' 18 c) — 
ei， system above the elevation, in connection with the uppermost view of t.lie top 
or 、 ’ which slioukl be tbe plan from above. Then, again, if the bridge be a large 
e ’ tlie height may bo so great that it will be impossible to show the plan below 
# a 10u > iu which case it will be necessary either to make separate drawings 
In F ai1 ail< ^ e lcvatioi], or to place one alongside of the other on the same sheet. 
so tracings of the working-drawing, the traciug-cloth can be shifted about 
:)s to group similar parts and so as to avoid too much intersection of different 
Potions. 
Provided that any piece be symmekical about a plane cutting it at tlie middle 
the S . en 抑 ailt l at i'ight angles thereto, it will be sufficient to show only oue-lialf of 
cent! 初 6 ’ aU( 】 仏 6 measuremeu も be referred to the end of the member, to tlie 
it is no/) aUe ’ 0r も 0 hotli. Where the same detail is used in more places than one, 
k a necessary to show it more than once, provided that it be exaclhj the same 
m every respect. 
bril aU ill Us 【 ra tion of liow to make a working. drawing, take the case of tlie 
four ぶ ea e d in the last chapter, and assume that the paper and table are each 
G ] eva ^ a 卜 l 】f feet wide. Using the scale of an inch to tlie foot, the depth of the 
five i °| U a b ou t two feofc two inches, and the width of tlio plan about one foot 
elevati 的 Allowing five inches above the elevation, and as much more between 
of tlie U aU( i l)l an , bring tlie lower side of the plan within an inch of the edge 
with t/ 沖 61 ’ 咖牙 arrangement will do very well. The first step is to draw a line 
parallel S ^ ra ^ 1 ^" e( ^® e, as near ^y as possible, without taking too much trouble, 
below tl わ 】 en gth of tlie paper, and at a distanco of two feet five inches 
be ma、 le 叩㈣’ e ^^ e# This line should be very fine and perfectly straight. It can 
War^ s 1 so by prolonging it half tbe length of the straiglit-edge at a time, mil after- 
^ left ^ ln ° ^ 8everft l places. On this line take a point a foot or move from 
alo n „ aiR l eiu l of the paper, as tbe centre of the end lower chord pin. Lay off 
0 f the d me W “l“] ie greatest possible accuracy the panel length, until tlie cenlie 
At th e -, 11 reac hed: in this case twenty one feet must bo laid off four times. 
penjj cu j aUe P 0 i n t s erect short perpendiculars with tlie triangles, anti on t.lie per- 
Cliapter マ/ 1 も — ceu ^ l，e ^ a y the camber, which in this case is three inches ( vide 
been ne • . 打 ^ the bridge contained an odd number of panels, it would have 
each eU( ^ SS f 1 ^. draw tliG middle panel, and lay off tlie camber of tlu.ee inches at 
the fa]} f 0 18 P aue 】. Tlieu, assuming the curve of the chord to be a parabola., 
也 1 * Uiplie ぶ 0 f 讼 10 cen 也 1 . e to any panel point is equal to the camber at the centre 
from .? -^ ie 8( l uar e of tlie ratio of the distanco of tlie panel point considered 
ThuJ^ 1 也 び 出 6 s P an to the balf-lengtli of span. 
Will ]^ e S m case considered, tbe falls at the first, second, and third panel points 
of these 1 eSpCC ^ ve ^ 8 ( そ) 2 , 3(i) 2 , and 3 ⑴ 2 , or 科 "， ami ず making tlie heights 
各 ”、 0X f 二 ts above the horizontal lino respectively 3"— 料", 3" — 普"， and 8" — 
Oculars ’ 2 】 and 2Ü"; which distances are to be laid out upon tbe perpen- 
Panels as 〇 十 aS 七 0 ^ oca ^ e centres of tlie lower chord pins. Tlie length of tlie 
happrecilbl U1S determined differ from those of their horizontal projections by an 
e < l uan tity. If there be any lengthening of the chord, it may go against 
