GRAPHICAL ALGEBRA. 



97 



engraved on the upper side of the lower sheet and on the under side of the upper 

 sheet. When using the instrument it will be best to have illumination from below. 

 Otherwise the pictures seen will be blurred by the shadows which the lines will throw 

 on the paper. 



Fig. 79. Resultautometer. 



When one of the divided sheets is made to rotate relatively to the other, the 

 two sets of parallel lines produce parallelograms of all possible shapes. The inter- 

 section of the lines gives at the same time equidistant divisions on each line, which 

 can be used for measuring the lengths which shall represent the scalar values of the 

 given vectors. In this manner we use a variable unit length, which varies with the 

 angle between the lines. This is a great advantage, for we get automatically the 

 construction performed on a large scale in the difficult case in which the given vectors 



