corresponds to a pressure of 21 kg/m on a surface perpendicular to the wind), a constant agitation 

 of the ice (irregular periods of 20 to 180 sec.) was observed. The amplitude reached 3 mm (figure 

 86). In calm weather, fluctuations of an equal order were not observed. 



/f*^<J» 09 10 11 12 13 14 



TIME (MINUTES) 



Figure 86. Wind fluctuation of ice on the Volga. 



15 



16 



LITERATURE: 17, 77. 



Section 94. Break-up of Ice Fields 



Let us assume that initially we have a large ice field. With diverse conditions existing in 

 the sea, the examined field cannot be equally durable and vigorous in its entire extent; this is not 

 possible since it usually consists of comparatively small ice blocks which are frozen together. 

 Even if the field is initially completely homogeneous, it will be gradually covered by a set of 

 thermal and dynamic fissures. The external forces of a different order, which have a varying 

 effect on individual parts of the field, cause tension of a distinct order. When these tensions ex- 

 ceed the limit of plasticity of the weakest parts of the examined field, it separates along the lines 

 of least resistance . 



Of the external forces which fracture ice fields, sea current and tidal phenomena are most 

 important in the Littoral Belt, while in the open sea the wind exerts the greatest influence on 

 floating ice. 



The appearance of fissures in the ice fields naturally preceeds the break-up. Sometimes 

 ice hummocks result from the contact of large ice fields and thus border the fields. Moreover, 

 large fissures are not observed. 



During powerful jammings, large fissures are found which run across the entire field, 

 fracturing it into large sections. 



It was established from a trial durability test of materials that the length of the test model 

 must not exceed five times the dimension of the smallest magnitude of its transverse section. 

 Otherwise, the model is bent and cracked with jamming even with the comparatively largest ice 

 blocks, where the horizontal dimensions often surpass the vertical. Thus it is necessary to re- 

 gard the formation of fissures accompanying ice janxming as entirely natural. 



247 



