268 



DEPARTMENT OF THE NATAL SERVICE 



taneous surveys o£ density conditions in the area. From a dynamic point of view, 

 Hjort's measurements leave absolutely nothing to be desired. 



In applying Bjerknes' theory to hydrographic observations, it will be well to keep 

 to one thoroughly connected system of units, as, for instance, the centimetregrammese- 

 cond system. Where these units do not suit, other convenient units can always be 

 arrived at by multiplying by suitable powers of 10. Pressure in the sea increases by 

 very nearly 100,000 c.g.s. units for every metre depth. If therefore, we take as unit of 

 pressure of 10'^ c.g.s. then the pressure will increase by one such unit for every metre 

 depth. Such iinit of pressure we may call a decibar. To the pressure of the sea-water 

 should be added that of the atmosphere, amounting to about 10 decibars. In the follow- 

 ing, however, we shall not have occasion to reckon with absolute pressures, but only with 

 differences of pressure, and (may therefore neglect this initial pressure, taking the 

 pressure at the surface of the sea as a starting point. The depth in metres will then 

 likewise serve to express the pressure in decibars; thus at 10 metres' depth, the pressure 

 is 10 decibars, at 300 meters, it is 300 decibars, and so on. This identity of values will, 

 as we shall subsequently see, serve greatly to simplify the graphical and numerical 

 operations involved. 



om. 



100 - 



200 



500 - 



400 - 



V,= 500 



V,=400 



Fig. 50. — Isobars, isosters and solenoids in 

 section IX. 



For specific volume, it is convenient to take 10-^ c.g.s. as the unit. Then instead 

 of writing, for instance, v=0-97261, we take 10^ v=97261, and as the two first figures 

 in values for specific volume are always 97, and we have only to reckon with differences 

 of the same, we can discard these, and write v== 261. Thus in the case of specific 

 volume, we have only to deal with three-figure values, which again serves to render the 

 necessary calculations easier. 



By thus taking the isobars as 10^ times too far apart, and the isosteres 10^ too close, 

 we obtain cgs. solenoids. 



