By this process the parallel ruler could be moved all over the chart without losing the course from one point to the other.
It will be of some interest to teachers to see one of the early forms of this parallel ruler, as shown in the illustration.
Mr. Spokesly laid the parallel ruler against the line and produced it clear across the chart.
The lieutenant followed him, and stood watching as the master again manipulated his parallel ruler and dividers.
Spreading it on the table he ran his parallel ruler over it.