42 TECHNOLOGY. 
to show the arrangement of the timbers. The boldness and simplicity 
of this bridge are equally to be admired. The greatest part of the thrust 
and the whole weight of the covering are thrown by the framework a} upon 
the abutment ¢ and the timbers de¢c', whilst the arch has only its own 
weight and that of the movable load to support. In 1838 this bridge was 
burned, and its place is now supplied by a wire bridge of single span. 
A new system of bridge-building recently come much into use should be 
here mentioned ; it was invented by Laves, chief architect to the court of 
Hanover. Laves had already invented a peculiar method of building 
beams, by which he had attained great strength at comparatively small cost. 
The girders were sawed longitudinally each way from the centre to within 
two feet from the ends, as seen in fig. 35. At each end where the cut 
commences the girder is bound with iron rings, a, two inches wide and half 
an inch thick, to prevent the entire splitting of the timber. The two por- 
tions of the bridge were then driven apart by wedges, 6, and a girder was 
obtained, having all the strength of a flat arch without the thrust, only 
wall-plates 4 B being required to give the ends an even bearing. Shortly 
afterwards the inventor carried the idea further and. constructed his girders 
of two timbers notched together at the ends, jig. 36; as in the former case 
no abutments were required and no thrust was axons 
Subsequently this method of construction was extended to bridge-building 
by the inventor. The principles of the application will be made apparent 
by an inspection of pl. 8, jig. 30, the cord below being united with the 
bridge by the braces ac and bc, whilst the diagonal braces serve to render 
the structure self-sustaining and stiff. 
The same system, carried out with rather more attempt at beauty and 
ornament, is seen in jigs. 31 and 32; jig. 33 is a cross-section, and jig. 34 a 
plan showing the arrangement of the braces. This system would be very 
limited in its capabilities of extension were it confined to a single beam ; 
this is not the case, however. Any number of beams may be scarfed 
together, as seen in jigs. 87 and 38, to form either the roadway or the tie- 
beam beneath. In fig. 39 is seen the method of giving the ends of the 
bridge-frames a solid bearing upon the abutments. 6 and @ are the upper 
and lower timbers bound together by iron rings, dd, and ¢ are wedges also 
notched to the lower beam and which serve to give it a firm bearing upon 
the head of the pier. 72g. 40 is a section through an arch of this descrip- 
tion ; a and 0 are the timbers, and g the blocks which serve to keep them in 
their places. A large bridge built entirely upon this principle is seen in 
jig. 41; fig. 42 is a plan showing the diagonal braces, and a portion of the 
road covering ; jig. 43 a cross-section of the bridge; jig. 44 the scarfing of 
the timbers of the tie-beam ; jig. 45 the joint at the end where the girders 
rest upon the abutment. 
C. Iron Bridges. 
When the arch of a bridge is constructed of iron it is called an iron 
bridge, although the piers and abutments may be of stone and the floor or 
roadway of wood. These bridges are variously constructed ; in very short 
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