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VIII.— ON THE PHYSICAL UNITS OF NATURE,* by G. 

 JOHNSTONE STONEY, d.sc, f.r.s., Secretary to the Society. 



[Read, February 16th, 1881.] 



When mathematicians apply the sciences of measurement to 

 the investigation of Nature, they find it convenient to select such 

 units of the several kinds of quantity with which they have to 

 deal as will get rid of any coefficients in their equations which 

 it is possible in this way to avoid. Every advance in our 

 knowledge of Nature enables us to see more distinctly that it 

 would contribute to our further progress if we could effect 

 this simplification not only with reference to certain classes of 

 phenomena, but throughout the whole domain of Nature. 



Hitherto the practice of mathematicians has been governed by 

 the demands of the science of mechanics, in the greater part 

 (though not in the whole), of which science it is possible to derive 

 the units of all the other kinds of quantity from any three which 

 may be chosen. A system built in this way upon a foundation 

 which is arbitrarily assumed is necessarily an artificial system. 

 The units which are usually selected as the fundamental units of 

 a series of arbitrary systematic measures, are — 



The metret for lengthine, or unit of length, 



The gramme for massine, or unit of mass, and 



The solar second for timine, or unit of time. 

 These three, and all the units which may be derived from them 

 may be called the metric series of units, and in this investigation 

 they will be represented by small letters. Thus — 



The fundamental metric units being, 



l^, the metre, the metric lengthine, or unit of length. 



ii, the solar second, the metric timine, or unit of time. 



mi, the gramme, the metric massine, or unit of mass, 



some of the derived units will be, 



Vi, the metric velocitine, or, unit of velocity, which is a 

 velocity of one metre per second. 



* This paper was read before section A., of the British Association at the Belfast 

 Meeting in 1 874. 



t Since this paper was written the centimetre has been suggested as a unit of length, 

 and has been largely made use of. 



