264 



DEPARTMENT OF THE NATAL SERVICE 



sucli parallelogram endeavours to bring about a rotary movement of the water, whereby 

 the isosteric lines would be brought into the horizontal, vide fig. 46. 



Fig. 46. — Bjerknes' diagram of the 

 Archimedean forces in the sea. 



It is interesting to pursue the study of this simple geometrical diagram so as to- 

 embrace the three dimensions. - We have, then, in place of the isobars and isosteric 

 lines shown in fig. 46, a series of isobar and isosteric surfaces, while the parallelograms 

 become tubes, each presenting a parallelogram in section, these striving to turn the 

 water round in such a manner as to render the isosteric surfaces horizontal. As the 

 isobar and isosteric surfaces cannot terminate in the water itself, but must continue 

 until they reach either surface or bottom, so also the isobar-isosteric tubes formed by 

 intersection of the two must either continue vuitil they reach one limit of the water 

 or else turn back upon themselves. As each tube lies between two adjacent isobar sur- 

 faces, its course becomes horizontal. 



These isobar-isosteric tubes are called solenoids. Each solenoid produces the same 

 whirling effect in the water, and the number of solenoids is therefore a measure of the 

 degree of intensity attained by the Archimedean forces. 



The number of solenoids depends upon the number of isosteric surfaces; i.e., upou 

 the stability of the sea-water, and upon the obliquity of the isosteric surfaces them- 

 selves. As the isosteric surfaces fall into a horizontal position, the solenoids disappear 

 entirely. Consequently, the greater the stability of the water layers, and the greater 

 the obliquity of the isosteric surfaces, the greater will be the number of solenoids. 



Fig. 47. — Solenoids in the separating surface 

 between two water layers. 



Where a homogeneous water layer rests upon another of higher specific gravity, 

 there will be no isosteric surfaces within the layers; such will, however, be found in 

 the separating .surface between them. In this boundary surface, then, the solenoids will 



