BUILDING. 
letter L ; a tee roof upon a plan in the 
form of the letter T; and an aitch roof 
upon a plan formed like the letter H ; but 
when two common roofs, having tlieir 
ridges parallel to each other, and a side of 
the one either joins one of the other, or 
these two sides intersect each other, and 
thereby leave a gutter above the roof ; 
then the roof which is thus compoimded of 
the two simple roofs, is called an em roof, 
as the verticle section is in the form of the 
letter M, or rather an inverted W as M : 
this is an instance wdiere the roof is deno- 
minated by the vertical section, and not by 
the plan. j\ll roofs whatever are said to be 
truncated, whether they terminate in a 
plane or raised platform, or have a voi.^ ..t 
the top, bounded by a level curb. 
When the sWe of a roof is a plape surface, 
except at the eaves, at which place it is 
concave, the roof is said to have a bell 
cast at that place. 
The general names of the timbers are 
straining pieces, tie pieces, and bearers; 
under straining pieces are included princi- 
pal rafters, camber beams, hip and valley 
rafters, collar beams, or straining beams, 
straining sills, struts, auxiliary rafters, or 
principal braces and studs. 
Under tie pieces are included, tie beams, 
diagonal ties, and truss posts ; and under 
bearers are included, plates, purlins, com- 
mon rafters, small rafters, ridge pieces, 
boarding and dragon beams. 
The sloping sides of roofs are of two 
kinds, single and doable, or plain, and car- 
case : single roofs are those which have one 
row of rafters upon t'oe same side ; double 
or carcase roofs are those which have two 
tie of rafters ; the lower tie supporting the 
upper by the intervention of transverse 
pieces called purlins. 
Stone Bridges. A stone bridge is a thick 
wall built across a hollow, with one, two, 
tliree, or a series of apertures, formed into 
arcades, which either serve to lighten the 
masonry, or to give passage to a stream of 
water, or both. 
When a stone bridge is resolved upon, the 
first consideration is its place : in this seve- 
ral particulars should be taken into consi- 
deration, and tlie advantages compared to 
the disadvantages. As the height of the 
bridge depends on the banks of the river, 
the expense will be increased according to 
their height, therefore a convenient situa- 
tion should be chosen, where the banks will 
be adequate in height to that necessary for 
the bridge, though the expense will be in- 
creased by the length of the bridge. lu 
most cases, where the river runs in the vab 
ley, a wide part of the stream must be pre- 
ferred to a narrow part, as the water at this 
narrow er part has not only a greater degree 
of velocity of itself, but the velocity would 
also be increased by the piers of the bridge ; 
in times of heavy floods it would be liable to 
be thrown down, and in a navigable river 
the navigations would be impeded. As the 
expense depends on the bed of the river, it 
must also be taken into the account. 
These being settled, the form and heights 
of the arches come next under considei-a- 
tiou ; the height of the arches, which deter- 
mine that of the bridge, depends on the rise 
of the water in time of floods ; and whether 
there is to be a navigation, and what kind of 
vessels there are to pass. 
Stone bridges ought to be constructed 
with as few arches as possible, which will 
not only give greater beauty, but will re- 
quire fewer foundations, piers, and center- 
ings, and also easier passage for craft. The 
piers ought to be so proportioned as to en- 
able them to withstand the thrust of the ad- 
jacent arches, though the rest were thrown 
down. The number of arches ought to be 
odd, in order that one may stand in the 
middle, where the stream has its greatest 
velocity. 
AThen the passage-way along the top of 
the bridge is a convex curve, the arches 
should diminish from the middle towards 
each extreme, so as to be similar to the 
middle one, tliis will allow a more flee 
passage to the water, the velocity being 
greatest in the middle. With respect to the 
choice of arches, the elliptical, cycloidal, 
and equilibrated arches, are not only conve- 
nient, in allowing more room for the pas- 
sage of ships at the handles ; but they are 
also more strong, and require fewer mate- 
rials tlian most other curves of the same 
dimensions. 
When the extrados is convex, and the 
height of the arch small in proportion to the 
span, a segment of a circle may be used 
with success : in this case the arch should 
not exceed 60 degrees. 
These particulars being fixed, the practice 
is as follows : — When the foundation of a 
stone bridge is to be laid in a river which is 
not very deep, a single or double inclosurc 
of wood is formed, and the intervening space 
is rammed well with clay or chalk, to pre- 
vent the water from coming in. These in- 
closures are eitlier made with piles driven 
closely together, and dovetailed, at their 
