Uses of a Line-Divider. 283 
which represents the mean pressure of the engine, divide the 
area EK H C G into ten parts by lines parallel to HC and HB, 
and equidistant from one another ; bisect each of the areas 
thus obtained by lines parallel to the last. Let the points 
where these last lines meet the diagram be aa’, bb’, &e..... 
Add together the lengths aa’, bb’, &., and divide by 10; the 
result will give the line representing the mean pressure of the 
engine. This process of dividing the diagram, first by ten 
equidistant lines and then bisecting the spaces between these 
lines, is generally a very troublesome one, effected by means 
of a system of parallel rulers and a T-square. In the figure 
the dotted lines are those not required when the line-divider 
is used. 
To divide the diagram by means of the rule, place the rule 
as if for dividing the area by 20 equidistant lines parallel to 
HC. Draw the first of these lines, and then every alternate 
one, and the diagram is divided as required. 
This method of dividing areas is also convenient for draw- 
ing treads and rises of stairs, joists, roof-timbers. 
In the third method the divider is used simply as a parallel 
ruler. As such it has the advantage of having a wide range, 
and of being firmly fixed at the same time. In graphical 
statics, where it is often necessary to draw a line parallel to 
another ata considerable distance from it, set squares have 
often to be placed and replaced five or six times before the 
necessary line can be drawn. This instrument will at once 
draw the line. The range as a parallel ruler may be still 
_ further increased by having the plain rule considerably longer 
than the hinged one. 
