OBSERVATIONS OF CHANGE OF POSITION 23 



more completely is the path of the moving body known. Having 

 made such measurements at sufficiently frequent intervals, we 

 shall find that a particle may move : 



1. In a straight line with alteration of distance from one 

 plane. 



2. In a straight line with alteration of distance from two 

 planes. 



3. In a straight line with alteration of distance from three 

 planes. 



4. In a curved line with alteration of distance from two 

 planes. 



5. In a curved line with alteration of distance from three 

 planes. 



20. Practical Measurement of the Paths of Moving Bodies, 

 The three necessary planes from which measurements of 

 position have to be taken, may be illustrated by three plane 

 pieces of wood, screwed together at right angles, as shown 

 below. For readiness in calculation and measurement these 

 pieces of wood should be covered with paper containing lines 

 ruled at right angles to each other vertically and horizontally 

 on each surface. By this means, the three inner faces are 

 covered with equal squares of, for example, a centimetre in 

 the side ; and if each of the lines so drawn is numbered, the 

 position of a body with regard to any plane is readily per- 

 ceived. The numbers may be written along the three lines of 

 junction as shown in the figure. 



A small body fixed upon a wire, which is curved at the 

 bottom so as to stand, will serve as the body of which the 

 position requires to be defined. Different values result from 

 the measurement as its position is changed. Several observa- 

 tions should be made and recorded. It will be noticed, how- 

 ever, that the distance from one plane, which is the one on 

 which it stands, does not vary from the nature of the support. 

 This may be varied by bending the wire. A little considera- 

 tion of the model will show that we may get sufficiently 

 accurate results by shutting one eye and looking with the 

 other at the position marked by the body on each surface in 

 succession. 



