114 SECTIONAL ADDRESSES 
on the resistance to compression offered by stone or brick. A com- 
plementary form of structure dependent on the resistance to rupture by 
the pulling asunder of its parts, is the suspension bridge, the stability of 
which depends almost entirely on the tensile resistance of the chains or 
cables. ‘The greatest structure of this form is undoubtedly the George 
Washington Bridge over the Hudson River, New York, with its span of 
3,400 ft. 
In most iron and steel structures the resistance of the material to both 
tension and compression contributes to their stability in equal proportions, 
as is found in the great girder and cantilever bridges. 
Reinforced concrete, in which the great strength of concrete to resist 
compression is combined with the power of steel to resist tension, owes 
its development largely to the facility with which it can be built and 
shaped. It has been applied to many large structures which present 
problems in stability of considerable interest. 
Tunnels of masonry or brickwork, and cast-iron lined tube tunnels, 
subject to the pressure of great depths of earth, are forms of structure 
the stabilities of which are not easy to calculate. 
In estimating the stability of a structure, the principal factors are the 
strength or resistance to rupture of the material and the balance or direc- 
tion of the forces or loads brought to bear on it. The ultimate strength 
of a simple part of a structure can be calculated without difficulty by 
applying to it the breaking stress determined by experiment for the 
material used. It is not easy, however, to estimate the strength of that 
element to withstand the long-continued action of loads and forces to 
which it might be subjected in use. Thus, the elementary part in 
question might be required to withstand, for an indefinite time, a load of 
ten tons applied in a particular way. ‘The part could be designed so that 
it would require 100 tons, ten times the working load, to break it, and it 
would no doubt fulfil the requirement and carry the ten tons satisfactorily. 
On the other hand, it might be made with a breaking strength of only 
thirty tons and be able to carry the imposed loading equally satisfactorily. 
The second design requiring so much less material, would be the more 
economical and more correct one. Although put simply and somewhat 
crudely, that is the essential problem underlying the measure of 
stability having regard to the economical use of material, and it is 
interesting to note that, so far as applies to civil engineering structures, 
the advance made in its solution during the last fifty years has not been 
great. 
The rupture or breaking down of a structural element by a force is 
dependent on the detail of its incidence and the resulting intensity of the 
stress induced in the material. To make clear the manner in which the 
stability of some forms of structure is gauged, it is necessary to outline 
the approach to the problem. 
To fix the directions of, and arrange for the balance of loads and forces, 
the conception of action along lines was introduced at an early stage. 
The position of such a line with respect to the boundary of a member 
offering resistance, governs the distribution and intensity of stresses in 
the material. In estimating the intensity of stress, the position of the line 
