 

 relation to each other of these phenomena merit a close investigation. 



The disturbance is here treated of as unperiodic. Its investigation is of too recent 

 date to allow of its regularity being proved. A glance at the representation on pag. 13 

 of the annual variation in the water- and air-temperature during the cold seasons of the 

 past 30 years will, however, satisfy us of the existence of such a regularity. The general 

 impression is that the maxima and minima of the water- and air-temperature during the 

 cold season reappear with great regularity every second year. This suggests a two-years 

 period for the disturbance in the hydrographie periodicity. Meteorologists have long 

 been acquainted with this phenomenon, which is known as "the even and odd years''^. 

 The "even" of the past 30 years have as a rule been characteristic of more temperate 

 winters than have the "odd". The diagrami shows that this rule applies to the oceanwater 



Fig. 13.' [Temperature 2 of the water March i£ 



Fig. 14. Mean monthly isobares for January 1898 



as well as to the atmosphere. Even the exceptions to the rule such as f inst. the period 



1886— 1887- 



\ appear in the hydrographie series If we take the proved regularity 



of the years 1874 — 84 and 1893 — 1899 to be the effect of some systematically operating 

 agent, we must regard the irregular period 1886— 1888 as due to the interference with the 

 two-years period of some stronger disturbing agent. The joint tendency of this and other, 

 biologic, phenomena seem to foreshadow the existence also of secular periodicity in the 

 oceanic circulation, the origin of which must be of cosmic nature. One phenomenon secuiar periods 

 of this class is the appearance and disappearance in the Skager Rak of the winter-herring. 

 Since 859, the earliest date recorded, this fishery has returned with intervals of on the 

 whole III years, though frequent irregularities in its appearance have occurred''. 



The elucidation of this problem belongs to the future. The all-engrossing task of 

 the present is the study of the annual periodicity, its disturbance and accompanying 



1 Woeikoff, Die Schneedecke in paaren und unpaaren Wintern. Metcorolog. Zeitschrift. Febr. 1895. 



2 N. B. The punctuated area means atlantic water. 



3 Known in météorologie littérature as "the cold spell". 



4 A. Ljungman, Nordisk Tidssl;rift for Fiskevi. 5. Aargang. Kobenhavn 1880. 



3' 



