— 186 — 
the play of the pins in the eye.«. 
Next join the consecutive pin centres producing them each way a little more 
than a panel length, so as to facilitate the erection of perpendiculars thereto. Then 
at encli of the different; centres erect a perpendicular to eacli centre line meeting 
there, and bisect the angle between the perpendiculars : the line of bisection will be 
tlie centre Jine of the post. Great cavo must be exercised in tui.niug these right 
angles with tlie beam compasses, two points on each of the perpendiculars being 
found, so that if these two points and tlie centre be iu exact line, the perpendicular 
may be relied on as correct. On each of these centre lines lay off tlie depth of the 
truss， and complete the skeleton diagram. 
A partial check on the accuracy of the construction may be had by measuring 
tlie panel length of tlie top chord, which should agree with the length calculated as 
follows. Let / 
j) = the increase in tlie panel leugfcli of tlie top chord above that of the 
bottom chord , 
c — the camber at tlie centre of the span, 
s = leu g tli of span, 
d = depth of truss, 
and 
d and 8 ruay be measured in feet, and c and p in inches. The panel length of tlie 
top chord will then be ^ = l -f- p f where l is the panel length of the bottom chord. 
This is not a certain proof of tlie accuracy of the work. Two consecutive post 
centre lines might be equally inclined from their correct positions, and on the same 
side, though this would be shown in the next panel. A certain check must be 
obtained by measuring the lengths of the diagonals, which should be equal to each 
other, and agree with that found by tlie formula 
where D is the length required. 
Iu double intersection bridges the exact lengths of the long diagonals can bo 
found by the following method. 
