86 FRIDTJOF NANSEN. M.-N. Kl. 



Ice- Pressures and the Tidal Wave. 



According to our observations the ice was comparatively tight at the 

 minima of movement, when the water was at rest, /. e. at high-water, while 

 the ice \vas comparatively slack, with much open water between the floes, 

 when there was a maximum of movement in the water, /. e. at low-water. 



This is in accordance with what might be expected to be the case 

 owing to the horizontal movements of the surface- layers, caused by the 

 tidal wave during its progress across the sea. Let us try to examine what 

 the horizontal distribution of the tidal movements over a sea-surface may 

 be at a certain moment, if we do not take the movements due to other 

 causes (prevailing sea current, wind, etc.) into consideration. This distri- 

 bution of the movements might then be something similar to what is indi- 

 cated in Fig. 69. 



Maximum of movement, going in the same direction as the tidal wave, 

 will be found where there is high-water at any given moment, /. e. along 

 the crest of the tidal wave (Fig. a - > ^ 



69, b); while maximum of move- \ ^ /i\ ^ > ^ /t\ ^ \ 

 ment going in the opposite direc- \ ^ J \\ ^J^s 1 /r\ *x< 

 tion will be found where there is \ / 1 \ ^* _ > /* / t \ "\< _ 



low-water at the same moment, i. e. ' a_ __ ^ '* a 



~~~ ~~ 



along the trough of the tidal wave 



/T- * Fig - 6g - 



(Fig. 69, aa). 



In the regions between the crest and the trough of the wave, the 

 velocities will be decreasing towards a minimum midway between them, or 

 midway between high-water and low-water. Owing to the effect of the 

 Earth's rotation (or under certain circumstances also owing to other causes) 

 the directions of the movements might change as indicated in Fig. 69. 

 As our figure clearly demonstrates, the movements will be converging in 

 the region between the trough and the crest on the front side of the advan- 

 cing wave, but they will be dispersing between the crest and the trough 

 on the back side of the wave. The converging movements will naturally 

 tighten the ice, and may produce ice-pressures, that will reach their maxi- 

 mum near high-water (or near the crest of the wave). The dispersing 

 movements will produce a slackening of the ice, that begins as soon as the 

 crest of the wave has passed, and will reach its maximum near the trough 

 of the wave (near low-water). As, however, the ice-masses are big and 

 heavy, there may be a lag in their movements; for instance, ice-pressures 

 caused by the tidal currents might also occur some short time after high- 

 water. 



