52 Scientific Proceedings, Royal Dublin Society. 



/i, the metric forcine, or unit of force, which is the Hyper- 

 decigramme,* this being the force which if it acted in a 

 fixed direction on a mass of one gramme for a second 

 would in that time change its velocity by one metre 

 per second. 



Pi, the metric unit of the coefficient in the expression H--^- 



for the gravitation of two masses towards one another. 

 This unit is the coefficient which should be used if each 

 gramme mass attracted other ponderable matter at a 

 metre distance with such intensity as would impress on 

 it an acceleration towards the attracting gramme of one 

 metre per second per second. 

 6].. the electromagnetic electrine, or the electromagnetic unit 

 quantity of electricity in the metric series, is that 

 quantity of each of the two kinds of electricity which 

 must be discharged every second in opposite directions 

 along a wire, in order to maintain in it the metric unit 

 current ; this currentine or unit current being defined as 

 the current which must exist in a wire a metre long 

 in order that it may exert a force of a Hyper-deci- 

 gramme on ponderable matter at a metre distance 

 charged with a unit of magnetism ; and the unit charge 

 of magnetism of either kind being defined as that 

 quantit}^ which acting on ponderable matter at a metre 

 distance, charged with an equal quantity of magnetism, 

 exerts on it the unit force, that is one Hyper-deci- 

 gramme. 



* The hyper-decigramme means the gravitation or downward force towards the earth 



of a mass which exceeds a decigramme in the ratio of — where g is the acceleration of 



gravity measured in metres per second per second. The appropriateness of the term 



/(^^e?'-decjgramme arises from the circumstance that the coefficient — evert/where 



exceeds unity, whether within the earth, outside it, or on its surface ; and the convenience 

 of the term arises from the circumstance that on the earth's surface the coefficient 

 nowhere exceeds unity by more than a small fraction, so that the hyper- decigramme is 

 a force which but little differs in value from that gravitation or weight of a decigramme 

 with whicli we, inhabitants of the earth, have become familiar ; so that the name suggests 

 to us the amount of the force. Gravitation is the downward force and gravity is the 

 downward acceleration towards the earth as olserved. They are chiefly due to the 

 attraction of the earth, and in a small degree, when the observation is made on or within 

 the earth, to the earth's rotation. This is the meaning of the word gravity as it is used 

 by the classical writers on mechanics, (see Pouillet's Mecanique, 2MSsim), and the practice 

 of some modern writers, who use this term to designate a force instead of an acceleration, 

 is to be deprecated. 



