METHOD OF DISCO VERING CA USAL LAWS 187 



it cannot assure us that &quot;neither is the cause of the other, both 

 alike being effects of the electrical condition of the atmo 

 sphere&quot;. 1 But when we can experiment we may succeed in 

 removing such sources of uncertainty. 



The following examples will illustrate the experimental appli 

 cation of the method : (i) The first law of motion states that all 

 bodies in motion tend to remain moving with uniform velocity 

 in a straight line until acted on by some interfering force. How 

 was this law proved ? The method of difference could not be 

 applied, for it is impossible to procure a negative instance, i.e. 

 an instance with no interfering force and no retardation of uni 

 form rectilinear motion. It was well known that all moving 

 bodies are being constantly influenced by interfering forces of 

 such a kind that all of these cannot be totally eliminated, e.g. 

 gravity, friction, resistance of the atmosphere, etc. How, then, 

 verify the hypothesis that retardation of motion is always due to 

 such interfering influences ? Obviously, by trying to vary the 

 influence of these obstacles, to diminish it, for instance (since 

 total elimination is impossible), and then see whether, by doing 

 so, the phenomenon itself, the retardation of motion, would be 

 diminished proportionately. And this is what Borda did in his 

 experiments with the pendulum. The influence of gravity on 

 the oscillation of a pendulum had already been calculated. If 

 the first law of motion were true, an oscillating pendulum, unin- 

 terfered with by any other force than gravity, should remain 

 oscillating indefinitely. Friction at the point of suspension, and 

 atmospheric resistance, were the supposed causes of the retarda 

 tion. &quot;The simple oscillation . . . which in ordinary circum 

 stances lasts but a few minutes, was prolonged in Borda s 

 experiments to nearly thirty hours, by diminishing as much as 

 possible the friction at the point of suspension, and by making 

 the body oscillate in a space exhausted as nearly as possible of 

 air.&quot; 2 The less of the supposed obstacles, the less retardation. 

 &quot;There could, therefore, be no hesitation in assigning the whole 

 of the retardation of motion to the influence of the obstacles.&quot; 3 

 It was concluded, therefore, that in the absence of all interference 

 moving bodies would continue to move with uniform velocity in 

 a straight line. (2) Another simpler experimental example of 

 the method may be exhibited by projecting a body successively, 

 with the same initial velocity, along specially prepared surfaces 



1 FOWLER, Inductive Logic, p. 177. 4 MILL, Logic, III., viii., 7. :i ibid. 



