LUCRETIUS AND THE ATOMIC THEORY 191 



speed, and in parallel lines, would relatively to one another have 

 been in perfect rest. A bag of marbles in a railway train could 

 not be employed as a source of energy in the train ; they lie at 

 rest ; and it is only when brought into collision with something 

 moving at a different pace from the train that they can develop 

 any power, which may then be considerable. But more than 

 this : How are we to conceive direction in space except 

 relatively to something? what is up and what is down in 

 space ? If it be answered, The place atoms come from is up 

 above us, we answer, How, when all atoms are all one relatively 

 to one another in a perfectly similar position, are we poor atoms 

 to know that they are coming from anywhere ? So far as we 

 can see, an absolute motion in space is devoid of all meaning. 

 We must conceive a shape or position for space before we can 

 conceive of motion relatively to space, and as we are at perfect 

 liberty to conceive any shape or position, or none at all, it fol- 

 lows that absolute motion in space is anything you please, that 

 is to say, a mere fancy. Lucretius unconsciously assumed the 

 world as his basis by which to measure direction and velocity. 

 The direction in which things fall on the earth was sheer down 

 in void ; but really his assumption was meaningless, or, at least, 

 explained in no way the power or force which he wished to 

 explain. Not so, by the way, the older conception of Demo- 

 critus, who thought atoms moved in all directions freely and 

 indifferently; a universe so constituted originally might at 

 least contain all the energy we require. One atom would then 

 exert its force on another, but the Lucretian atoms would have 

 remained in profound stillness, except for that occasional swerve 

 at quite uncertain times and places, the cause of which he 

 leaves wholly unaccounted for. This swerving seems but a silly 

 fancy, and yet consider this : It is a principle of mechanics 

 that a force acting at right angles to the direction in which a 

 body is moving does no work, although it may continually and 

 continuously alter the direction in which the body moves. No 

 power, no energy, is required to deflect a bullet from its path, 

 provided the deflecting force acts always at right angles to that 

 path an apparent paradox, which is, nevertheless, quite true 

 and familiar to the engineer. It is clear to us that Epicurus, 

 when he devised his doctrine of a little swerving from the 



