xiv.] UNITS AND STANDARDS OF MEASUREMENT. 323 



natural unit of energy will then be that which overcomes 

 a unit of force acting through a unit of space ; when we 

 lift one kilogram through one metre, against gravity, we 

 therefore accomplish 9-80868 . . units of work, that is, we 

 turn so many units of potential energy existing in the 

 muscles, into potential energy of gravitation. In lifting 

 one pound through one foot there is in like manner a con- 

 version of 32' 1 889 units of energy. Accordingly the 

 unit of energy will be in the English system, that required 

 to lift one pound through about the thirty-second part of 

 a foot ; in terms of metric units, it will be that required to 

 lift a kilogram through about one tenth part of a metre. 



Every person is at liberty to measure and record 

 quantities in terms of any unit which he likes. He 

 may use the yard for linear measurement and the litre 

 for cubic measurement, only there will then be a com- 

 plicated relation between his different results. The 

 system of derived units which we have been briefly con- 

 sidering, is that which gives the most simple and natural 

 relations between quantitative expressions of different 

 kinds, and therefore conduces to ease of comprehension 

 and saving of laborious calculation. 



It would evidently be a source of great convenience if 

 scientific men could agree upon some single system of 

 units, original and derived, in terms of which all quantities 

 could be expressed. Statements would thus be rendered 

 easily comparable, a large part of scientific literature would 

 be made intelligible to all, and the saving of mental labour 

 would be immense. It seems to be generally allowed, too, 

 that the metric system of weights and measures presents 

 the best basis for the ultimate system; it is thoroughly 

 established in Western Europe; it is legalised in England ; 

 it is already commonly employed -by scientific men; it is 

 in itself the most simple and scientific of systems. There 

 is every reason then why the metric system should be 

 accepted at least in its main features. 



Provisional Units. 



Ultimately, as we can hardly doubt, all phenomena 

 will be recognised as so many manifestations of energy ; 

 and, being expressed in terms of the unit of energy, will 



Y 2 



