294 
STATE AGRTCULTUEAL SOCIETY. 
suitable sand and lime or cement are conveniently attainable, 
the span required not more than thirty feet, rock foundations 
for the abutments within reasonable depth, and the banks of a 
proper height, the stone arch with stone parapets is, perhaps, 
as economical a structure as can be adopted. Where greater 
spans are required, and the banks are low, stone abutments 
and well constructed frame covered bridges are preferable. 
Not a doubt exists of the economy of siding and roofing 
wooden bridges, and of extending t-oth over the abutments, 
so as to effectually protect from rain the timbers and planking 
at these points, as they are known to decay first when not 
protected. There is a frame covered bridge in Hartford 
county, Maryland, which was built more than fifty years since, 
and is still safe. 
Among the errors in bridge construction, those most com¬ 
mon are the injudicious distribution of material, particularly 
of timber; the contraction of the water way, so as to expose 
the superstructure to liability to be swept from the abut¬ 
ments; neglecting to bolt the superstructure to the abut¬ 
ments; laying the flooring with close joints, instead of with 
proper openings, to prevent water from standing on the floors ; 
using perishable varieties of timber, and even allowing the 
sapwood to be used in part, by which all is reduced to its 
ephemeral character. No errors perhaps more common, and 
none results in so needless and speedy destruction of the longi¬ 
tudinal timbers of bridges, as the want of attention to keeping 
them dry, where they rest on the abutments, and especially at 
the ends where they support the earth-filling of the road-bed. 
The durability of the timbers may be increased by intro¬ 
ducing a light back sill and short light joist about two feet in 
length, with a plank on edge resting against them, to support 
the filling independent of the main, horizontal timbers, that 
air may circulate around the ends; and by covering the ends 
of all the timbers resting oij abutments and piers with several 
thicknesses’of tarred paper, these being the points where decay 
often destroys when the other parts are unaffected. 
