C 67 3 
least with any othet in common use. The large¬ 
ness of the circle on which the graduations are 
made contributes much to its accuracy : Be¬ 
sides, the mind is entirely relieved from that con¬ 
stant attention, which is required in the usual way 
of protracting, to determine whether a given course 
is to be laid oif to the right or left of the me¬ 
ridian. The operation is more simply mechanical 
and therefore less ^iresome and less liable to mis¬ 
takes. 
DESCRIPTION 
OF A FIELD PROTRACTOR, 
By the same» 
A, B. C. D. is a box constructed like a back¬ 
gammon box, but not so deep, having the sides ex¬ 
actly at right angles to each other : The size when 
shut is 8 by 14 inches, or 17 by 14 inches, when 
open. A a Sc a B are plates of brass laid into the 
back of the box along its upper edge, a b is the 
line on which the two parts of the box when opened 
are made exactly to join, c d e is a bevel, the limb 
c d of which moves on the centre d and is fastened 
or loosened by means of a screw, when folded down 
it falls into a groove made to receive it in the edge 
of the part e d. To graduate the instrument, bring 
the end of the bevel even with the edge B D keep, 
ing the part e d pressed close against C D. Then 
from the centre d on which the limb of the bevel 
