2. In the surface layers the direction of the tidal currents is clockwise. (This also occurred 

 at 100 m. Only at the very bottom, at a depth of 250 m, did the current shift in a counterclock- 

 wise direction). 



3. The orbits of the particles increased rather than decreased with depth. The greatest 

 speeds of the tidal current were observed at a depth of 25 m. These phenomena are explained 

 firstly by the fact that the water is not able to attain a position of equilibrium and secondly by the 

 inertia of the water masses. Thus, in the open sea and in the central parts of wide straits there 

 is not found what is commonly referred to along the shores as "change of currents" or "slack 

 water. " Here the tidal currents never cease. 



LITERATURE: 50, 51, 53, 54. 



Section 120. Influence of Ice on Tidal Phenomena 



In the case of a sea surface free of ice, the energy of tidal waves is expended in the end on 

 destruction of the bottom and the shores and on heating of the sea. In a sea which is covered with 

 ice this energy is expended, in addition, on all sorts of deformations of the ice cover — hummock 

 formation, fracture, etc. It follows, therefore, that with the presence of ice all elements of the 

 tidal phenomena are significantly changed. 



In a sea free of ice the speed of tidal currents decreases with depth approximately paraboli- 

 cally. If the sea is covered with fast ice the tidal current is similar to the flow of a liquid between 

 two plates; i.e., the graph of the distribution of speeds will also be approximately parabolic, but 

 with its horizontal axis situated at the middle of the depth of the sea (if we consider friction with 

 the ice and with the bottom as equal and the fluid as homogeneous). From the above, it follows that 

 the average speed of the tidal current under the ice will be considerably less than the speed of the 

 tidal current in a sea free of ice. 



But the decrease in speed of tidal currents results in a proportional decrease in amplitude 

 of the tide. The amplitude is also decreased in turn by the presence of the ice cover for creating 

 the appropriate variations of level there is required a periodic bending of the ice cover first in one 

 direction, then in the other, and part of the tidal energy is expended in this. 



The investigations conducted by port discovery parties in the White Sea serve as a typical 

 example in this respect (table 93). 



TABLE 93. AMPLITUDES OF THE TIDE (IN METERS) IN THE KAMENAK AND PYA RIVERS 



In February at Mezenski Bay at the mouth of the Pya River there was loose ice, while at the 

 mouth of the Kamenka fast ice. If we accept the fact that the ratio between the amplitudes in these 

 two points should always be about 1. 6, then in February the amplitudes of tide at Kamenka should 

 have been about 3 m, while only 0.68 m was observed. 



328 



