48 



DIAGRAMS. 



Methods in which diagrams are used for purposes of measurement are called 



(Jraphkml method*. 



of illustration, if sufficiently accurate, may be used for purjxxsea 



of measurement; and diagrams for measurement, if sufficiently clear, may be 

 used for purposes of demonstration. 



There are some diagrams or schemes, however, in which the form of the 

 parts is of no importance, provided their connections are properly shewn. Of 

 tfrfr fcjnd are the diagrams of electrical connections, and those belonging to 

 that department of geometry which treats of the degrees of cyclosis, periphraxy, 

 linkednen, and knottedne 



Diagrams purely Graphic and mixed Symbolic and Graphic. 



Diagrams may also be classed either as purely graphical diagrams, in which 

 DO symbols are employed except letters or other marks to distinguish particular 

 points of the diagrams, and mixed diagrams, in which certain magnitudes are 

 represented, not by the magnitudes of parts of the diagram, but by symbols, 

 such as numbers written on the diagram. 



Thus in a map the height of places above the level of the sea is often 

 indicated by marking the number of feet above the sea at the corresponding 

 places on the map. 



There is another method in which a line called a contour line is drawn 

 through all the places in the map whose height above the sea is a certain 

 number of feet, and the number of feet is written at some point or points 

 of this line. 



By the use of a series of contour lines, the height of a great number of 

 places can be indicated on a map by means of a small number of written 

 symbols. Still this method is not a purely graphical method, but a partly 

 symbolical method of expressing the third dimension of objects on a diagram 

 in two dimensions. 



Diagrams in Pairs. 



In order to express completely by a purely graphical method the relations 

 of magnitudes involving more than two variables, we must use more than one 

 diagram. Thus in the arts of construction we use plans and elevations and 



