MUTUAL CHANGES COMMON TO ALL MATTER 55 



of the earth, or gravitation, but it is more exact to say that 

 a mutual action, or a stress, exists between the two. The 

 phrase attraction of the earth fails to convey the mutual nature 

 of the change. 



We have seen that displacement between two bodies can 

 only be measured by considering one of them to be at rest. So 

 also mutual action is generally measured by its effect on one 

 body. 



We may say that the earth moves to the displaced body, 

 and consider the displaced body to remain fixed after it is re- 

 leased ; or we may consider the earth to be fixed, and the body 

 to move. We have no reason, so far, for taking one aspect of 

 the change in preference to the other. We shall learn later 

 that, relatively to any body not taking part in the mutual 

 change, the earth must be considered to move, although to an 

 infinitesimal degree, at the same time as the other body under- 

 goes its much more apparent change. The displacements are, 

 in fact, inversely proportional to the masses. 



Taking the customary and convenient conception of force, it 

 may be defined as whatever changes or tends to change the 

 motion of a body by altering either its direction or its magnitude. 

 But since observations show that change of motion in one body 

 is always accompanied by change of motion, or change of 

 some other kind, in another body, we must not forget that in 

 using the term or idea, ' force,' we confine our attention to one 

 side only of a two-sided event. This two-sided event is the 

 mutual change that always occurs wherever stress manifests 

 itself. 



It is to be noted in the case of the mutual action here 

 investigated that the motion takes place in the straight line 

 joining the centres of the bodies in other words, the body 

 moves towards the centre of the earth. 



40. The Moveable Pulley. A cord is fixed at one end, then 

 passes over the moveable pulley A, and next over the fixed 

 pulley B. The free end of the cord c and the pulley A are 

 each furnished with a hook. If a known mass is attached to 

 the free end of the cord it may be made to raise a mass 

 nearly double its own in quantity. We must take into 



