., to be drawn with the means
at the draughtsman's disposal. Draw a curve as shown in fig. 72 with arcs
of the length l_1, l_2, l_3, &c., and with the radii r_1, r_2, &c. (note,
for a length 1/2l_1 at each end the radius will be infinite, and the curve
must end with a straight line tangent to the last arc), then let v be the
measured deflection of this curve from the straight line, and V the actual
deflection of the bridge; we have V = av/b, approximately. This method
distorts the curve, so that vertical ordinates of the curve are drawn to a
scale b times greater than that of the horizontal ordinates. Thus if the
horizontal scale be one-tenth of an inch to the foot, a = 120, and a beam
100 ft. in length would be drawn equal to 10 in.; then if the true radius
at the centre were 10,000 ft., this radius, if the curve were undistorted,
would be on paper 1000 in., but making b = 50 we can draw the curve with a
radius of 20 in. The vertical distortion of the curve must not be so great
that there is a very sensible difference between the length of the arc and
its chord. This can be regulated by altering the value of b. In fig. 72
distortion is carried too far; this figure is merely used as an
illustration.

38. _Camber._--In order that a girder may become straight under its working
load it should be constructed with a camber or upward convexity equal to
the calculated deflection. Owing to the yielding of joints when a beam is
first loaded a smaller modulus of elasticity should be taken than for a
solid bar. For riveted girders E is about 17,500,000 lb per sq. in. for
first loading. W.J.M. Rankine gives the approximate rule

Working deflection = [delta] = l squared/10,000h,

where l is the span and h the depth of the beam, the stresses being those
usual in bridgework, due to the total dead and live load.

(W. C. U.)

[1] For the ancient bridges in Rome see further ROME: _Archaeology_, and
such works as R. Lanciani, _Ruins and Excavations of Ancient Rome_ (Eng.
trans., 1897), pp. 16 foll.

BRIDGET, SAINT, more properly BRIGID (c. 452-523), one of the patron saints
of Ireland, was born at Faughart in county Louth, her father being a prince
of Ulster. Refusing to marry, she chose a life of seclusion, making her
cell, the first in Ireland, under a large oak tree, whence the place was
called Kil-dara, "the church of the oak." The city of Kildare is supposed
to derive its name from St Brigid's cell. The year of her death is
g