CHAPTER \Y 



SYSTEM OF UNITS. 



1. Meter-Ton-Second System. In quantitative physical investigations the abso- 

 lute units of the centimeter-gram-second system are now in general use. Sometimes 

 these units are used directly. But, as one set of units can not have the proper 

 magnitude for all sorts of measurements, special practical units are in many cases 

 introduced which are derived from the corresponding fundamental units by the 

 multiplication by suitable powers of 10. The choice of practical units is a question 

 of great importance. It is of great advantage if they themselves form a connected 

 system, or if they are at least in some simple relation to a connected system which 

 can be used as a system of reference, the incessant troublesome return to the c.g.s. 

 system being thus avoided. 



For the purpose of dynamic meteorology and hydrography, the centimeter and 

 gram are too small as units of length and mass. If tor unit-length we choose the 

 meter, and for unit-mass the metric ton, i. e., the mass of a cubic meter of water 

 at maximum density, great advantages are gained. The choice of a convenient 

 unit of time unfortunately implies difficulties. Evidently the second is far too small 

 a length of time for the measurement of changes in the state of the atmosphere and 

 still more so in that of the sea. But the circumstance that the division of time is 

 not decimal makes every change in the fundamental unit of time inadvisable. As 

 fundamental units of reference we shall therefore use consistently 



Meter = io 2 centimeters. Metric ton = io 6 grams. Second. 



We shall refer to this system as the meter-ton-second system, or the m.t.s. system. 



To the fundamental mechanical units we have to add, finally, the fundamental 

 thermal unit. For this we shall choose the degree of the centigrade thermometer. 



Unfortunately there is great confusion as to the units to which meteorological 

 and hydrographical observations are referred. As we proceed we shall, therefore, 

 give the tables required to derive from the observations recorded in the principal 

 publications the results we wish to express in the units used in this treatise. These 

 auxiliary tables, which would be superfluous if all observations were recorded 

 in absolute units, are collected in the annexed " Appendix to meteorologic and 

 hydrographic tables." 



2. Simplest Derived Units. From the values of the fundamental units those of 

 the derived units are easily deduced. For completeness we shall add the dimen- 

 sions of each derived quantity, expressed in the usual way in terms of length Z, 

 mass M, and time T. 



