122 SECTIONAL ADDRESSES 
comparing angles on the two plates. Our investigations enabled us to 
plot curves of stress-distribution on section lines at various heights. 
The curves were of quite different shape. We found no evidence of the 
reputed tensile stress at the downstream toe. The shear stress diagram 
was practically a triangle with the maximum at the downstream edge, and 
the vertical stress distribution agreed substantially with the‘ trapezium 
law.’ 
These experiments helped materially to clear up the situation and to 
re-establish confidence in the method that had been in general use for 
estimating the stability of masonry dams. 
During the last few years investigations, both experimental and mathe- 
matical, of problems relating to the design of large concrete dams and 
curved dams, have been made in the United States. The influence of 
heat, both natural and that generated by the setting of cement, on stresses 
and stability, has received much attention. In these gigantic structures, 
monolithic construction and the use of too large masses of concrete has 
been found accountable for serious cracking. In discussions on these 
problems, the investigations I made with Gore have been referred to as 
‘the English Tests.’ 
The suspension-bridge or ‘ philosopher’s bridge,’ as it has been called, 
is a fascinating type of structure. In the course of the development of 
its design and stability there have been some astonishing occurrences. 
In its most elementary form the suspension-bridge formed of strong 
flexible climbing trees or roots has been used by primitive peoples for 
centuries. Examples made of wrought iron appear to have been in exist- 
ence in the eighteenth century. Samuel Brown, who was granted a patent 
in 1817, designed and constructed some of the earliest in this country. 
His main suspension chains were made of long open welded links or long 
plate links connected together with round bars and shorter plate links. 
The chains, which were made up with several links side by side, connected 
with common hinge pins, were of uniform strength throughout their 
length, and the road or platform was suspended by vertical rods from the 
short links. In addition to the main chains Brown put chains at the 
platform level to prevent undulation. Within its limitations it was a 
satisfactory form of construction. In a bridge which carries a series of 
loads on a flexible chain, the loads and the chains are only in equilibrium 
when the chain assumes an appropriate shape, and to support any addi- 
tional weight or rearrangement of weights the chain changes its shape 
slightly. With a moving load the tendency of the platform of a suspension- 
bridge to undulate with the passage of the load has handicapped the 
development of this type of bridge. An early attempt to use it for a 
railway proved a complete failure. 
Telford’s famous bridge across the Menai Straits, with a span of 57oft., 
completed in 1826, is of the simple suspension type. At first the platform 
was too flexible and caused anxiety, but that part was altered and made 
stiffer. The bridge is still in service, and is standing proof that in 
principle and construction it was sound. A few years later, a stage in the 
quest for stability in suspension-bridges reflects in a remarkable degree 
