22 CORRELATION OF PHYSICAL FORCES. 



resist or arrest the motion of the mass of matter appended 

 to it. 



It is difficult, in such cases, not to recognise a reality in 

 force. We need some word to express this state of tension ; 

 we know that it produces an effect, though the effect be nega- 

 tive in character : although in this effort of inanimate matter 

 we can no more trace the mode of action to its ultimate ele- 

 ments than we can follow out the connection of our own 

 muscles with the volition which calls them into action, we 

 are experimentally convinced that matter changes its state 

 by the agency of other matter, and this agency we call 

 force. 



In placing the weight on the glass, we have moved the 

 former to an extent equivalent to that which it would again 

 describe if the resistance were removed, and this motion of 

 the mass becomes an exponent or measure of the force exert- 

 ed on the glass ; while this is in the state of tension, the 

 force is ever existing, capable of reproducing the original 

 motion, and while in a state of abeyance as to actual motion, 

 it is really acting on the glass. The motion is suspended, 

 but the force is not annihilated. 



But it may be objected, if tension or static force be thus 

 motion in abeyance, there is at all times a large amount of 

 dynamical action subtracted from the universe. Every stone 

 upon a hill, every spring that is bent, and has required force 

 to upraise or bend it, has for a time, and possibly for ever, 

 withdrawn this force, and annihilated it. Not so ; what 

 takes place when we raise a weight and leave it at the point 

 to which it has been elevated? we have changed the centre 

 of gravity of the earth, and consequently the earth's position 

 with reference to the sun, planets, and stars ; the effort we 

 have made pervades and shakes the universe ; nor can we 

 present to the mind any exercise of force, which is thus not 

 permanent in its dynamical effects. If, instead of one weight 

 being raised, we raise two weights, each placed at a point 



