98 THE ANATOMY OF SCIENCE 



mutual velocity diminishes, or, in other words, 

 as the two paths approach one another they 

 experience a curvature. 



Now, from a knowledge of the direction of 

 the paths at the bottom of the figure, can we 

 make any prediction as to their directions at the 

 top of the figure? In one corner of the diagram 

 let us draw an arrow equal and parallel to the 

 arrow A, and another from the same point 

 parallel and equal to the arrow B, and complete 

 the parallelogram, at the upper corner of which 

 we now draw an arrow equal to A' and another 

 equal to B', and complete this parallelogram. 

 Now we may announce the extremely simple law 

 that the diagonals of the two parallelograms so 

 dra^Ti must lie in the same line and be equal to 

 one another. This law, which Wilson and I first 

 stated and called the law of conservation of 

 extended momentum, comprises Newi:on's law of 

 the conservation of momentum. Expressed in 

 another way, the lower diagonal represents the 

 locus of the center of gravity of the two bodies 

 before they come together, and the upper diag- 

 onal represents the locus of the center of grav- 

 ity after they have separated. That these two 

 diagonals are in the same line means that the 

 center of gravity proceeds with a constant 



