VALUE OF MEASUREMENTS. 389 



between the gaseous and liquid states was worked out 

 by many years of laborious and minute measurement of 

 phenomena scarcely sensible to the naked eye.' l 



6 Here, then, we have a very full recognition of the im- 

 portance of accurate measurement, by one who has a per- 

 fect right to speak authoritatively on such a subject. It 

 may indeed be maintained that no accurate knowledge of 

 anything or any law in nature is possible, unless we 

 possess a faculty of referring our results to some unit of 

 measure, and thus it might truly be said " to know is to 

 measure" ' 2 



After having ascertained the existence of a fact or 

 principle in science, the next most important step is to 

 determine its amount under various conditions of time, 

 space, direction, &c., and its quantitative relations to its 

 causes and effects; to other facts and principles, &c. 

 Often by the aid of measurements we are enabled to deter- 

 mine the nature, causes, and conditions of phenomena. 

 One of the commonest methods of discovering whether 

 two different phenomena are connected together is to 

 ascertain whether they simultaneously vary in amount ; if 

 one varies in the same proportion as the other which 

 accompanies it, the two are probably related to each 

 other, either as cause and effect, or as coincident effects 

 of the same cause. Each force can only act in accord- 

 ance with its own laws ; the quantitative relations and 

 properties of forces, therefore, often enable us to detect 

 and distinguish the action of those forces in any new 

 phenomena which we have discovered, and to ascertain 

 also the extent to which such forces exist and operate 



1 Address to the British Association, 1871. 



2 Address by Dr. C. W. Siemens. Conferences, Special Loan Collec- 

 tion, London, 1876, p. 206. 



