30 THE REALITIES OF MODERN SCIENCE 



apparatus, through which opposing effects are balanced, 

 are to be eliminated by a reversal, as in double weighing, 

 the number of operations required for equally accurate 

 results in the two methods becomes the same. For 

 many measurements, as for example in astronomy, 

 it is impossible to apply the substitution method and 

 results of marvelous accuracy are obtained by the 

 opposition method. 



Both the substitution method and the opposition 

 method with reversals, are inconvenient because they 

 take so much time, as is evident from the illustration 

 of weighing. To-day the opposition method of weigh- 

 ing is used only where especially precise results are 

 required, as for example, in chemical analysis, phar- 

 maceutical work, and in the weighing of precious stones 

 and metals. For ordinary purposes the use of spring 

 scales makes possible a more convenient form of sub- 

 stitution method. 



A spring scale depends for its operation upon elas- 

 ticity, 1 which is "the property by virtue of which a 

 body requires the continued application of a deform- 

 ing stress to prevent the recovery, entire or partial, 



1 This definition, as the reader recognizes, is a good example of 

 the expression of an abstract idea. Thus the use of the word 

 "body" meaning a "definite amount of matter" does not limit the 

 definition to any particular substance or form. Again in the word 

 "deforming" we get away from the concrete by using a word ex- 

 pressing an abstract idea, for it covers stretching, bending, twisting, 

 compressing, distending, in fact, any change in form. To stretch 

 we must pull in opposite ways, to compress we push from opposite 

 sides, to distend we push outward in every direction. A single 

 force will not produce a deformation. To convey the idea of com- 

 binations of oppositely directed equal forces the physicist uses the 

 word "stress." 



