CLASSICAL AND NEW MECHANICS 145 



is to be found in the saying of Lord Kelvin, that no 

 scientific statement is understood until it is measured. 

 If we grant this, then we must first examine, in such 

 a discussion as this, our methods of measurement. 



The statement of a phenomenon invariably contains 

 two terms, called qualitative and quantitative factors; 

 the former expressing " what kind " and the latter, 

 " how much." Thus, when we speak of a distance as 

 ten centimeters, we mean that we are to consider a 

 quantity, ten, of the quality, length. Now it is not 

 really the province of science to seek for absolute 

 knowledge of either qualities or quantities, and the 

 attempt to do so is the excuse for hypothesis. What 

 science is concerned with is the relative knowledge, or 

 comparison, of different quantities of any quality and 

 the reduction of complex qualities into combinations 

 of simpler ones. 



It thus becomes of prime importance to settle on the 

 simplest and most fundamental qualities which may 

 serve as a foundation for our system of measurement. 

 Because of the fact that mechanical motions and me- 

 chanical forces are the most readily perceived by us 

 and are most easily expressed in mathematical for- 

 mulae, the fundamental units of quality are always 

 selected from mechanical concepts. Of these, length, 

 time, and mass or inertia, are found to be incapable 

 of further simplification and have been adopted as the 

 units of measurement. As these qualities must enter 



