THE ROYAL ARTILLERY INSTITUTION". 
405 
What is it? 
It is a rectangular piece of cardboard. Parallel to its longer edge, 
equidistant straight lines are ruled, the interval between them repre¬ 
senting a certain number of feet on the scale of the plan. Call this 
set of lines A. 
Perpendicular to these, another set of lines (not necessarily equi¬ 
distant) is ruled. Call this set B. 
For the sake of convenience, it is better to mark the lower long 
edge of the cardboard as an ordinary linear scale for the plan, and to 
prolong the divisions upwards to form the B set of lines. (This is 
done in the figure.) 
The accompanying figure shows a sectograph lying on a contoured 
plan. 
How is it Used? 
1. Suppose you have a plan of ground contoured for every 25 ft., 
and that the distance between the lines of the A set in the sectograph 
corresponds to 25 ft. on the scale of the plan. It is required to make 
a section of this ground along a given line XY. (See figure.) 
Place the lower edge of the sectograph along the given line, and 
mark on the sectograph with a pencil the elevation of each point 
where a contour cuts the given line. 
Thus the elevation of the point “ ar” (see fig.) where the 400 con¬ 
tour cuts the given line, will be on the 400 line of the A set; and its 
position “ a ” on that line will be determined by the lines of the B set 
on either side of it. Similarly, the elevations, of b, c, d, e,f> and g, will 
be at b', d, d', d ,f, and g' respectively. By joining on the sectograph 
the points, thus determined, the required section is obtained. 
In practice, a piece of tracing paper is laid on the sectograph, the 
points marked, and the section drawn on the former. 
2. Suppose the plan and sectograph to be as in the first case. It 
is required to find whether a point “a,” (400ft.) on the plan, is in view 
of another point (( g ,} (250 ft.) Place the lower edge of the sectograph 
so that it passes through both points, and adjust a “ straight-edge 33 
to pass through “a! 33 and “ g' 33 (the elevations of “a” and “g” 
respectively). Then the “ straight-edge 33 represents the elevation of 
the line of vision from “a” to “ g. 33 
Now, if the elevation (V, d , d !, <?', or f) of any point ( b , c, d , e, or/) 
is above the “ straight-edge/ the ground at that point obstructs the 
line of vision from “a 33 to “g” and these two points are not in view 
of each other. Otherwise they are. 
It is obvious that it is not necessary to mark any point on the secto¬ 
graph itself; for in the first case the points are marked on the tracing 
paper, and in the second case they do not require to be marked at all. 
