PRODUCT OF MAGNETIC ENERGIES. 23 



force exerted between the two magnetic systems must 

 contain as factor the product of the energies of the two 

 systems. 



The experiments cited in this Article have been 

 carefully verified by the writer of this Treatise. 



It is necessary now to fix with precision the units of 

 the different elements which we have to employ. For 

 the unit of time, 1 second of mean solar time is uni- 

 versally adopted: for the unit of measure of length, 

 1 foot is commonly used in England, and 1 millimetre 

 by the nations which adopt the Metrical system : for 

 the measure of mass, reference is made to weight, and 

 the received units are, 1 grain in England, and 1 milli-" 

 gramme in the Metrical system. For the measure of 

 statical force, it is found convenient to depart from the 

 custom usually followed in mechanical investigations 

 (in which the unit of pressure is considered to be the 

 pressure produced by a unit of mass under the action 

 of terrestrial gravity), and to adopt, instead, that pres- 

 sure which, acting through the time 1 upon the mass 

 1, would produce in it the velocity 1. (This unit, in 



English experiments, is about -^- of the ordinary unit 



O*u A. 



of pressure.) This selection of unit of pressure amounts 

 to the same as saying that the unit of accelerative force 

 will be that which produces the velocity 1 in the 

 time 1. 



