224 REPORT— 1873. 



cardinal number, if the exponent be positive, and by a prefixed ordinal 

 number if the exponent be negative. 



Thus 10" grammes constitute a gramme-nine ; ^^^ of a gramme constitutes 



a ninth-gramme ; the approximate length of a quadrant of one of the earth's 

 meridians is a metre-seven, or a centimetre -nine. 



For multiplication or division by a million, the prefixes mer/a * and micro 

 may conveniently be employed, according to the present custom of electricians. 

 Thus the megohm is a million ohms, and the microfarad is the millionth part 

 of a farad. The prefix mega is equivalent to the affix six. The prefix micro 

 is equivalent to the prefix sixth. 



The prefixes hilo, hecto, deca, deci, centi, milVi can also be employed in their 

 usual senses before all new names of units. 



As regards the name to be given to the C. G. S. xm.it of force, we recom- 

 mend that it be a derivative of the Greek Zvi a ins. The form dynamy appear* 

 to be the most satisfactory to etymologists. Dynam is equally intelligible, 

 but awkward in sound to English ears. The shorter form, dyjie, though not 

 fashioned according to strict rules of etymology, will probably be generally 

 preferred in this country. Bearing in mind that it is desirable to construct 

 a system with a view to its becoming international, we think that the termi- 

 nation of the word should for the present be left an open question. But we 

 would earnestly request that, whichever form of the word be employed, its. 

 meaning be strictly limited to the unit of force of the C. G. S. system — that 

 is to say, the force which, acting tqjon a gramme of matter for a second, gene- 

 rates a velocitif of a centimetre per second. 



The C. G. S. unit of ivork is the work done by this force worJcing through a 

 centimetre ; and we propose to denote it by some derivative of the Greek 

 epyoi'. The forms ergon, ergal, and erg have been suggested ; but the second 

 of these has been used in a difi'erent sense by Clausius. In this case also wc 

 propose, for the present, to leave the termination unsettled ; and we request 

 that the word ergon, or erg, be strictly limited to the C. G. S. unit of work, 

 or what is, for purposes of measurement, equivalent to this, the C. G. S. unit 

 of energy, energy being measured by the amount of work which it represents. 



The C. G. S. unit of power is the power of doing work at the rate of one er(j 

 per second ; and the power of an engine, under given conditions of working, 

 can be sjiecified in ergs per second. 



For rough comparison with the vulgar (and variable ) units based on ter- 

 restrial gravitation, the following statement will be useful : — 



The iveight of a gramme, at any part of the earth's surface, is about 980 

 dynes, or rather less than a kilodyne. 



The iveight of a hilogramme is rather less than a megadyne, being about 

 980,000 dynes. 



Conversely, the dyne is about 1-02 times the weight of a milligramme at 

 any part of the earth's surface; and the megadyne is about 1"02 times the 

 weight of a Icilogramme. 



The lilogrammetre is rather less than the ergon-eight, being about 98 

 million ergs. 



The gramme-centimetre is rather less than the KZer^, being about 980 ergs. 



For exact comparison, the value of r; (the acceleration of a body falling in 

 vacuo) at the station considered must of course be known. In the above 

 comparisons it is taken as 980 C. G. S. units of acceleration. 



* Before a Towel, either viey or mcgal, as euphony may suggest, may be employed 

 instead of mega. 



