134 DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



It may be useful in this state of indetermination of the results to remark that we 

 can give a slightly changed interpretation to these charts. Let us, instead of sea- 

 pressure, consider total pressure, obtained by addition of the atmospheric pressure 

 upon the sea's surface. Let us further, to simplify the conditions, imagine the 

 atmosphere to be removed and be replaced by a layer of sea-water of the proper 

 thickness to exert the actual atmospheric pressure. In this case the isobaric sur- 

 face of absolute pressure 10 decibars will always very nearly coincide with the 

 physical sea-level, passing a little below it in places where the atmospheric pressure 

 upon the physical sea-level is smaller than 10 decibars, and a little above it in the 

 artificially introduced water-layer, where the atmospheric pressure is of smaller 

 value. Now, the working out of the soundings gave the distance from the physical 

 sea-level to a surface of the constant sea-pressure p. But this distance will be 

 essentially the same as the distance from the defined ideal isobaric surface of the 

 total pressure 10 decibars to the isobaric surface where the total pressure is p -f- 10 

 decibars. 



We can thus also interpret the charts of fig. 27 as representing the topography 

 of true isobaric surfaces of a total pressure of p -f- 10 decibars, taken relatively to 

 the unknown topography of the ideal 10-decibar surface. 



Whichever view we take of the chart representing the distribution of pressure 

 in the sea, the representation remains incomplete. But of the distribution of mass, 

 on the other hand, we are able to give as complete a representation as in the case 

 of the atmosphere. 



Forming the differences of depth between two isobaric surfaces of the standard 

 pressures of p and -p -f- 1 decibars, we get the numbers representing the specific 

 volume of the water in the standard sheet between the two surfaces. But the 

 thickness of these sheets being only about 1 meter, we would get too many charts 

 by taking every sheet. We have therefore introduced for this purpose dynamic 

 decameters as units of dynamic depth in the upper sheets of the sea, to a depth of 

 6 dynamic decameters. Simultaneously we use the bar as unit-pressure. For the 

 deeper strata, where the changes with the depth are slower, we have used dynamic 

 hectometers as units of dynamic depth and the decabar as corresponding unit of 

 pressure. The charts representing the specific volume in the corresponding sheets 

 are given in fig. 28 . 



82. Pressure along Level Surfaces. Suppose us, on the other hand, to have 

 determined the sea-pressure at a given dynamic depth. We are then able to draw 

 a chart representing the distribution of sea-pressure in this depth. But it must be 

 remembered that this depth is measured from the physical sea-level. The chart 

 thus gives the distribution of the sea-pressure not along a true level surtace, but 

 along a surface of constant dynamic depth below the physical sea-level. 



As in the preceding case, we may take a different view of the chart, giving 

 another definition of the indeterminate element. We then imagine the atmosphere 

 to be replaced by a layer of water having the density of the water at the sea's sur- 

 face and of the proper thickness to exert the pressure of the atmosphere against the 



