THE AIM AND ACHIEVEMENTS OF SCIENTIFIC METHOD. 97 



objective* " force." Thus, though the force required to stretch 

 a given elastic cord, or to bend a given rod to a definite extent, 

 will seem very different when one is fatigued and when one 

 is fresh, very different in the cases of a man and a child, we 

 admit instinctively that it must " really " be the same in every 

 case where the same change has been brought about by 

 manipulating the cord or the rod in the same manner. Such 

 changes in the cord or rod would readily be regarded, like the 

 positions of the index of the thermometer, as objective records 

 of the forces applied, and would need only to be correlated 

 with numbers by means of an arbitrary scale (such as the 

 divisions of a foot rule arranged suitably in connection with the 

 cord or the rod) to be practically useful. In whatever way 

 the change in the cord or the rod has been brought about it is 

 always possible to find a body whose weight applied in the 

 same manner as the force in question can produce the game 

 change in length or shape, so that we pass readily to the 

 practice" of equating forces to the weights of bodies. When 

 this stage is reached it is possible by a very natural convention 

 of a character previously discussedf to proceed beyond the 

 mere recording of forces to their meastwement : a force equal 

 to the weight of 2 Ibs. is thought of as twice a force equal to 

 the weight of 1 Ib. 



- 42. 



At this stage it is clear that a different method must be 

 used for estimating the "force" which is ejectively thought 

 of as present in a moving body ; and the method just described 

 loses its theoretical security even for static measurements when 

 it is discovered that the weight of the same body is different 

 at different points of the earth's surface.^ The need is now 



* In the sense explained on p. 86. 



t Supra, p. 28. 



| Vaguer notions of the distinction between the mass and the weight 

 of a body arose historically from simpler experiences (cf. Maxwell, Theory 

 of Heat, 9th ed., p. 86). 



H 



