124 GRAVITATION— THE LAW 



he blandly invents a new force to account for the devia- 

 tion. We can improve on his enunciation of the First 

 Law of Motion. What he really meant was — 



"Every body continues in its state of rest or uniform 

 motion in a straight line, except in so far as it doesn't." 



Material frictions and reactions are visible and abso- 

 lute interferences which can change the motion of a 

 body. I have nothing to say against them. The mole- 

 cular battering can be recognised by anyone who looks 

 deeply into the phenomenon no matter what his frame 

 of reference. But when there is no such indication of 

 disturbance the whole procedure becomes arbitrary. On 

 no particular grounds the motion is divided into two 

 parts, one of which is attributed to a passive tendency 

 of the body called inertia and the other to an interfer- 

 ing field of force. The suggestion that the body really 

 wanted to go straight but some mysterious agent made 

 it go crooked is picturesque but unscientific. It makes 

 two properties out of one; and then we wonder why they 

 are always proportional to one another — why the gravi- 

 tational force on different bodies is proportional to 

 their inertia or mass. The dissection becomes untenable 

 when we admit that all frames of reference are on the 

 same footing. The projectile which describes a parabola 

 relative to an observer on the earth's surface describes 

 a straight line relative to the man in the lift. Our 

 teacher will not easily persuade the man in the lift who 

 sees the apple remaining where he released it, that the 

 apple really would of its own initiative rush upwards 

 were it not that an invisible tug exactly counteracts this 

 tendency.* 



Einstein's Law of Motion does not recognise this 

 dissection. There are certain curves which can be 



* The reader will verify tkat this is the doctrine the teacher would have 

 to inculcate if he went as a missionary to the men in the lift. 



