SYSTEM OF UNITS. 7 



But it should be emphasized that the dynamic meter and its subdivisions are 

 units of gravity potential, not of length. In every given locality, however, they 

 represent definite lengths measured along the plumb-line, and for this reason they 

 can be used as full equivalents for the common length-measure when distances 

 measured along the plumb-line are concerned. 



5. Units of Pressure. The unit-pressure of the m.t.s. system is the pressure 

 of the unit-force defined above when it is exerted over the area of a square meter, 

 and, as mentioned already, is equal therefore to 10,000 c.g.s. units of pressure, or 

 10,000 dynes per square centimeter. To avoid circumlocution, it will be necessary 

 for us to have names for the employed units of pressure. The megadyne per 

 square centimeter is approximately equal to the present practical unit, the atmos- 

 phere. It has often been proposed to introduce the megadyne per square centi- 

 meter as a practical unit of pressure, and to designate it by some name derived from 

 the word " barometer." We shall choose the name bar as being the shortest, and 

 designate the decimal parts of it as the decibar, centibar, and millibar. The m.t.s. 

 unit of pressure will then be the centibar, while the c.g.s. unit will be the microbar. 



Very simple rules are obtained for the columns of water exerting these pressures 

 if we agree to have the heights of the water-columns represented by their values in 

 dynamic meters, their multiples or subdivisions. Taking pure water at maximum 

 of density, and neglecting its compressibility, we get these relations: 



1 bar = pressure of 1 dynamic decameter of water. 



1 decibar = pressure of 1 dynamic meter of water. 

 1 centibar = pressure of 1 dynamic decimeter of water. 

 1 millibar = pressure of 1 dynamic centimeter of water. 



Finally, the c.g.s. unit, the microbar, is equal to the pressure of 10 dynamic 

 microns of water. 



Among these units we shall use often the decibar as a technical unit, on 

 account of its correspondence to the dynamic meter as unit of gravity potential. 

 Completing our terminology, we shall denote pressures represented by an integer 

 number of decibars as standard pressures. In cases where we have to do with 

 the relations between pressures and gravity potentials we shall often refer to these 

 standard values of both quantities, using thus dynamic meter and decibar as con- 

 nected units. But when the relation to other quantities comes in, we shall have to 

 return to the m.t.s. units, the centibar, and the dynamic decimeter. 



We shall also make frequent use of the millibar as that technical unit which is 

 most convenient in reading the barometer. It will replace the present practical 

 units, the millimeter or the inch of mercury. Using 13.59545 * or tne density of 

 mercury at o C.,* and 9.80617 for the standard value of gravity (compare sec- 

 tion 8 below), we find that 1 meter of mercury of o C. at a place where gravity 

 has this standard value exerts the pressure of 1.333193 bars. Thus, a mercury 



*Thiesen und Scheel : Tatigkeitsbericht derPhys. Techn. Reichsanstalt, I Feb., 1897-31)311., 1898. Berlin, 189S. 



