10 



apart (about one-tenth of a meter) and calculate tlie pressure per 

 unit area on such a plane at a depth of about 1 decimeter. The pres- 

 sure of the atmosphere being subject to comparatively slight and 

 compensating variations can be totally disregarded throughout 

 hydrodynamic works. (See p. 45.) We have given by definition 

 values of pressure = weight per unit area = 



area, height, density, acceleration of gravity , 

 area 



Since the area values cancel, we have 



pressure = 7t g q. 



But it has been determined that g 7i = D, where D equals 1 dynamic 



decimeter. 



Substituting : 



p=q D 



Now it remains to find a suitable system of units of pressure based 

 upon the value equal to a water column 1 dynamic decimeter high 

 and possessing a mean density q. 



The most conmion example of natural pressure with which we are 

 familiar is that of the atmosphere. It has been a practice, long 

 established, to balance the perpendicular column of the atmospheric 

 envelope against an equal cross-sectional area of mercury. Tliis is a 

 well-known experiment of any physics laboratory in which mercury 

 has come to be adopted because of its great density; other liquids 

 being forced to too great a height by the balance. We employ exactly 

 the same equation, of course, as evolved in the case of a motionless 

 ocean; in fact, we might imagine finding the pressure at various 

 depths in the sea, theoretically, by means of a balanced column of 

 mercury. 



It has been found that at 0° C. and 45° latitude at sea level, the 

 normal height to which mercury is forced by the ever pressing air 

 envelope, is 0.76 meters, sometimes termed an ''atmosphere." 

 Since the acceleration of gravity at 45° latitude is known, viz, 9.8 

 meters, and the density of mercury at 0° C. is 13.59, let us calculate 

 the pressure p in meter-ton-second units — i. e., the system upon 

 which previous dynamic figures have been based. Substituting in 



p = qgh,we have p = 13.59 X 9.8 X 0.76 = 101.218. 



V. Bjerknes has used this quantity of 101.218 as a guide in deciding 

 upon the value ascribable to p. He has selected as a unit suitable 

 for hydrodynamic computations, the nearest integral number of 

 10 to 101.218, viz, 100, and has called this a bar. A bar is approxi- 

 mately the pressure exerted by a column of water 10 meters in height; 

 therefore the pressure of 1 meter of water is very nearly equal to the 



