8 DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



barometer which gives, for standard value of gravity, direct readings in millibars 

 is a barometer whose scale has its divisions at the mutual distance of 0.750079 mm. 

 or 0.75 mm., practically, instead of at the distance of integer millimeters. 



In meteorology it is common to give the barometric pressure either in milli- 

 meters or in inches of mercury. The millimetric division is not in the least more 

 rational than the division into inches. Neither of them has anything to do with the 

 system of absolute units. The consequences of this irrationality have not yet been 

 seriously felt, because the barometric records have until now served for qualitative 

 purposes mainly. But the further development of dynamic meteorology will 

 compel us to introduce rational units sooner or later. Meanwhile we shall be 

 obliged to change from the one system of units to the other by auxiliary tables. 



The tables required for the direct passage from millimeters or inches of mercury 

 to millibars are given in the Appendix. In many cases, however, it will be a saving 

 of time and labor for a while to retain the units to which the original observations 

 are referred, in order to carry out the transition to the rational units at a later 

 stage of the work of computation, as will be developed in the proper places below. 



