20 



ELEMENTS OF LABORATORY WORK 



each from a given origin, and drawing corresponding lines on 

 paper. 



We shall now proceed to show that such a representation 

 is only partially true. It describes relative distances only. 



Fig. 5. 



17. Means of Defining the Position of a Small Body with 

 regard to a Fixed Point. It is clear that we know nothing 

 of position except by reference to some point taken as fixed ; 

 and, since our description is always relative, it is of no import- 

 ance to determine the real condition of this point, whether fixed 

 or moving. Our statements and measurements are made on 

 the assumption that the point of reference is fixed, although 

 further investigation might show that the point of -reference 

 is far from fixed. 



It must not be supposed, however, that the measurement 

 of the linear distance of a body from a point, considered 

 as fixed, completely describes position. It is necessary to 

 state the direction in which the distance has been measured. 

 For example, the circumference of a circle consists of an 

 infinite number of points equidistant from the centre of the 

 circle. And all positions on the surface of a sphere are the 



same linear distance 

 from the centre of the 

 sphere. Something 

 more than linear dis- 

 tance is needed. We 

 require to know the 

 direction in which the 

 operation of linear 

 measurement has been, 

 or is to be, made. 



An inspection of 

 the diagram (fig. 6) 

 will show that if it be merely a question of the position of 

 the particle A with regard to the point o, each of them being 

 upon this flat sheet of paper, we may understand what is 



M 



Fig. 6. 



