ON BRIDGES. 12& 



Figure 59, is frequently employed, for reasons that we shall consider 

 further on. Such curves are called by the French engineers basket- 

 handles, (anse de panier.) For arches which bear no load except that 

 of their own weight, the catenary curve — that which a chain assumes 

 when fastened at both ends and allowed to hang 

 freely — is sometimes used. 



Circular arches may either be semicircular, 

 (full centre,) as in Figure 49, or segmental, as in 

 Figure 48. 



Two segments of a circle of equal radius are 

 sometimes joined at an angle more or less acute, 

 as in Figure 56, a form known as the Gothic arch. 

 This form is frequently used in architecture, but 

 rarely in bridge-building. The aqueduct of Spo- 

 letto is a rare instance of its employment in an engineering structure. 



Suspension bridges, formed by stretching a rope or chain over a 

 chasm, and supporting the foot-way from it, were very early used, 

 since the principles involved in their construction are simple, and 

 the materials for them are readily obtained in new countries. In 

 parts of South America they are frequently seen across mountain 

 chasms and streams, the material used being either flexible rods, or 

 poles properly joined, or tough vines, such as the grape, forming a 

 sort of rope, from two of which the foot-way is suspended or on 

 which it is laid. 



There are accounts of very ancient iron chain bridges found in 

 India, constructed upon the same principle. 



The suspension bridge may be regarded as the converse of the 

 arch, since it is not only inverted in form, but all its parts are sub- 

 jected to a tensile strain instead of one of compression. Wrought 

 iron is the material which is naturally the best adapted to this method 

 of construction, and is generally used. 



A suspension structure, to bear a given weight, may be made with 

 but little waste of material, since tensile strains are usually trans- 

 mitted with great directness, and, being easily calculated, the mini- 

 raum of material may be used. For this reason suspension works 

 are characterized by lightness and cheapness, two great advantages. 



The principal objections to them are, first, their flexibility, which 

 can be provided against with proper care, and the space which they 

 occupy beyond their true span — only objectionable in certain locali- 

 ties. 



The weight of the bridge and its load tend to draw the extremities 

 of the chains together, and if they were merely fastened to the tops 

 of the towers which support them, these towers would be overturned. 

 It is necessary, therefore, to extend them on the land side of the 

 towers, and to anchor them in the ground at a considerable distance 

 from the foot of the tower to prevent such action. In a city this is 

 an objectionable feature, since the land chains interfere with the free 

 passage in the streets. The fundamental principles governing\the 

 suspension bridges are so simple that it will not be necessary to dis- 

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