With the fingers of the right hand he moved the lower half down, which, in its turn, he kept firmly in place, while he slipped the upper half over the paper, thus preserving the direction between the points.

By this process the parallel ruler could be moved all over the chart without losing the course from one point to the other.
On every chart there are one or more diagrams of the compass, with lines diverging from a centre, representing all the points.

The parallel ruler is worked over the chart to one of these diagrams, where the direction to which it has been set nearly or exactly coincides with one of the lines representing a point of the compass.
The first master of the Young America worked the ruler down to a diagram, and found that it coincided with the line indicating east by north; or one point north of east.
"That's the course," said Thomas Ellis, the third master--"east by north." "I think not," added Foster.

"If we steer that course, we should go forty or fifty miles south of Cape Sable, and thus run much farther than we need.

What is the variation ?" "About twelve degrees west," replied Martyn.
The compass does not indicate the true north in all parts of the earth, the needle varying in the North Atlantic Ocean from thirty degrees east to nearly thirty degrees west.