200 THE PHENOMENA OF MOTION 



force is recognized by two conditions of activity, 

 entirely different in the phenomena they offer to the 

 attention of the observer. 



The force of gravitation inherent in the particles 

 of a stone, gives to them a continual tendency to 

 move towards the centre of the earth. 



This effect of gravitation becomes inappreciable 

 to the senses when the stone, for example, rests 

 upon a table, the particles of which oppose a resist- 

 ance to the manifestation of its gravitation. The 

 force of gravity, however, is constantly present, and 

 manifests itself as a pressure on the supporting 

 body ; but the stone remains at rest ; it has no mo- 

 tion. The manifestation of its gravity in the state 

 of rest we call its weight. 



That which prevents the stone from falling is a 

 resistance produced by the force of attraction, by 

 which the particles of the wood cohere together ; a 

 mass of water would not prevent the fall of the stone. 



If the force which impelled the mass of the stone 

 towards the centre of the earth were greater than 

 the force of cohesion in the particles of the wood, 

 the latter would be overcome ; it would be unable 

 to prevent the fall of the stone. 



When we remove the support, and with it the 

 force which has prevented the manifestation of the 

 force of gravity, the latter at once appears as the 

 cause of change of place in the stone, which acquires 

 motion, or falls. Resistance is invariably the result 

 of a force in action. 



