328 THE PRINCIPLES OF SCIENCE. 



interesting quantitative research P, in which we shall 

 doubtless at some future time discover the operation of 

 causes now most mysterious and unaccountable. 



The Methods of Accurate Measurement. 



In studying the modes by which physicists have ac- 

 complished very exact measurements, we find that they 

 are very various, but that they may perhaps be reduced 

 under the following three classes : 



1. The increase or decrease of the quantity to be 

 measured in some determinate ratio, so as to bring it 

 within the scope of our senses, and to equate it with the 

 standard unit, or some determinate multiple or sub-mul- 

 tiple of this unit. 



2. The discovery of some natural conjunction of events 

 which will enable us to compare directly the multiples of 

 the quantity with those of the unit, or a quantity related 

 in a definite ratio to that unit. 



3. Indirect measurement, which gives us not the quan- 

 tity itself, but some other quantity connected with it by 

 known mathematical relations. 



Conditions of Accurate Measurement. 



Several conditions are requisite in order that a mea- 

 surement may be made with great accuracy, and that 

 the result may be closely accordant when several inde- 

 pendent measurements are made. 



In the first place the magnitude must be exactly defined 

 by sharp terminations, or precise marks of inconsiderable 

 thickness. When a boundary is vague and graduated, 

 like the penumbra in a lunar eclipse, it is impossible to 

 say where the end really is, and different people will come 



P Gauss, 'General Theory of Terrestrial Magnetism'; Taylor's 

 ' Scientific Memoirs/ vol. ii. p, 228. 



