C 65 3 
perfectly This pin or pivot enters a box of 
brass exactly fitted to it and let into the bottom of 
the circular board exactly in its centre. The cir¬ 
cular board is 22 inches diameter, precisely of the 
thickness of the frame, made of well seasoned wood, 
secured in the best manner against warping and 
graduated as represented in the draught. At thes 
bottom of the letters N. S. E. W. are cavities made 
with a centrebit about half way through the circu¬ 
lar board to serve the fingers in turning it. At H 
a plate of brass or ivory is laid in the frame with a 
nonius graduated on it to adjust a course to parts of 
a degree. , ’ 
To USE THE In STRUM EKT, fasten a sheet of pa¬ 
per on the circular board by means of a little bees 
wax applied to its corners ; then turn the degree of 
the course to be protracted as marked on the edge 
of the circle to the o on the nonius : Have a square 
with a projecting shoulder which slide along the 
lower edge of the frame till it touch the point from 
which the course is to be drawn and then draw the 
course along the edge of the square. For e^ampk. 
If a course south 30 degrees east is required to be 
drawn from the point 5 in the traverse represented 
on the draught ; bring the division marked SO in 
the graduated circle between the letters S & E to o 
on the nonius. The square (represented by the 
dotted lines) with its shotilder pressed etose against 
the frame is then moved till it touch the point 5 aitd 
the line 5, Is drawn along it, which^ as is 
