i 



growth of exentricity. In perigee the moon approaches the earth 

 and in apogee it withdraws from the earth and approaches the sun. 

 It is impossible on account of the complicated nature of the lunar 

 disturbances to get any approximate estimate of the influence of 

 the excentricity on the value of the maximum of the tidegenerating 

 force 500 — 600 years ago. 



In contrast to the maximum-years mentioned: 3 500, 1 900, 250 

 b. C and 1 433, 3 300 a. C. we get the years 2 800 and 1 200, b. C. 

 and 550 and 2 400 a. C. when the constellation perihelion-node-apside 

 is changed into its opposite: aphelion-node-apside. 



If we represent the variation in the tide-generating force graphi- 

 cally by a continuous curve it would appear like a succession of 

 big oceanic waves with intervals of some thousand years, their 



crests rippled by numerous lesser wave-systems each with their 

 proper crests and hollows. 



The amplitudes of all wavesystems increase as they approach years 

 of absolute maximum and decrease near the minimum years 1 260 b. 

 C. 530 a. C. a. s. o. The difference in springtide and eap-tide, as well 

 as of the sea-level in general at ebb and flood, in winter and in summer 

 must also be greatest at the time of maxima and smallest at the time 

 of minima. This variation in the tide-water at the surface need not 

 neccessarily be great to work destruction on alowlying coast-line guar- 

 ded by sand-dunes. Formed by the action of the weaves on the beach 

 in centuries of weak ebb and flood these sand-dunes may in succeeding 

 centuries of a growing tide become gradually undermined till at last 

 easily broken through by a slight increase in the destructive forces. 



savMCh, ite mr.Ck, i'ryte,m 



Fig. 2. 



If we find that the absolute maxima are followed by catastrophes 

 such as floods, outbursts of polar ice, and climatically by sharp con- 

 trasts of temperature, excessive drought and excessive precipitation, we 

 should expect the minimum-years to be noted for the reverse condi- 

 tions. In such an investigation the sea neccessarily comes first, for 

 it is far more susceptible to the influence of the tide-generating force 

 than is the atmosphere or the solid earth's crust. What we should 

 expect from the tidegenerating force at the time of absolute maxima 

 is this: 



1) In the surface: Greater tidal phenomena with bigger flood- 

 waves and a greater difference between spring-tide and neap-tide 

 capable of breaking through dikes and flooding lowlying coasts. 



2) In the border-layer (when a less heavy waterlayer super- 

 poses salter and heavier water, as is always the case in arctic seas 

 and in the Skagerak, Cattegat and Baltic): strong pulsations in 

 the undercurrent appearing as submarine waves, which enter into 

 fjords and inlets like the s. c. »moon-waves » we have observed in 

 the Belt, the Cattegat and the Gullmarfjord. 



This diagram, shows how submarine waves, 2 5 — 30 m. in height 

 enter the fjord in the border-layer between the »Baltic» water of the 

 surface and the salt deepwater which rises and falls causing the 

 surfacelayer to retract or expand in thickness. 



The instrument which records these movements of the deeplayer 

 consists of a float of sheet-copper of 500 1. capacity filled with water 



and so constructed as to float in the border-layer and partake in 

 its movements. 



The undulations of the border-layer are evidently of tidal nature 

 and have hitherto been overlooked. This is not the time to settle 

 whether they should be classed as »wave-movements » or »seiches». 

 In any case they are not local oscillations originating within the 

 fjord, for their period is 13 — 14 days (or 7 days) and follows the 

 lunar periods, while the longest independant oscillation of the fjord 

 is 1 hour 49 min. for the surface seiche and 2 — 3 days for the imi- 

 nodal seiche of the deepw^ater. In all probability these submarine 

 waves are formed by the impact of the oceanic tidal wave on the 



Fig. 6. 



submarine ridges in the North Atlantic, f. inst. the Faroe-Shetland 

 ridge or the North-sea bank in lat. 60°, rising out of the depth of 

 the Norwegian Sea. The phenomenon is akin to that reproduced in 

 Dr. Zeilon's experiment 1 . The advancing wave is stopped .and broken 

 by the wall in the experiment (the suboceanic ridge) and pours over 

 the lip of the wall in little cascades of water then to travel along 

 the bottom on the other side as a succession of submarine solitary- 

 waves*. According to Zeilon these submarine waves preserve their 

 original period of undulation, though otherwise modified as to length, 

 face and amplitude. If the original undulation contains several pe- 

 riods as in tidal phenomena some of these may be suppressed while 

 others develop and become dominant. The total of the energy in 

 the wave-motion will remain constant, of course, but it may be un- 

 equally divided among the induced wave-systems. This seems to 

 be the case in the Cattegat where the diurnal tidal waves are blotted 

 out by the longperiodic waves ( »the moon-waves »), the amplitude 

 of which is so great that they measure up to 30 m. between crest 

 and hollow in the Gullmarfjord and even more at the Skagen light 

 ship. 



When these giant-waves enter the fjord they appear as an influx 

 of water at 33 m. depth while there is an outflow of surface-water. 

 The strengh of both currents is registered in cm /sec. by two propeller- 

 wheels at Borno Station. The velocity of the undercurrent may on 

 occasions (as shown in the diagram fig. 7) amount to 8 — 17 cm. /sec. 

 When the submarine wave subsides the currents are reversed. 



The strongest currents observed were those of the 16th and 17th 

 Nov. 1910. 



The submarine wave, which had entered in the 3 previous 

 days then sank more than 25 m. in 24 hours. The astronomic 

 constellation which caused this was the total lunar eclipse the 

 night between the 16th and 17th Nov. or in other words the 

 combination of fullmoon with perigee and nodeapside of the lunar 

 orbit, which constellation for the past 4% years observations in the 

 Gullmarfjord has been found to bring strong ebb in the moon- 

 waves ». 



i N. Zeilon, on the Seiches of the Gullmarfjord Sv. Hydrogr. Biol. Kommis Skrifter. 



— 3 — 



