378 THE PRINCIPLES OF SCIENCE. 



and not only this science but almost every other. The 

 true course evidently is to select, as the provisional unit 

 of light, some light of convenient intensity, which can be 

 reproduced from time to time in exactly the same in- 

 tensity, and which is defined by physical circumstances. 

 All the phenomena of light may be experimentally investi- 

 gated relatively to this unit, for instance that obtained 

 after much labour by Bunsen and Eoscoe a . In after 

 years it will become a matter of inquiry what is the 

 energy exerted in such unit of light ; but it may be long 

 before the relation is exactly determined. 



A provisionally independent unit, then, means one which 

 is assumed and physically defined in a safe and repro- 

 ducible manner, in order that particular quantities may 

 be compared inter se more accurately than they can yet 

 be referred to the primary units. In reality almost all 

 our measurements are made by such independent units. 

 Even the unit of mass is practically an independent one, 

 as we have seen (p. 373). 



Similarly the unit of heat ought to be simply the 

 unit of energy, already described. But a weight can 

 be measured to the one-millionth part, and temperature 

 to less than the thousandth part of a degree Fahrenheit, 

 and to less therefore than the five-hundredth thousandth 

 part of the absolute temperature, whereas the mechanical 

 equivalent of heat is probably not known to the thousandth 

 part. Hence the need of a provisional unit of heat, which 

 is often taken as that requisite to raise a unit weight of 

 water (say one gramme) through one degree Centigrade 

 of temperature, that is from o to i. This quantity of 

 heat is capable of approximate expression in terms of 

 time, space, and mass ; for by the natural constant, 

 determined by Dr. Joule, and called the mechanical 



a ' Philosophical Transactions ' (1859), vol. cxlix. p. 884, &c. 



