— 190 — 
some other m'omment part. Show a section of each member, and write tlie dimen- 
sions of all channels, angles, I-beams, etc., near the section. Write along eaclx 
piece its extreme length or lengths, its length from centre to centre of eyes, and of wliat 
ic is composed, Tlie ends of the two pieces of an adjustable rod should be separated 
by at least three or four inches in the turn buckle or sleovo nut. Mark what rivets are 
countersunk, and at which end. It* the scale of the drawing be large enough, the 
rivetting can bo thus represented — draw full parallel lines across the rivet for 
countersinking on the upper side, dotted parallel lines for countersinking on the lower 
side, two sets of parallel lines crossing each other at right angles for countersinking 
on both sides, and solid black circles for rivet holes to be left open. Be careful to 
always note how many pieces are to be made as shown and liow many opposite 
hand, when there are both rights and lefts. 
Lay out all bevelled ends on an enlarged scale say from half to full size, anti 
mark their dimensions along the edges, referring all measurements to a transverse 
line through some well defined point as the centre ot* the pin liolo. These measure- 
ments should be checked by calculation. The slight bevels at the joints of the top 
chord sliould bo treated with as much accuracy as the bevels at the liip joints ; but» 
as the bevel is very slight, it will be legitimate to put it all on one of the abutting 
ends making the other a square cut. 
The centre lines for lacing bars on the under side of a strut should be dotted* 
In laying out a long row of rivets, for instance kttice rivets or those for the top 
plate of a chord or batfcor brace, calculate the distances of some of the intermediate 
rivet holes from one end of the strut, then interpolate the other holes ； because, 
the spacing be laid out continuously from one end with the dividers, any error iw 
the span of the dividers will be multiplied by the number of times that the distance 
is laid off. 
After laying out a complete system ot rivets for any member, check by seeing 
that the sum of the distances between rivet holes plus the distance of each end rivet 
from the end of tlie member is equal to the total length of the member. 
Make duplicates of as many parts of the bridge as possible even at tlie expense 
of a small amount of iron, not only to save time in draughting but also in the shop 
and to facilitate tlie work in erection. Arrange to have as few loose pieces ior shipment 
as possible, and mark on the drawing of each connecting piece to what ib is to be atta- 
died or whether it is to be left loose. Thus the hip connecting platos should bß 
attached to the chords and braces and tliose for the top chord to that portion tl ば 0 • 
ugh which the pin hole is bored. If there bo any reason to fear rougli liaudliug 
tJie iron in transit, it may bo necessary to send some of the connecting plates sep^* 
rately, but the more loose pieces the more field rivetting, and the more field rivettinf?» 
tlie greater the erecting expenses and the longor the time and the greater tho 
in raising tho bridge. 
Rivet spacing should bo as regular as circumstances will permit, and all cliu n * 
ges in spacing should be made suddenly instead of gradually so as to facilitate th c 
punching of the holes by machine. 
All measurements should be in feet, inches and the following vulgar fi.ncti ⑽ 13 
