24 



ELEMENTS OF LABORATORY WORK 



Instead of varying the position of a body we may support 

 a wire, a 6, as shown in the diagram, in such manner as to 

 represent the path through which an imaginary body has 



been, or is being, dis- 

 placed. The position of 

 the wire may be varied in 

 any manner, and it may 

 be made to assume any 

 shape. A number of 

 measurements should be 

 taken of various possible 

 paths. 



If we now introduce 

 an irregular-shaped body 

 of considerable dimen- 

 sions, it will at once be 

 seen that three linear 

 measurements are not 

 sufficient to define its 

 position. They would 

 in fact define nothing 

 more than the position of that portion of the body from whicli 

 they were made, and no information would be supplied as to 

 the rest of the body. In order to obtain this information 

 accurately each portion of the body would require to be defined 

 by its distance from the three planes ; but if the body is a 

 rigid body, it is generally sufficient to make measurements from 

 various portions of its surface. The more numerous the por- 

 tions measured, the more accurate will be the description 

 given by them of the position and shape of the body. It 

 will be readily noticed that by shape we mean relative position 

 of parts. 



21. Observation of Rotation. It must not be supposed that a 

 knowledge of the exact position of any given portion of a body 

 at two different times is sufficient for the purpose of describing 

 the movement which the body has undergone in the interval, 

 however small this interval may be. "We might find that in 

 the interval the separate portions of the body have moved 



Fig. 7. 



