4v6 Description and Use of The Sectograph. 
drawn, and extend the extremes to any distance along the 
line, as from A to C; then the central point will fall in the 
perpendicular line at D. 
To find the measure of an angle, 
Rule.—Fix the central point in the intersection of the 
lines, or angular point, and bring the extreme points to the 
containing lines. Then this extent or distance of the ex- 
tremes applied to the line of chords, will show the measure 
of the angle required. 
To make an angle of any number of degrees less than 90. 
Rule.—Apply the extreme points to the line of chords, 
and take between them the measure of the angle. Then 
fix the central point in the angular point, and the extremes 
will fall in the lines to be drawn from that point. 
N.B. If it be required to make an angle less than can be 
taken between the extreme points of the instrument,—draw 
a perpendicular or angle of 90°, and then lay down the 
complement of the required angle.- 
2. Of the Sines, Tangents, and Semi-Tangents. 
The lines of tangents and semi-tangents are particularly 
useful in the projection of the sphere, and the line of sines 
in drawing the parabola by points. No further explanation 
secms to be necessary here, as that is given in every trea- 
tise where their use is required. 
3. Of the Line of Polygons. 
This line is numbered the contrary way, because the 
Jengths of the sides increase as their numbers decrease : it is 
regularly divided from 3 to 24, and numbered; but the 
numbers 11, 13, 15,17, 19, 21 and 22 are left out for want 
of room. 
When a polygon of several sides is inscribed in a circle, 
by continuing the division with one side only taken from 
the scale, the error (if any) will increase with the number ; 
and will consequently at iast be very considerable. It will 
therefore be necessary to divide the circle into parts of 2, 
3, or more sides each; and then take one side between the 
extremes from the line of polygons, and finish the poly- 
gon by subdivisions. This may be done very correctly for 
composite numbers. 
Thus the number 21 is composed of 3 and 7: therefore, 
divide the circle first into 3 parts, and then take the side of 
21 from the line, and subdivide each division into 7. 
Again, 
